Publications by authors named "Russell Greenberg"

28 Publications

  • Page 1 of 1

Competition and habitat quality influence age and sex distribution in wintering rusty blackbirds.

PLoS One 2015 6;10(5):e0123775. Epub 2015 May 6.

Smithsonian Migratory Bird Center, National Zoological Park, Washington, D. C., United States of America.

Bird habitat quality is often inferred from species abundance measures during the breeding and non-breeding season and used for conservation management decisions. However, during the non-breeding season age and sex classes often occupy different habitats which suggest a need for more habitat-specific data. Rusty Blackbird (Euphagus carolinus) is a forested wetland specialist wintering in bottomland hardwood forests in the south-eastern U. S. and belongs to the most steeply declining songbirds in the U.S. Little information is available to support priority birds such as the Rusty Blackbird wintering in this threatened habitat. We assessed age and sex distribution and body condition of Rusty Blackbirds among the three major habitats used by this species in the Lower Mississippi Alluvial Valley and also measured food availability. Overall, pecan groves had the highest biomass mainly driven by the amount of nuts. Invertebrate biomass was highest in forests but contributed only a small percentage to overall biomass. Age and sex classes were unevenly distributed among habitats with adult males primarily occupying pecan groves containing the highest nut biomass, females being found in forests which had the lowest nut biomass and young males primarily staying in forest fragments along creeks which had intermediate nut biomass. Males were in better body condition than females and were in slightly better condition in pecan groves. The results suggest that adult males occupy the highest quality habitat and may competitively exclude the other age and sex classes.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0123775PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4422684PMC
February 2016

Habitat type and ambient temperature contribute to bill morphology.

Ecol Evol 2014 Mar 13;4(6):699-705. Epub 2014 Feb 13.

Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute National Zoological Park, Washington, District of Columbia, 20008.

Avian bills are iconic structures for the study of ecology and evolution, with hypotheses about the morphological structure of bills dating back to Darwin. Several ecological and physiological hypotheses have been developed to explain the evolution of the morphology of bill shape. Here, we test some of these hypotheses such as the role of habitat, ambient temperature, body size, intraspecific competition, and ecological release on the evolution of bill morphology. Bill morphology and tarsus length were measured from museum specimens of yellow warblers, and grouped by habitat type, sex, and subspecies. We calculated the mean maximum daily temperature for the month of July, the hottest month for breeding specimens at each collecting location. Analysis of covariance models predicted total bill surface area as a function of sex, habitat type, body size, and temperature, and model selection techniques were used to select the best model. Habitat, mangrove forests compared with inland habitats, and climate had the largest effects on bill size. Coastal wetland habitats and island populations of yellow warblers had similar bill morphology, both of which are larger than mainland inland populations. Temperate but not tropical subspecies exhibited sexual dimorphism in bill morphology. Overall, this study provides evidence that multiple environmental factors, such as temperature and habitat, contribute to the evolution of bill morphology.
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http://dx.doi.org/10.1002/ece3.911DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3967896PMC
March 2014

Experimental support for food limitation of a short-distance migratory bird wintering in the temperate zone.

Ecology 2013 Dec;94(12):2803-16

Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA.

The Winter Food Limitation Hypothesis (WFLH) states that winter food abundance is a dominant source of population limitation of migratory birds. Evidence is accumulating that long-distance migratory birds wintering in tropical climates have high overwinter survival probabilities and that winter food limitation mainly affects their fitness nonlethally by limiting energetic reserves necessary for successful reproduction. In contrast, the relative roles of direct mortality vs. indirect effects caused by food limitation have not been investigated thoroughly on short-distance migratory birds wintering in temperate zones, where they face thermal challenges. We performed the first test of the WFLH for a temperate migratory bird in the wild on the Swamp Sparrow (Melospiza georgiana), with a replicated plot-wide food supplementation experiment. In contrast to tropical, but consistent with other temperate-wintering migrants, Swamp Sparrows on unmanipulated plots showed relatively low apparent survival across the winter. Following food addition, birds (1) immigrated to experimental plots, which subsequently supported approximately 50% higher abundances, (2) experienced increases of within-season apparent survival of 8-10%, depending on age/sex class, and (3) had higher-scaled mass index values, all supporting winter food limitation. The last two findings are interrelated because birds with higher scaled mass had higher survival probabilities, further supporting direct effects of winter food limitation. Food limitation of fat reserves might also have indirect effects on reproductive success by limiting migration timing and survival during migration. Increases in scaled mass were higher in females, suggesting that they are disproportionately affected by food limitation, possibly through competition. Based on Robust Design Modeling, we found no support for emigration prior to food addition, indicating that our estimates of mortality are unbiased.
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http://dx.doi.org/10.1890/13-0337.1DOI Listing
December 2013

Bird community comparisons of four plantations and conservation concerns in South China.

Integr Zool 2014 Jan;9(1):97-106

Guangdong Entomological Institute, Guangzhou, China; South China Institute of Endangered Animals, Guangzhou, China.

Plantations of non-native, fast-growing trees are increasing in the tropics and subtropics, perhaps with negative consequences for the native avifauna. We studied bird diversity in 4 types of plantations in South China to determine which plantation types are especially detrimental, and compared our findings with studies in nearby natural forests to assess the magnitude of the negative impact. A total of 57 species was recorded. The mean capture rate of understory birds was 1.7 individuals 100-net-h(-1). Bird richness and capture rate were lower in plantations than in nearby natural forests. Babblers (Timaliidae), primarily forest-dependent species in South China, were particularly under-represented in plantations. Species richness, composition and bird density, particularly of understory birds, differed between plantation types. Plantations of Schima, which is native to South China, had the highest species richness according to point count data. Plantations of Acacia (non-native) supported the highest understory species richness and produced the highest capture rate of understory birds, probably because of their complex structure and high arthropod abundance. If bird diversity is to be considered, we strongly recommend that future re-afforestation projects in South China should, as far as possible, use mixed native tree species, and especially Schima, ahead of the other species.
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http://dx.doi.org/10.1111/1749-4877.12037DOI Listing
January 2014

Climate, ecological release and bill dimorphism in an island songbird.

Biol Lett 2013 Jun 24;9(3):20130118. Epub 2013 Apr 24.

Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC 20008, USA.

Sexual size dimorphism is expected to be more pronounced in vertebrates on islands, particularly in trophic characters, as a response to decreased interspecific competition for food. We found (based on measurements of 1423 museum specimens) that bill size dimorphism was greater in island than mainland populations of song sparrows. However, dimorphism varied among islands and was positively correlated with high summer temperature and island size. Island song sparrow bills follow the overall positive temperature bill size relationship for California song sparrows, which includes larger bills on large, warmer islands. Large bills dissipate more heat and may be an adaptation to summer heat stress. Dimorphism increases because the slope for males is greater than females. Thus, the greater magnitude of bill dimorphism on islands with warmer summers may result from males experiencing greater thermal stress during territorial activity, creating different thermal optima. In contrast, bill dimorphism was unrelated to climate on the mainland. We hypothesize that reduced interspecific competition releases island populations from a constraint so that sex-specific physiological optima can be achieved, whereas mainland birds are constrained.
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http://dx.doi.org/10.1098/rsbl.2013.0118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3645045PMC
June 2013

Seasonal dimorphism in the horny bills of sparrows.

Ecol Evol 2013 Feb 11;3(2):389-98. Epub 2013 Jan 11.

Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute National Zoological Park, Washington, District of Columbia, 20008.

Bill size is often viewed as a species-specific adaptation for feeding, but it sometimes varies between sexes, suggesting that sexual selection or intersexual competition may also be important. Hypotheses to explain sexual dimorphism in avian bill size include divergence in feeding niche or thermoregulatory demands, intrasexual selection based on increased competition among males, or female preference. Birds also show seasonal changes in bill size due to shifts in the balance between growth rate and wear, which may be due to diet or endogenous rhythms in growth. Insight into the function of dimorphism can be gained using the novel approach of digital x-ray imaging of museum skins to examine the degree to which the skeletal core or the rhamphotheca contribute to overall dimorphism. The rhamphotheca is ever-growing and ever-wearing, varying in size throughout life; whereas the skeletal core shows determinant growth. Because tidal marsh sparrows are more dimorphic in bill size than related taxa, we selected two marsh taxa to investigate dimorphism and seasonality in the size of the overall bill, the skeletal core, and the rhamphotheca. Bill size varied by sex and season, with males having larger bills than females, and bill size increasing from nonbreeding to breeding season more in males. Skeletal bill size varied with season, but not sex. The rhamphotheca varied primarily with sex; males had a larger rhamphotheca (corrected for skeletal bill size), which showed a greater seasonal increase than females. The rhamphotheca, rather than the skeletal bill, was responsible for sexual dimorphism in overall bill size, which was particularly well developed in the breeding season. The size of the rhamphotheca may be a condition-based character that is shaped by sexual selection. These results are consistent with the evidence that bill size is influenced by sexual selection as well as trophic ecology.
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http://dx.doi.org/10.1002/ece3.474DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3586648PMC
February 2013

Migratory New World blackbirds (icterids) are more neophobic than closely related resident icterids.

PLoS One 2013 27;8(2):e57565. Epub 2013 Feb 27.

School of Natural Sciences & Psychology, Liverpool John Moores University, Liverpool, United Kingdom.

Environments undergo short-term and long-term changes due to natural or human-induced events. Animals differ in their ability to cope with such changes which can be related to their ecology. Changes in the environment often elicit avoidance reactions (neophobia) which protect animals from dangerous situations but can also inhibit exploration and familiarization with novel situations and thus, learning about new resources. Studies investigating the relationship between a species' ecology and its neophobia have so far been restricted to comparing only a few species and mainly in captivity. The current study investigated neophobia reactions to experimentally-induced changes in the natural environment of six closely-related blackbird species (Icteridae), including two species represented by two distinct populations. For analyses, neophobic reactions (difference in number of birds feeding and time spent feeding with and without novel objects) were related to several measures of ecological plasticity and the migratory strategy (resident or migratory) of the population. Phylogenetic relationships were incorporated into the analysis. The degree of neophobia was related to migratory strategy with migrants expressing much higher neophobia (fewer birds feeding and for a shorter time with objects present) than residents. Furthermore, neophobia showed a relationship to diet breadth with fewer individuals of diet generalists than specialists returning when objects were present supporting the dangerous niche hypothesis. Residents may have evolved lower neophobia as costs of missing out on opportunities may be higher for residents than migrants as the former are restricted to a smaller area. Lower neophobia allows them approaching changes in the environment (e.g. novel objects) quickly, thereby securing access to resources. Additionally, residents have a greater familiarity with similar situations in the area than migrants and the latter may, therefore, initially stay behind resident species.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0057565PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3583864PMC
September 2013

The influence of the California marine layer on bill size in a generalist songbird.

Evolution 2012 Dec 27;66(12):3825-35. Epub 2012 Jul 27.

Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC 20008, USA.

The hypothesis is tested that birds in hotter and drier environments may have larger bills to increase the surface area for heat dissipation. California provides a climatic gradient to test the influence of climate on bill size. Much of California experiences dry warm/hot summers and coastal areas experience cooler summers than interior localities. Based on measurements from 1488 museum skins, song sparrows showed increasing body-size-corrected bill surface area from the coast to the interior and declining in the far eastern desert. As predicted by Newton's convective heat transfer equation, relative bill size increased monotonically with temperature, and then decreased where average high temperatures exceed body temperature. Of the variables considered, distance from coast, average high summer temperature, and potential evapotranspiration showed a strong quadratic association with bill size and rainfall had a weaker negative relationship. Song sparrows on larger, warmer islands also had larger bills. A subsample of radiographed specimens showed that skeletal bill size is also correlated with temperature, demonstrating that bill size differences are not a result of variation in growth and wear of keratin. Combined with recent thermographic studies of heat loss in song sparrow bills, these results support the hypothesis that bill size in California song sparrows is selected for heat dissipation.
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http://dx.doi.org/10.1111/j.1558-5646.2012.01726.xDOI Listing
December 2012

Mercury in wing and tail feathers of hatch-year and adult tidal marsh sparrows.

Arch Environ Contam Toxicol 2012 Nov 5;63(4):586-93. Epub 2012 Aug 5.

Department of Entomology and Wildlife Ecology, University of Delaware, Newark, 19717, USA.

We estimated mercury exposure and bioaccumulation in sparrow feathers to determine variation among age groups, between sparrow species, and between feather types. Results of feather mercury studies in piscivorous birds indicate that mercury concentrations tend to increase with age and differ between feather types; however, data for insectivorous birds are lacking. We estimated mercury exposure of two insectivorous and sympatric tidal marsh sparrows: coastal plain swamp sparrow (Melospiza georgiana nigrescens), and seaside sparrow (Ammodramous maritimus). Tidal marshes have favorable conditions for mercury methlyation, thus it is likely that tidal marsh sparrows are exposed to methylmercury. We found no difference in mercury concentrations between males and female birds of both species. Adult swamp sparrow feather mercury concentrations did not differ among adult age groups; therefore, mercury was not found to increase with age in sparrows at the site. Hatch-year birds had significantly greater feather mercury concentrations compared with adult birds for both species. Mercury concentrations in adult seaside sparrows were twice as high as those in adult swamp sparrows suggesting species-specific variation, although concentrations in hatch-year sparrow species did not differ. Mercury concentrations differed between feather types in adults of both species. The first primary feather of both species had at least three times greater mercury concentrations than the outer tail feather possibly reflecting varying depuration rates with feather type.
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http://dx.doi.org/10.1007/s00244-012-9783-2DOI Listing
November 2012

Heat loss may explain bill size differences between birds occupying different habitats.

PLoS One 2012 25;7(7):e40933. Epub 2012 Jul 25.

Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC, United States of America.

Background: Research on variation in bill morphology has focused on the role of diet. Bills have other functions, however, including a role in heat and water balance. The role of the bill in heat loss may be particularly important in birds where water is limiting. Song sparrows localized in coastal dunes and salt marsh edge (Melospiza melodia atlantica) are similar in size to, but have bills with a 17% greater surface area than, those that live in mesic habitats (M. m. melodia), a pattern shared with other coastal sparrows. We tested the hypotheses that sparrows can use their bills to dissipate "dry" heat, and that heat loss from the bill is higher in M. m. atlantica than M. m. melodia, which would indicate a role of heat loss and water conservation in selection for bill size.

Methodology/principal Findings: Bill, tarsus, and body surface temperatures were measured using thermal imaging of sparrows exposed to temperatures from 15-37°C and combined with surface area and physical modeling to estimate the contribution of each body part to total heat loss. Song sparrow bills averaged 5-10°C hotter than ambient. The bill of M. m atlantica dissipated up to 33% more heat and 38% greater proportion of total heat than that of M. m. melodia. This could potentially reduce water loss requirements by approximately 7.7%.

Conclusions/significance: This >30% higher heat loss in the bill of M. m. atlantica is independent of evaporative water loss and thus could play an important role in the water balance of sparrows occupying the hot and exposed dune/salt marsh environments during the summer. Heat loss capacity and water conservation could play an important role in the selection for bill size differences between bird populations and should be considered along with trophic adaptations when studying variation in bill size.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0040933PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3405045PMC
April 2013

Avian conservation practices strengthen ecosystem services in California vineyards.

PLoS One 2011 9;6(11):e27347. Epub 2011 Nov 9.

Department of Environmental Studies, University of California Santa Cruz, Santa Cruz, California, United States of America.

Insectivorous Western Bluebirds (Sialia mexicana) occupy vineyard nest boxes established by California winegrape growers who want to encourage avian conservation. Experimentally, the provision of available nest sites serves as an alternative to exclosure methods for isolating the potential ecosystem services provided by foraging birds. We compared the abundance and species richness of avian foragers and removal rates of sentinel prey in treatments with songbird nest boxes and controls without nest boxes. The average species richness of avian insectivores increased by over 50 percent compared to controls. Insectivorous bird density nearly quadrupled, primarily due to a tenfold increase in Western Bluebird abundance. In contrast, there was no significant difference in the abundance of omnivorous or granivorous bird species some of which opportunistically forage on grapes. In a sentinel prey experiment, 2.4 times more live beet armyworms (Spodoptera exigua) were removed in the nest box treatment than in the control. As an estimate of the maximum foraging services provided by insectivorous birds, we found that larval removal rates measured immediately below occupied boxes averaged 3.5 times greater than in the control. Consequently the presence of Western Bluebirds in vineyard nest boxes strengthened ecosystem services to winegrape growers, illustrating a benefit of agroecological conservation practices. Predator addition and sentinel prey experiments lack some disadvantages of predator exclusion experiments and were robust methodologies for detecting ecosystem services.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027347PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3212556PMC
April 2012

Bill size and dimorphism in tidal-marsh sparrows: island-like processes in a continental habitat.

Ecology 2010 Aug;91(8):2428-36

Smithsonian Migratory Bird Center, National Zoological Park, Washington, D.C. 20008, USA.

Conditions favoring population divergence in trophic features, such as the low levels of species richness and interspecific competition found on islands, can be similar to conditions that increase their sexual dimorphism or overall variance. Male emberizid sparrows of tidal marshes have undergone parallel evolution of large bills. We tested for parallel increases between dimorphism and overall variation in bill size by comparing three groups totaling 30 sparrow subspecies: tidal-marsh sparrows, nontidal relatives of tidal-marsh taxa, and representative sparrow taxa. Bill size (and not other features) showed the following patterns in tidal-marsh sparrows compared to nontidal relatives or sparrows at large: (1) an increase; (2) a greater increase in males than females; (3) an increase in sexual dimorphism; and (4) greater variation in females. A high degree of sexual dimorphism in bill size is consistent with the hypothesis that low levels of interspecific and high levels of intraspecific competition select for intraspecific niche divergence. Alternatively, increased sexual selection in tidal-marsh sparrows, vis-a-vis high densities and hence increased male-male competition, may account for the differentially large increase in bill size in males. Relaxed natural selection due to high ecosystem productivity and low interspecific competition may explain why, in tidal-marsh sparrows, female bills have diverged less than males and show higher levels of variability at larger sizes. Both the niche divergence and sexual selection hypotheses depend upon processes, particularly increases in population density, that are similar to those often reported for island passerines. However, the low species diversity and increased intraspecific competition of salt marsh faunas is probably a result of abiotic constraints on colonization (tides and salinity) rather than the isolating distances of island biotas. Thus, both a shift in bill size and increases in its dimorphism and variability may be favored by high productivity and abiotic constraints.
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http://dx.doi.org/10.1890/09-1136.1DOI Listing
August 2010

Bird community response to fruit energy.

J Anim Ecol 2010 Jul 23;79(4):824-35. Epub 2010 Apr 23.

Odum School of Ecology, University of Georgia, Athens, GA 30602, USA.

1. The abundance and predictability of food resources have been posited as explanations for the increase of animal species richness in tropical habitats. However, the heterogeneity of natural ecosystems makes it difficult to quantify a response of animal species richness to these qualities of food resources. 2. Fruit-frugivore studies are especially conducive for testing such ecological theories because fruit is conspicuous and easily counted. Fruit-frugivore research in some locations has demonstrated a relationship between animal abundance and fruit resource abundance, both spatially and temporally. These studies, which typically use fruit counts as the variable of fruit abundance, have never documented a response of species richness at the community level. Furthermore, these studies have not taken into account factors influencing the detection of an individual within surveys. 3. Using a combination of nonstandard approaches to fruit-frugivore research, we show a response of bird species richness to fruit resources. First, we use uniform and structurally similar, one-ha shade-grown coffee plots as replicated experimental units to reduce the influence of confounding variables. Secondly, we use multi-season occupancy modelling of a resident omnivorous bird assemblage in order to account for detection probability in our analysis of site occupancy, local immigration and local emigration. Thirdly, we expand our variable of fruit abundance, Fruit Energy Availability (FEA), to include not only fruit counts but also fruit size and fruit quality. 4. We found that a site's average monthly FEA was highly correlated (0.90) with a site's average bird species richness. In our multi-season occupancy model 92% of the weight of evidence supported a single model that included effects of FEA on initial occupancy, immigration, emigration and detection. 5. These results demonstrate that fruit calories can broadly influence the richness of a neotropical bird community, and that fluctuations of FEA explains much of the site occupancy patterns of component species. This study shows that in depauperate, managed landscapes fruit resource abundance supports more species and fruit constancy allows for higher levels of avian persistence, an important practical concept for conservation planning.
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http://dx.doi.org/10.1111/j.1365-2656.2010.01699.xDOI Listing
July 2010

Common garden experiment reveals genetic control of phenotypic divergence between swamp sparrow subspecies that lack divergence in neutral genotypes.

PLoS One 2010 Apr 19;5(4):e10229. Epub 2010 Apr 19.

Smithsonian Migratory Bird Center, National Zoological Park, University of West Georgia, Carrollton, Georgia, United States of America.

Background: Adaptive divergence between populations in the face of strong selection on key traits can lead to morphological divergence between populations without concomitant divergence in neutral DNA. Thus, the practice of identifying genetically distinct populations based on divergence in neutral DNA may lead to a taxonomy that ignores evolutionarily important, rapidly evolving, locally-adapted populations. Providing evidence for a genetic basis of morphological divergence between rapidly evolving populations that lack divergence in selectively neutral DNA will not only inform conservation efforts but also provide insight into the mechanisms of the early processes of speciation. The coastal plain swamp sparrow, a recent colonist of tidal marsh habitat, differs from conspecific populations in a variety of phenotypic traits yet remains undifferentiated in neutral DNA.

Methods And Principal Findings: Here we use an experimental approach to demonstrate that phenotypic divergence between ecologically separated populations of swamp sparrows is the result of local adaptation despite the lack of divergence in neutral DNA. We find that morphological (bill size and plumage coloration) and life history (reproductive effort) differences observed between wild populations were maintained in laboratory raised individuals suggesting genetic divergence of fitness related traits.

Conclusions And Significance: Our results support the hypothesis that phenotypic divergence in swamps sparrows is the result of genetic differentiation, and demonstrate that adaptive traits have evolved more rapidly than neutral DNA in these ecologically divergent populations that may be in the early stages of speciation. Thus, identifying evolutionarily important populations based on divergence in selectively neutral DNA could miss an important level of biodiversity and mislead conservation efforts.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0010229PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2856683PMC
April 2010

Interactions among predators and the cascading effects of vertebrate insectivores on arthropod communities and plants.

Proc Natl Acad Sci U S A 2010 Apr 5;107(16):7335-40. Epub 2010 Apr 5.

Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697-2525, USA.

Theory on trophic interactions predicts that predators increase plant biomass by feeding on herbivores, an indirect interaction called a trophic cascade. Theory also predicts that predators feeding on predators, or intraguild predation, will weaken trophic cascades. Although past syntheses have confirmed cascading effects of terrestrial arthropod predators, we lack a comprehensive analysis for vertebrate insectivores-which by virtue of their body size and feeding habits are often top predators in these systems-and of how intraguild predation mediates trophic cascade strength. We report here on a meta-analysis of 113 experiments documenting the effects of insectivorous birds, bats, or lizards on predaceous arthropods, herbivorous arthropods, and plants. Although vertebrate insectivores fed as intraguild predators, strongly reducing predaceous arthropods (38%), they nevertheless suppressed herbivores (39%), indirectly reduced plant damage (40%), and increased plant biomass (14%). Furthermore, effects of vertebrate insectivores on predatory and herbivorous arthropods were positively correlated. Effects were strongest on arthropods and plants in communities with abundant predaceous arthropods and strong intraguild predation, but weak in communities depauperate in arthropod predators and intraguild predation. The naturally occurring ratio of arthropod predators relative to herbivores varied tremendously among the studied communities, and the skew to predators increased with site primary productivity and in trees relative to shrubs. Although intraguild predation among arthropod predators has been shown to weaken herbivore suppression, we find this paradigm does not extend to vertebrate insectivores in these communities. Instead, vertebrate intraguild preda-tion is associated with strengthened trophic cascades, and insectivores function as dominant predators in terrestrial plant-arthropod communities.
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http://dx.doi.org/10.1073/pnas.1001934107DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2867756PMC
April 2010

Causes of reduced clutch size in a tidal marsh endemic.

Oecologia 2008 Dec 30;158(3):421-35. Epub 2008 Sep 30.

Smithsonian Migratory Bird Center, National Zoological Park, 3001 Connecticut Ave. NW, Washington, DC 20008, USA.

We tested three hypotheses of clutch size variation in two subspecies of the swamp sparrow (Melospiza georgiana georgiana and M. g. nigrescens). Swamp sparrows follow the pattern of other estuarine endemics, where clutch size is smaller among tidal salt marsh populations (M. g. nigrescens) than their closest inland relatives (M. g. georgiana). Our results support predation risk and temperature, but not adult survival, as explanations of this pattern in swamp sparrows. Coastal nests were twice as likely to fail as inland nests, and parental activity around the nest site was positively related to clutch size at both sites. When brood size was controlled for, coastal adults visited nests less often and females vocalized less frequently during visits than inland birds, which may decrease nest detectability to predators. Coastal parents waited longer than inland birds to feed offspring in the presence of a model nest predator, but there was no difference in their response to models of predators of adults, as would be expected if coastal birds possessed increased longevity. Additionally, coastal females laid more eggs than inland females over a single season, following a within-season bet-hedging strategy rather than reducing within-season investment. Coastal territories experienced ambient air temperatures above the physiological zero of egg development more often, and higher temperatures during laying correlated with smaller clutches and increased egg inviability among coastal birds. Similar effects were not seen among inland nests, where laying temperatures were generally below physiological zero. Both subspecies showed an increase in hatching asynchrony and a decrease in apparent incubation length under high temperatures. Coastal individuals, however, showed less hatching asynchrony overall despite higher temperatures. Both air temperatures during laying and predation risk could potentially explain reduced clutch size in not only coastal plain swamp sparrows, but also other tidal marsh endemics.
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http://dx.doi.org/10.1007/s00442-008-1148-1DOI Listing
December 2008

Biodiversity loss in Latin American coffee landscapes: review of the evidence on ants, birds, and trees.

Conserv Biol 2008 Oct;22(5):1093-1105

Smithsonian Migratory Bird Center, Washington, DC 20013, USA.

Studies have documented biodiversity losses due to intensification of coffee management (reduction in canopy richness and complexity). Nevertheless, questions remain regarding relative sensitivity of different taxa, habitat specialists, and functional groups, and whether implications for biodiversity conservation vary across regions.We quantitatively reviewed data from ant, bird, and tree biodiversity studies in coffee agroecosystems to address the following questions: Does species richness decline with intensification or with individual vegetation characteristics? Are there significant losses of species richness in coffee-management systems compared with forests? Is species loss greater for forest species or for particular functional groups?and Are ants or birds more strongly affected by intensification? Across studies, ant and bird richness declined with management intensification and with changes in vegetation. Species richness of all ants and birds and of forest ant and bird species was lower in most coffee agroecosystems than in forests, but rustic coffee (grown under native forest canopies) had equal or greater ant and bird richness than nearby forests. Sun coffee(grown without canopy trees) sustained the highest species losses, and species loss of forest ant, bird, and tree species increased with management intensity. Losses of ant and bird species were similar, although losses of forest ants were more drastic in rustic coffee. Richness of migratory birds and of birds that forage across vegetation strata was less affected by intensification than richness of resident, canopy, and understory bird species. Rustic farms protected more species than other coffee systems, and loss of species depended greatly on habitat specialization and functional traits. We recommend that forest be protected, rustic coffee be promoted,and intensive coffee farms be restored by augmenting native tree density and richness and allowing growth of epiphytes. We also recommend that future research focus on potential trade-offs between biodiversity conservation and farmer livelihoods stemming from coffee production.
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http://dx.doi.org/10.1111/j.1523-1739.2008.01029.xDOI Listing
October 2008

Birds as predators in tropical agroforestry systems.

Ecology 2008 Apr;89(4):928-34

Smithsonian Tropical Research Institute, Unit 0948, APO, AA 34002, USA.

Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.
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http://dx.doi.org/10.1890/06-1976.1DOI Listing
April 2008

An experimental study of habitat selection by birds in a coffee plantation.

Ecology 2008 Apr;89(4):921-7

Instituto de Ecologia, A.C. Departamento de Ecologia Funcional, Xalapa, Veracruz, México.

Unique components of tropical habitats, such as abundant vascular epiphytes, influence the distribution of species and can contribute to the high diversity of many animal groups in the tropics. However, the role of such features in habitat selection and demography of individual species has not been established. Understanding the mechanisms of habitat selection requires both experimental manipulation of habitat structure and detailed estimation of the behavioral and demographic response of animals, e.g., changes in movement patterns and survival probabilities. Such studies have not been conducted in natural tropical forest, perhaps because of high habitat heterogeneity, high species diversity, and low abundances of potential target species. Agroforestry systems support a less diverse flora, with greater spatial homogeneity which, in turn, harbors lower overall species diversity with greater numerical dominance of common species, than natural forests. Furthermore, agroforestry systems are already extensively managed and lend themselves easily to larger scale habitat manipulations than protected natural forest. Thus, agroforestry systems provide a good model environment for beginning to understand processes underlying habitat selection in tropical forest animals. Here, we use multistate, capture-recapture models to investigate how the experimental removal of epiphytes affected monthly movement and survival probabilities of two resident bird species (Common Bush-Tanager [Chlorospingus ophthalmicus] and Golden-crowned Warbler [Basileuterus culicivorus]) in a Mexican shade coffee plantation. We established two paired plots of epiphyte removal and control. We found that Bush-Tanagers were at least five times more likely to emigrate from plots where epiphytes were removed compared to control plots. Habitat-specific movement patterns were not detected in the warbler. However, unlike the Golden-crowned Warbler, Common Bush-Tanagers depend upon epiphytes for nest sites and (seasonally) for foraging. These dispersal patterns imply that active habitat selection based on the presence or absence of epiphytes occurs in C. ophthalmicus on our study area. Survival rates did not vary with habitat in either species. Interestingly, in both species, survival was higher in the nonbreeding season, when birds were in mixed-species flocks. Movement by Common Bush-Tanagers into areas with epiphytes occurred mostly during the breeding season, when mortality-driven opportunity was greatest.
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http://dx.doi.org/10.1890/07-0164.1DOI Listing
April 2008

Agroforests as model systems for tropical ecology.

Ecology 2008 Apr;89(4):913-4

Smithsonian Migratory Bird Center, USA.

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http://dx.doi.org/10.1890/07-1578.1DOI Listing
April 2008

Field-testing ecological and economic benefits of coffee certification programs.

Conserv Biol 2007 Aug;21(4):975-85

Smithsonian Migratory Bird Center, National Zoological Park, 3001 Connecticut Avenue NW, Washington, DC 20008, USA.

Coffee agroecosystems are critical to the success of conservation efforts in Latin America because of their ecological and economic importance. Coffee certification programs may offer one way to protect biodiversity and maintain farmer livelihoods. Established coffee certification programs fall into three distinct, but not mutually exclusive categories: organic, fair trade, and shade. The results of previous studies demonstrate that shade certification can benefit biodiversity, but it remains unclear whether a farmer's participation in any certification program can provide both ecological and economic benefits. To assess the value of coffee certification for conservation efforts in the region, we examined economic and ecological aspects of coffee production for eight coffee cooperatives in Chiapas, Mexico, that were certified organic, certified organic and fair trade, or uncertified. We compared vegetation and ant and bird diversity in coffee farms and forests, and interviewed farmers to determine coffee yield, gross revenue from coffee production, and area in coffee production. Although there are no shade-certified farms in the study region, we used vegetation data to determine whether cooperatives would qualify for shade certification. We found no differences in vegetation characteristics, ant or bird species richness, or fraction of forest fauna in farms based on certification. Farmers with organic and organic and fair-trade certification had more land under cultivation and in some cases higher revenue than uncertified farmers. Coffee production area did not vary among farm types. No cooperative passed shade-coffee certification standards because the plantations lacked vertical stratification, yet vegetation variables for shade certification significantly correlated with ant and bird diversity. Although farmers in the Chiapas highlands with organic and/or fair-trade certification may reap some economic benefits from their certification status, their farms may not protect as much biodiversity as shade-certified farms. Working toward triple certification (organic, fair trade, and shade) at the farm level may enhance biodiversity protection, increase benefits to farmers, and lead to more successful conservation strategies in coffee-growing regions.
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http://dx.doi.org/10.1111/j.1523-1739.2007.00728.xDOI Listing
August 2007

A biogeographic pattern in sparrow bill morphology: parallel adaptation to tidal marshes.

Evolution 2005 Jul;59(7):1588-95

Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, USA.

The study of ecological convergence, the evolution of similar traits on multiple occasions in response to similar conditions, is a powerful method for developing and testing adaptive hypotheses. However, despite the great attention paid to geographic variation and the foraging ecology of birds, surprisingly few cases of convergent or parallel feeding adaptations have been adequately documented. In this study, we document a biogeographic pattern of parallel bill morphology across 10 sparrow taxa endemic to tidal marshes. All North American tidal marsh sparrows display parallel differentiation from close relatives in other habitats, suggesting that selection on bill morphology is strong. Relative to their body mass, tidal marsh sparrows have longer, thinner bills than their non-tidal marsh counterparts, which is likely an adaptation for consuming more invertebrates and fewer seeds, as well as for probing in sediment crevices to capture prey. Published data on tidal marsh food resources and diet of the relevant taxa support this hypothesis. This morphological differentiation is most pronounced between sister taxa with the greatest estimated divergence times, but is found even in taxa that show little or no structure in molecular genetic markers. We, therefore, speculate that tidal marsh ecosystems are likely settings for ecological speciation.
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http://dx.doi.org/10.1554/04-502DOI Listing
July 2005

Impacts of major predators on tropical agroforest arthropods: comparisons within and across taxa.

Oecologia 2004 Jun 17;140(1):140-9. Epub 2004 Apr 17.

Department of Ecology and Evolutionary Biology, University of Michigan, 830 N. University, Ann Arbor, MI 48109, USA.

In food web studies, taxonomically unrelated predators are often grouped into trophic levels regardless of their relative importance on prey assemblages, multiple predator effects, or interactions such as omnivory. Ants and birds are important predators likely to differentially shape arthropod assemblages, but no studies have compared their effects on a shared prey base. In two separate studies, we excluded birds and ants from branches of a canopy tree ( Inga micheliana) in a coffee farm in Mexico for 2 months in the dry and wet seasons of 2002. We investigated changes in arthropod densities with and without predation pressure from (1) birds and (2) ant assemblages dominated by one of two ant species ( Azteca instabilis and Camponotus senex). We first analyzed individual effects of each predator (birds, Azteca instabilis, and C. senex) then used a per day effect metric to compare differences in effects across (birds vs ants) and within predator taxa (the two ant species). Individually, birds reduced densities of total and large arthropods and some arthropod orders (e.g., spiders, beetles, roaches) in both seasons. Azteca instabilis did not significantly affect arthropods (total, small, large or specific orders). Camponotus senex, however, tended to remove arthropods (total, small), especially in the dry season, and affected arthropod densities of some orders both positively and negatively. Predators greatly differed in their effects on Inga arthropods (for all, small, large, and individual orders of arthropods) both in sign (+/-) and magnitudes of effects. Birds had stronger negative effects on arthropods than ants and the two dominant ant species had stronger effects on arthropods in different seasons. Our results show that aggregating taxonomically related and unrelated predators into trophic levels without prior experimental data quantifying the sign and strengths of effects may lead to a misrepresentation of food web interactions.
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http://dx.doi.org/10.1007/s00442-004-1561-zDOI Listing
June 2004

Divergence in foraging behavior of foliage-gleaning birds of Canadian and Russian boreal forests.

Oecologia 1999 Aug;120(3):451-462

State Darwin 57, Vasilova, Moscow 117292, Russia, , , , , , RU.

We compared foraging behavior of foliage-gleaning birds of the boreal forest of two Palaearctic (central Siberia and European Russia) and two Nearctic (Mackenzie and Ontario, Canada) sites. Using discriminant function analysis on paired sites we were able to distinguish foliage-gleaning species from the Nearctic and Palaearctic with few misclassifications. The two variables that most consistently distinguished species of the two avifaunas were the percentage use of conifer foliage and the percentage use of all foliage. Nearctic foliage-gleaner assemblages had more species that foraged predominantly from coniferous foliage and displayed a greater tendency to forage from foliage, both coniferous and broad-leafed, rather than twigs, branches, or other substrates. The greater specialization on foliage and, in particular, conifer foliage by New World canopy foliage insectivores is consistent with previously proposed hypotheses regarding the role of Pleistocene vegetation history on ecological generalization of Eurasian species. Boreal forest, composed primarily of spruce and pine, was widespread in eastern North America, whereas pockets of forest were scattered in Eurasia (mostly the mountains of southern Europe and Asia). This may have affected the populations of birds directly or indirectly through reduction in the diversity and abundance of defoliating outbreak insects. Loss of habitat and resources may have selected against ecological specialization on these habitats and resources.
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http://dx.doi.org/10.1007/s004420050878DOI Listing
August 1999

Constant density and stable territoriality in some tropical insectivorous birds.

Oecologia 1986 Jul;69(4):618-625

Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama.

Four species of understory antbirds (Formicariidae: Myrmotherula fulviventris, M. axillaris, Microrhopias quixensis, and Thamnophilus punctatus) had stable populations over eight rainy seasons on Barro Colorado Island, Panama. The co-defended territories of M. fulviventris and Microrhopias quixensis, were essentially identical from year to year on our intensive study site, despite a moderate turnover of territory owners. The location of the territories of T. punctatus was also similar between years. This stability occurred in the face of considerable annual variation in the survivorship of adult M. fulviventris and T. punctatus. This variation was not significantly correlated with patterns of rainfall. Stable territoriality has rarely been reported from relatively-short-lived insectivorous birds. The annual production of young was significantly variable only in M. axillaris. Because BCI is an island comprised of one habitat (tropical forest) and so supports a closed population of antbirds, and because it is unlikely that natality equaled mortality on our study site during the entire eight years of the study, we suggest that these breeding populations are socially regulated at a constant level below the limits directly set by food supply.
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http://dx.doi.org/10.1007/BF00410372DOI Listing
July 1986

Dissimilar bill shapes in new world tropical versus temperate forest foliage-gleaning birds.

Oecologia 1981 May;49(2):143-147

Museum of Vertebrate Zoology and Department of Zoology, University of California, 94720, Berkeley, California, USA.

The bill shape of foliage-gleaning birds in temperate and tropical new world forests is dissimilar. Tropical species have longer and narrower bills than their temperate zone counterparts. In addition, their bills are longer for a given body size. These differences cannot be readily explained as phylogenetic artifacts. I suggest that the distinct bill morphology of the two assemblages is determined by the type of insects that comprise the largest size classes of potential prey. These large insects are particularly important since they generally comprise the bulk of the nestling diet for insectivorous birds. In tropical forests Orthoptera are probably the most abundant large soft-bodied arthropods; they form an important resource for foliage-gleaning birds during the breeding (rainy) seasons. Most temperate zone foliage-gleaning birds rely almost entirely upon caterpillars when breeding. Long, narrow bills are thought to close more rapidly than shorter, broader bills. These long, "fast" bills may be required to efficiently harvest active Orthoptera. Migrant warblers may face morphological constraints from breeding successfully in lowland tropical forests. While the short-billed temperate zone birds can survive the tropical dry season by foraging on small arthropods, they may be inefficient at handling large Orthoptera to feed to nestlings.
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http://dx.doi.org/10.1007/BF00349180DOI Listing
May 1981

Leaf surface specializations of birds and Arthropods in a Panamanian forest.

Oecologia 1980 Jul;46(1):115-124

Museum of Vertebrate Zoology, University of California, 94720, Berkeley, California, USA.

Most species of Panamanian lowland forest birds specialize on leaf undersurfaces when hunting foliage insects. The few species of leaf surface generalists and leaf upper surface specialists are omnivorous gleaners. We estimate that while over 90% of the avian understory insectivory is directed towards leaf undersurfaces, only 50% of canopy foliage insectivory is directed towards the undersides of leaves. In the low understory we found 70-80% of the arthropods on leaf undersides. The excess use of leaf-bottoms by understory birds may be a result of their greater visibility. It is hypothesized that less proficient insectivores are unable to take advantage of the greater effective density of underleaf insects because they can only efficiently attack the closest leaf surfaces; these closest surfaces will usually be the leaf tops from the branch on which the bird is perched. Alternatively, leaf-top specialists may have special foraging adaptations for overcoming the disadvantages of leaf-top foraging. These adaptations may involve attack behavior (Tachyphonus luctuosus) or searching behavior (Dacnis cayana). Dacnis often used leaf damage as a foraging cue; this may be the first report of a bird using leaf damage for searching for insects. The greater use of leaf upper surfaces by canopy birds may be influenced by four factors: greater seasonality of insects in the canopy favoring omnivores which may be less efficient insectivores; more insects on leaf tops; fewer planar leaf arrangements in canopy plants; or the greater visibility of leaf upper surfaces of the outer shell of foliage of massive trees. Based on the greater number of arthropods on leaf bottoms in the dry season, the higher abundance of smaller insects on leaf bottoms, as well as the greater proportion of insects on leaf tops at cooler higher elevations, we suggest that arthropods prefer leaf bottoms in tropical areas for physiological, not predator avoidance reasons.
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http://dx.doi.org/10.1007/BF00346975DOI Listing
July 1980