Scientific Papers

Dr. Richard Holmes began studying birds at Hubbard Brook in 1969, and since then he and his colleagues have published a tremendous number of scientific papers on the bird community as a whole and on species like the Black-throated Blue Warbler in particular. Below is a selection of papers that are relevant to the ideas and data discussed here. They are listed in chronological order and appear with abstracts written by the authors of the various papers, summarizing the goals of their study, the method used, and their main findings.

RT Holmes, RE Bonney, and SW Pacala (1979) Guild structure of the Hubbard Brook bird community: a multivariate approach. Ecology 60 (3): 512–520.

We examined the similarities and differences in the foraging patterns of 22 insectivorous bird species during their breeding season in the Hubbard Brook Experimental Forest, New Hampshire, USA. Using multivariate techniques (clustering of hyperdimensional Euclidean distances, principal components analysis, and Varimax rotated factor analysis), we distinguish 4 groups of species or guilds, each of which exploits food resources in a distinctly different way. Partitioning occurs primarily by (1) foraging height and height—related characters, (2) foraging locations within the forest canopy, and (3) differential use of tree species, foraging substrates and foraging maneuvers. The results indicate that the importance of vegetation height to bird species diversity is related (1) to foraging opportunities which differ along a gradient from ground level to the upper canopy and which are roughly indexed by measures of foliage height diversity (FHD), and (2) to the presence of the supporting branch and bole framework which provides a major distinct foraging region. We suggest that foraging opportunities vary with height in a forest and are influenced by the physical and chemical characteristics of the plant species, which in turn affect the kinds and distributions of foraging substrates, the ways in which birds search for and find food, and the abundances of food resources. The implications of these findings for understanding the structure of forest bird communities are discussed.

RT Holmes, JC Schultz, and P Nothnagle (1979) Bird predation on forest insects: an exclosure experiment. Science 206 (4417): 462–463.

Exclusion experiments show that birds significantly reduce densities of larval Lepidoptera on forest understory vegetation. When insect densities are already low, bird predation may act both as a population regulator and as a strong agent of natural selection.

RT Holmes and SK Robinson (1981) Tree species preferences of foraging insectivorous birds in a northern hardwoods forest. Oecologia 48: 31-35.

Birds searching for insects in the canopy of a northern hardwoods forest depart significantly from random in their use of tree species, even when these trees are generally similar in life form. All 10 foliage-dwelling bird species in the Hubbard Brook forest showed preferences for Yellow Birch, most had an aversion to Beech and Sugar Maple, and a few had special preferences for conifers or White Ash. Birds that glean prey from leaves had stronger tree species preferences than those that often hover for their prey, and were more influenced by tree species differences in foliage structure. The less common bird species and those for which northern hardwoods are marginal habitat had the most pronounced tree-species preferences. Food densities which are higher on Yellow Birch and specific adaptations to foraging in trees with particular foliage structures are considered major factors responsible for the observed tree species preferences. The implications of these findings for bird community structure and for forest management practices are discussed.

RT Holmes, TW Sherry, and FW Sturges (1986) Bird community dynamics in a temperate deciduous forest: long-term trends at Hubbard Brook. Ecological Monographs 56 (3): 201-220.

Changes in species composition and abundance of birds breeding in an unfragmented temperate deciduous forest in New Hampshire, USA, were studied intensively during 16 consecutive breeding seasons, 1969-1984. The number of species breeding in the 10-ha study area in any one year varied from 17 to 28, and averaged 24. Total numbers of individuals breeding on the 10-ha plot ranged from 214 to 89, with many species (70%) declining during the 16-yr period. Overall, there was significant positive covariation among population trends of all species, suggesting a major, perhaps single, factor affecting population levels, such as weather or food supply. No two species, however, had identical patterns of change across all 16 yr. Thus, populations in this forest fluctuated largely independently of one another, suggesting a different combination of regulatory factors for each species. The major factors pulsing or stressing bird populations in the field included: (1) changes in food abundance due largely to irruptions of defoliating Lepidoptera (most bird species, but especially vireos and warblers), (2) harsh late spring and summer weather (Scarlet Tanager, American Redstart), (3) changes in habitat structure related to forest succession (Least Flycatcher, Philadelphia Vireo), (4) interspecific interactions, particularly interference competition (Least Flycatcher-American Redstart, Red-eyed Vireo-Philadelphia Virreo), and (5) mortality during winter for both resident and migrant populations (Hermit Thrush, Dark-eyed Junco, permanent resident species such as woodpeckers and nuthatches. Some, but not all, irruptions of defoliating Lepidoptera significantly influenced many bird populations in this forest and contributed to the observed positive covariation in abundances. These irruptions produced pulses of food for breeding birds, but occurred at long and variable time intervals. Between outbreaks, food may regularly limit reproductive output of these forest birds, and low food abundance, along with the effects of predators, weather, and other mortality factors, contributes to high variability in bird reproductive success, which ultimately influences population size. Competition may also become important during these periods of food scarcity. We therefore propose that birds in these temperate deciduous forests experience periods of prolonged food limitation, interrupted by relatively brief periods of superabundant food, a pattern contrasting with that described for other temperate systems (e.g., by Wiens 1977). From this examination of bird community dynamics on one site over many years, it is apparent that each species responds to its environment in a unique way, as determined by a variety of influences on its populations. Some of these operate on a local scale (e.g., vegetation structure, food abundance, interspecific competitors), while others function at regional (e.g., some weather effects) and global (e.g., winter events) geographic scales. These influences also differ as to their temporal persistence and predictability, and thus are difficult to detect in short-term studies. This pluralistic view of community structure for birds in temperate forests represents a compromise between nonequilibrial and competitionist models, and argues against the existence of a tightly organized community at any one spatial or temporal scale.

RT Holmes and TW Sherry (1988) Assessing population trends of New Hampshire forest birds: local vs. regional patterns. Auk 105 (4): 756-768.

We examined the changes in abundance between 1969 and 1986 of 19 forest-dwelling, mostly migratory bird species breeding in New Hampshire at 2 different scales: one local (an intensively studied 10-ha plot in unfragmented forest) and the other regional (Breeding Bird Surveys statewide). Twelve of the 19 species exhibited similar trends at both scales. Eight neither increased nor decreased, and 4 (Least Flycatcher, Winter Wren, Wood Thrush, and Swainson's Thrush) declined significantly. Others increased, decreased, or remained steady at one or the other scale. Overall, more species declined than increased both locally (8 vs. 1) and regionally (5 vs. 1). Comparisons of these patterns, combined with results of intensive studies at the local level, suggest that changes in food abundance and in vegetation structure related to forest succession on the breeding grounds, along with other processes that influence bird reproductive success and survivorship, are the most plausible explanations for most of the observed trends. Winter mortality was also identified as affecting breeding abundances, but only in short-distance migrant and permanently resident species. We have no evidence to indicate that the numbers of long-distance migrants were affected by events in their Neotropical wintering areas, although this possibility is difficult to assess from breeding-ground data. We urge caution in attributing declines of breeding forest migrant birds to tropical deforestation or similar causes until we either can eliminate alternate explanations that involve breeding-season events or have available critically needed demographic information on migrant populations in their wintering areas.

NL Rodenhouse (1992) Potential effects of climatic change on a Neotropical migrant landbird. Conservation Biology 6 (2): 263-272.

Significant climatic changes due to the "green-house" effect are predicted to occur in breeding and wintering areas of neotropical migrant landbirds within 50 years. A simulation model was used to quantitatively evaluate the impact of predicted climatic changes on the annual productivity [number of fledgling · (1/pair · season)] of Black-throated Blue Warblers (Dendroica caerulescens) breeding in northern hardwoods forests. Results suggested that where mean summer rainfall increases by 20%, greater egg and nestling mortality may reduce annual breeding productivity by as much as 26%. Warmer temperatures, however, will tend to enhance food abundance for insectivorous birds and lengthen the avian breeding season. These changes will act to increase breeding productivity, but they can only compensate for about a 10% increase in precipitation. Precipitation, however, is predicted to decline over most of the range of Black-throated Blue Warblers. Where mean temperature is warmer and average summer precipitation is lower by as little as 10%, breeding productivity of Black-throated Blue Warblers may be enhanced by up to 25%. Because significant regional differences in climatic change are likely, and because migratory bird species will differ in their response to climatic change, the emphasis of avian research should be on intensive regional studies of the factors influencing avian reproduction and survival.

NL Rodenhouse and RT Holmes (1992) Results of experimental and natural food reductions for breeding Black-throated Blue Warblers. Ecology 73 (1): 357-372.

We examined effects of natural and experimentally created reductions in food abundance on the reproductive ecology of Black-throated Blue Warblers (Dendroica caerulescens). The study was carried out between 1982 and 1985 on four 30-ha plots of temperate deciduous forest within and near the Hubbard Brook Experimental Forest, New Hampshire, USA. Experimental food reduction using aerially sprayed Bacillus thuringiensis created significant differences in caterpillar biomass between one sprayed and two unsprayed plots throughout the breeding season in 1983. Where caterpillar abundance was reduced, Black-throated Blue Warblers made significantly fewer nesting attempts and diets of neslings included fewer caterpillars. Clutch size, hatching success, and numer of young fledging per nest did not differ among the food reduction site and controls. Also, the reduced number of nesting attempts per pair on the food reduction site in 1983 did not significantly lower production of young per pair. No detectable differences in caterpillar biomass between sprayed and unsprayed sites were created in 1984 or 1985 because natural caterpillar abundances were already low. Natural declines in food abundance for Black-throated Blue Warblers occurred from 1982 through 1985, primarily because of a decrease in caterpillar abundances. Significant reductions in number of young fledging per nest, nestling growth rates and survival, and number of nests attempted per pair corresponded with the natural decline in food abundance. Of those measures, a reduced number of nesting attempts per pair lowered annual production the most. When effects of food limitation were calculted separately from those of nest predation, food limited annual breeding productivity to below that needed to balance annual mortality in at least one of the four years of this study. These findings indicate that neotropoical migrant bird species are probably limited periodically by food when breeding in north-temperate habitats.

RT Holmes, PP Marra, and TW Sherry (1996) Habitat-specific demography of breeding Black-throated Blue Warblers (Dendroica caerulescens): implications for population dynamics. Journal of Animal Ecology 65: 183-195.

The distribution of individuals among habitats and their relative success in those habitats can have important consequences for population dynamics. To examine these processes for a long-distance migratory bird species, we studied the population structure, age-specific reproductive output, and local survival of black-throated blue warblers (Dendroica caerulescens, Gmelin) in two breeding habitats differing in shrub density within northern hardwoods forests in New Hampshire, USA. On forest plots with dense shrubs, warblers occurred at higher densities, and fledged significantly more young per capita per season than those occupying areas with lower shrub density. This differential productivity was due to higher reproductive output, mainly through double-brooding, of older (>= 2 years of age) individuals, which were disproportionately more abundant in high shrub density sites. Clutch initiation dates, clutch sizes, and predation rates at individual nests did not differ significantly between habitats. Mean body mass of nestlings on day 6 following hatching were higher on average on plots with high shrub density, but differences were not significant. Annual return rates, as indices of local survival, did not differ between habitats for older males or for females. Yearling males, however, returned in subsequent years at a significantly lower rate to low shrub density plots, a result of either lower survival or, more likely, dispersal to more suitable habitat in their second year of breeding. Parental age and habitat suitability interact in that older individuals, through their experience and/or dominance, acquire sites of higher quality, which results in higher reproductive output and probably higher survival. These differences between habitats in density, reproductive performance and local survival are consistent with an ideal-despotic/preemptive distribution of individuals, and suggest that this population could be regulated by the availability, distribution, and extent of high and low quality breeding habitats.

TS Sillett, RT Holmes, and TW Sherry (2000) Impacts of a global climate cycle on population dynamics of a migratory songbird. Science 288: 2040-2042.

Progress toward understanding factors that limit abundances of migratory birds, including climate change, has been difficult because these species move between diverse locations, often on different continents. For black-throated blue warblers (Dendroica caerulescens), demographic rates in both tropical winter quarters and north temperate breeding grounds varied with fluctuations in the El Niño Southern Oscillation. Adult survival and fecundity were lower in El Niño years and higher in La Niña years. Fecundity, in turn, was positively correlated with subsequent recruitment of new individuals into winter and breeding populations. These findings demonstrate that migratory birds can be affected by shifts in global climate patterns and emphasize the need to know how events throughout the annual cycle interact to determine population size.

RT Holmes and TW Sherry (2001) Thirty-year bird population trends in an unfragmented temperate deciduous forest: importance of habitat change. Auk 118 (3): 589–609.

Abundances of forest birds in an unfragmented, undisturbed, and relatively mature temperate deciduous forest at the Hubbard Brook Experimental Forest, New Hampshire, changed markedly between 1969 and 1998. Total numbers of birds (all species combined) declined from 210–220 individuals/10 ha in the early 1970s to 70–90/10 ha in the 1990s. Of the 24 regularly occurring species, 12 decreased significantly (four to local extinction), three increased significantly, and nine remained relatively constant in abundance. Nine of the 12 declining species were Neotropical migrants. Most species exhibited similar trends on Breeding Bird Survey (BBS) routes in New Hampshire during the same 30 year period and on three replicate study sites in nearby sections of the White Mountains from 1986–1998. Probable causes of trends were diverse and differed among species. Most could be accounted for by individual species' responses to events occurring primarily in the local breeding area. The most important local factor affecting bird abundance was temporal change in forest vegetation structure, resulting from natural forest succession and local disturbances. Four species that declined markedly and in some cases disappeared completely from the study plot (Least Flycatcher, Empidonax minimus; Wood Thrush, Hylocichla mustelina; Philadelphia Vireo, Vireo philadelphicus; and American Redstart, Setophaga ruticilla) appear to attain peak abundance in early or mid successional forests. Species preferring more mature forests, such as Black-throated Green Warbler (Dendroica virens) and Ovenbird (Seiurus aurocapillus), increased significantly in abundance over the 30 year study. Other important factors influencing bird abundances were food availability and events in the migratory and winter periods. Nest-predation rates, although varying among years, showed no long-term pattern that would account for population declines, and brood parasites were absent from this forest. Findings from this study demonstrate that major changes in bird abundances occur over time even in undisturbed and relatively mature forests, and illustrate the need for considering habitat requirements of individual species and how habitat suitability changes over time when trying to assess the causes of their long-term population trends. The results also imply that any conclusions about the effects of other factors affecting forest bird abundances, such as increased nest predation or brood parasitism associated with habitat fragmentation, must also account for successional changes that may be affecting habitat suitability.

TS Sillett and RT Holmes (2002) Variation in survivorship of a migratory songbird throughout its annual cycle. Journal of Animal Ecology 71 (2): 296-308.

Demographic data from both breeding and non-breeding periods are needed to manage populations of migratory birds, many of which are declining in abundance and are of conservation concern. Although habitat associations, and to a lesser extent, reproductive biology, are known for many migratory species, few studies have measured survival rates of these birds at different parts of their annual cycle. Cormack-Jolly-Seber models and Akaike's information criterion model selection were used to investigate seasonal variation in survival of a Nearctic - Neotropical migrant songbird, the black-throated blue warbler, Dendroica caerulescens. Seasonal and annual survival were estimated from resightings of colour-ringed individuals on breeding grounds in New Hampshire, USA from 1986 to 2000 and on winter quarters in Jamaica, West Indies from 1986 to 1999. Warblers were studied each year during the May-August breeding period in New Hampshire and during the October-March overwinter period in Jamaica. In New Hampshire, males had higher annual survival (0.51 ± 0.03) and recapture probabilities (0.93 ± 0.03) than did females (survival: 0.40 ± 0.04; recapture: 0.87 ± 0.06). In Jamaica, annual survival (0.43 ± 0.03) and recapture (0.95 ± 0.04) probabilities did not differ between sexes. Annual survival and recapture probabilities of young birds (i.e. yearlings in New Hampshire and hatch-year birds in Jamaica) did not differ from adults, indicating that from the time hatch-year individuals acquire territories on winter quarters in mid-October, they survive as well as adults within the same habitat. Monthly survival probabilities during the summer (May-August) and winter (October-March) stationary periods were high: 1.0 for males in New Hampshire, and 0.99 ± 0.01 for males in Jamaica and for females in both locations. These annual and seasonal survival estimates were used to calculate warbler survival for the migratory periods. Monthly survival probability during migration ranged from 0.77 to 0.81 ± 0.02. Thus, apparent mortality rates were at least 15 times higher during migration compared to that in the stationary periods, and more than 85% of apparent annual mortality of D. caerulescens occurred during migration. Additional data from multiple species, especially measures of habitat-specific demography and dispersal, will improve our understanding of the relative impacts of the breeding, migratory, and winter periods on population dynamics of migratory birds and thus enhance future conservation efforts.

J Jones, PJ Doran, and RT Holmes (2003) Climate and food synchronize regional forest bird abundances. Ecology 84 (11): 3024-3032.

Analysis of synchrony in population fluctuations can help to identify factors that regulate populations and the scales at which these factors exert their influence. Using 15 years of data on the abundances of songbirds at four replicate forest sites in New Hampshire, USA, we addressed two main questions: (1) Are forest bird populations synchronous at the scale measured (tens of kilometers), and if so, (2) what environmental factors are responsible for the synchrony? Nine of the 10 bird species we examined exhibited significant spatial synchrony across the four sites. Within nesting and foraging species groups, tree nesters and foliage gleaners exhibited the highest spatial synchrony. Long-distance (Neotropical) migrants exhibited higher spatial synchrony than did short-distance migrants or year-round residents. Synchrony within and among six species of long-distance, migratory, insectivorous birds was correlated with synchronous fluctuations in the abundance of lepidopteran larvae, a primary food type during the breeding season, which in turn have been shown to be influenced by El Niño/La Niña global climate patterns. Abundances of year-round resident species were related to another large-scale climatic phenomenon, the North Atlantic Oscillation. Winter weather can have both direct (e.g., via temperature-mediated mortality) and indirect (e.g., via winter food availability) effects on year-round resident species. We do not believe that predation on adults or nests accounted for the observed synchrony. Dispersal among regional populations in this system may have played a role but is likely a product of the influence of regionally synchronous caterpillar fluctuations on bird reproduction. Long-term regional population trends may have contributed to the observed synchrony for some species, but we do not consider these trends to be primary factors. Our findings of population synchrony support the importance of food and climate in influencing forest bird abundances and have broad implications for potential responses of bird and insect populations to climate change.

NL Rodenhouse, TS Sillett, PJ Doran, and RT Holmes (2003) Multiple density-dependence mechanisms regulate a migratory bird population during the breeding season. Proceedings of the Royal Society of London B 270: 2105-2110.

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969-2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997-1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.

TS Sillett, NL Rodenhouse, and RT Holmes (2004) Experimentally reducing neighbor density affects reproduction and behavior of a migratory songbird. Ecology 85 (9): 2467-2477.

Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic–Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that individuals with more neighbors (i.e., in neighborhoods with greater conspecific density) have reduced breeding productivity. The results also suggest that a crowding mechanism that mediates interactions among territory-holders could generate the density dependence needed to regulate local abundance, at least in areas of homogeneous, high-quality habitat. The effect of the neighbor-density reduction on warbler fecundity and behavior varied with annual fluctuations in weather and food availability, and was strongest in 1997, an El Niño year, when conditions for breeding were least favorable. This variation in our experimental results among years implies that density dependence due to crowding may have its strongest impact on local abundance when environmental conditions are relatively poor.

LR Nagy and RT Holmes (2004) Factors influencing fecundity in migratory songbirds: is nest predation the most important? Journal of Avian Biology 35 (6): 487-491.

Nest predation often is reported to be the major cause of nest failure in birds and by extension the predominant influence on annual fecundity. However, other factors such as the ability of some individuals to renest after a failed nest or to lay additional clutches after successful ones, both of which may in turn be influenced by food availability, can also contribute to individual fecundity. We evaluated the relative importance of nest predation, multiple brooding, renesting, and food availability on mean annual fecundity of a migratory songbird population, using AICc from multiple regression models. In a 16-yr study of black-throated blue warblers Dendroica caerulescens, these four variables combined explained 87% of the annual variance in fecundity. Individually, however, food abundance explained more of the variability (35%) than did predation (29%), double-brooding (19%), or renesting (15%). These results suggest that measuring food availability may be essential to understanding annual differences in reproductive output. They also demonstrate that failing to account for the contribution of food availability, renesting, and multiple brooding in studies of avian demography can result in serious underestimates of mean annual fecundity, potentially biasing calculations of population growth rate, source-sink dynamics, habitat quality, or population viability.

LR Nagy and RT Holmes (2005) Food limits annual fecundity of a migratory songbird: an experimental study. Ecology 86 (3): 675-681.

In short-lived species, fecundity strongly influences population size. For those species with multiple breeding attempts per breeding season, variance in fecundity is best explained by the number of breeding attempts. For birds, multiple brooding may be influenced by food availability. Here, we report results of a food supplementation experiment that tests the role of food as a mechanism driving variation among individuals in the frequency of multiple brooding in a Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). Supplementally fed females produced more second broods, spent less time foraging and more time loafing, and stayed closer to their nests than did control females. Fed and control females did not differ in the number or mass of young fledged from the first nesting attempt. Supplemental food increased the probability that females would initiate second broods in both a low and an average food year, suggesting that this population is food limited during the breeding season in most years. Our results thus demonstrate that food availability can strongly influence annual fecundity in migratory bird species breeding in temperate forests, which, in turn, affects annual recruitment rates and population size.

LR Nagy and RT Holmes (2005) To double brood or not? Individual variation in the reproductive effort in Black-throated Blue Warblers (Dendroica caerulescens). Auk 122 (3): 902-914.

Individuals within a population vary in important fitness components, such as reproductive success. In general, females can maximize the number of young they produce by altering either the number of young per breeding attempt or the number of breeding attempts per season. In short-lived species, and especially in small passerine birds, number of breeding attempts per season varies markedly among individuals. Here, we evaluated factors influencing whether female Blackthroated Blue Warblers (Dendroica caerulescens) initiated additional nests after a successful breeding attempt (i.e. double-brooded). The percentage of females that laid a second clutch after successfully fledging a first brood ranged from 0 to 87% and averaged 53% (n = 7 years). Multiple logistic regression and AICc model selection indicated that double-brooded females bred in territories with greater food availability and produced heavier nestlings than single-brooded females. Female age, male age, date of first breeding attempt, and number of young in the first clutch were not included in the best-fit model. Older females, however, produced heavier fledglings, and females mated to older males occurred on territories with greater food availability, indicating that age contributed to individual variation in reproductive output. Because the proportion of females that produce multiple broods within a season can have a substantial effect on the annual fecundity of a population, variation among females and among the territories they occupy (i.e. habitat quality) are key factors influencing population dynamics in this and other multibrooded, shortlived species.

RC Dobbs, TS Sillett, NL Rodenhouse, and RT Holmes (2007) Population density affects foraging behavior of male Black-throated Blue Warblers during the breeding season. Journal of Field Ornithology 78 (2): 133-139.

Foraging behavior often reflects food availability, a resource that may increasingly limit breeding birds as intraspecific crowding increases. Measuring foraging behavior, therefore, provides a way to investigate effects of population density on food limitation, an important link in understanding how crowding functions to regulate populations. We quantified three components of foraging behavior (prey attack rate, foraging speed, and relative use of morphologically constrained attack maneuvers) for male Black-throated Blue Warblers (Dendroica caerulescens) breeding under experimentally manipulated density conditions. Building on the previous work showing the density of conspecific neighbors affects territory size, reproductive success, and the time budgets of males (Sillett et al. 2004, Ecology 85: 2467–2477), we further show that density affects male foraging strategies. Although not differing in attack rate or foraging speed, male Black-throated Blue Warblers on territories with reduced neighbor densities used energetically expensive aerial attack maneuvers significantly less frequently than males in control (high-density) territories during both the incubation period and when provisioning nestlings and fledglings. We conclude that males altered their foraging behavior to compensate for density-related reductions in time available for foraging and that population density may constrain the time available for foraging.

RT Holmes (2007) Understanding population change in migratory songbirds: long-term and experimental studies of Neotropical migrants in breeding and wintering areas. Ibis 149 (2): 2-13.

Effective conservation and management of migratory bird species requires an understanding of when and how their populations are limited and regulated. Since 1969, my colleagues and I have been studying migratory songbird populations in their breeding quarters at the Hubbard Brook Experimental Forest in north-central New Hampshire, USA, and since 1986, in their winter quarters in the Greater Antilles (Jamaica). Long-term data on the abundance and demography of these populations, coupled with experimental tests of mechanisms, indicate that processes operating in the breeding area (e.g. density-dependent fecundity, food limitation) are sufficient to limit and regulate the local abundance of these species. At the same time, limiting factors operating in the non-breeding season (e.g. climate-induced food limitation in winter quarters and especially mortality during migration) also have important impacts on migrant populations. Furthermore, recent studies have shown that limiting processes during the winter period can carry over into the breeding season and affect reproductive output. These findings clearly demonstrate that to understand changes in abundance of long-distance migrant species requires knowledge of events operating throughout the annual cycle, which presents a challenge to researchers, managers and others concerned with the welfare of these species.

NL Rodenhouse, SN Matthews, KP McFarland, JD Lambert, LR Iverson, A Prasad, TS Sillett, and RT Holmes (2008) Potential effects of climate change on birds of the Northeast. Mitigation and Adaptation Strategies for Global Change 13: 517-540.

We used three approaches to assess potential effects of climate change on birds of the Northeast. First, we created distribution and abundance models for common bird species using climate, elevation, and tree species variables and modeled how bird distributions might change as habitats shift. Second, we assessed potential effects on high-elevation birds, especially Bicknell’s thrush (Catharus bicknelli), that may be particularly vulnerable to climate change, by using statistical associations between climate, spruce-fir forest vegetation and bird survey data. Last, we complemented these projections with an assessment of how habitat quality of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens) might be affected by climate change. Large changes in bird communities of the Northeast are likely to result from climate change, and these changes will be most dramatic under a scenario of continued high emissions. Indeed, high-elevation bird species may currently be at the threshold of critical change with as little as 1°C warming reducing suitable habitat by more than half. Species at mid elevations are likely to experience declines in habitat quality that could affect demography. Although not all species will be affected adversely, some of the Northeast’s iconic species, such as common loon and black-capped chickadee, and some of its most abundant species, including several neotropical migrants, are projected to decline significantly in abundance under all climate change scenarios. No clear mitigation strategies are apparent, as shifts in species’ abundances and ranges will occur across all habitat types and for species with widely differing ecologies.