Abstract. Understanding habitat quality for birds is crucial for ecologists and managers, but few papers have explored the advantages and disadvantages of different ways to measure it. In this review I clarify terminology and distinguish habitat quality from related terms, differentiate habitat quality at the levels of individual birds and populations, and describe different field methods for measuring habitat quality. As much as feasible, biologists concerned with habitat quality should emphasize demographic variables while recognizing that reproduction, survival, and abundance may not all be positively correlated. The distribution of birds can also reveal habitat quality (e.g., through patterns of habitat selection), but researchers should first investigate how closely their subjects follow ideal distributions because numerous ecological factors can lead birds to select poor and avoid rich habitats. Measures of body condition can provide convenient measures of habitat quality, but to be useful they must be a consequence, rather than a cause, of habitat selection. Habitat ecologists should use caution before relying on shortcuts from more labor-intensive demographic work. To increase the reliability of our habitat quality measurements, we should work to develop new methods to assess critical assumptions of nondemographic indicators, such as whether birds follow ideal distributions under natural conditions and whether spatial variation in body condition manifests in differential fitness.

Abstract. Habitats are composed of multiple variables, each of which may affect one or more population demographic rates. The patterns of correlation among habitat variables can introduce correlations among demographic rates, even if any single habitat variable affects only a single demographic rate and is independent of others. I present a simple model in which habitat is composed of two continuous habitat variables, one of which affects fecundity but not survival, and the other of which affects survival but not fecundity, such that correlations between survival and fecundity are due exclusively to correlations between the habitat variables. When individuals are able to select habitat with complete information about habitat quality, habitat selection always leads to higher growth rates at small population sizes. With complete information about habitat quality, patterns of distribution and population growth rates are strongly affected by the correlation between habitat variables. In contrast, when individuals have information about only fecundity or survival but not both, distributions become insensitive to correlations among habitat variables, which can lead to an ecological trap. This simple model demonstrates why explicitly distinguishing between habitat choices and the consequences of choices will be necessary if the complex interplay between habitat structure, habitat choice, and habitat quality is to be fully understood.

Abstract. Nest predation may influence habitat selection by birds at multiple spatial scales. We blended population and community ecology to investigate this possibility for 15 species of forest songbirds and their diurnal nest predators (corvids and sciurids) in 28 1 km² sites near Seattle, Washington, from 1998 to 2004. We determined whether songbirds were positively or negatively associated with nest predators at three spatial scales, and whether their co-occurrence affected reproductive success. At the largest ‘neighborhood’ scale (1 km² areas that included suburban and exurban development and second-growth forest remnants), nest predators and their prey were positively or negatively correlated according to general species-specific habitat associations. At the intermediate ‘forest patch’ scale (among remnant forested areas 0.5 to 70 ha), associations between predators and prey were generally weak. At the smallest ‘within patch’ scale (multiple 50 m radius survey plots within each forest patch), some songbird species avoided areas with greater predator use, particularly by Steller's Jays (Cyanocitta stelleri). Failed nests and territories tended to be in locations of higher predator occurrence (especially of corvids) than successful ones, but at the largest 1 km² neighborhood scale relative abundance of nest predators was not correlated with the fate of nesting attempts or annual reproductive success. Reproductive success was generally high, with 52% of all nests and 49% of all territories fledging at least one young (for all species and years combined). Nest predation influenced some species' use of resources, but was not a strong influence on overall reproductive success or community structure.

Abstract. Historically, studies of habitat selection have focused on quantifying how current patterns of habitat occupancy influence condition and survival within a season. This approach, however, is overly simplistic, especially for migratory birds that spend different periods of the year in geographically distinct places. Habitat occupancy and the resulting condition of individual birds is likely to be affected by events in the previous season, and the consequences of habitat occupancy will influence individuals and populations in subsequent seasons. Thus, for migratory birds, variation in habitat quality (and quantity) needs to be understood in the context of how events interact throughout periods of the annual cycle. Seasonal interactions can occur at the individual level or population level. Individual-level interactions occur when events in one season produce nonlethal, residual effects that carry over to influence individuals the following season. Population-level interactions occur when a change in population size in one season influences per capita rates the following season. We review various methods for estimating seasonal interactions and highlight a number of examples in the literature. Using a variety of techniques, including intrinsic and extrinsic markers, the vast majority of studies to date have measured seasonal interactions at the individual level. Obtaining estimates of density and changes in per capita rates across multiple seasons to determine population-level interactions has been more challenging. Both types of seasonal interactions can influence population dynamics, but predicting their effects requires detailed knowledge of how populations are geographically connected (i.e., migratory connectivity). We recommend that researchers studying habitat occupancy and habitat selection consider how events in previous seasons influence events within a season.

Abstract. In migrant birds, winter social systems reflect strategies of habitat use. The best-studied strategy is that of a fixed home range or territory. Alternative strategies to holding a territory, such as floating and flocking behaviors, suggest different processes of habitat selection. Whereas flocking, like territoriality, is a well-studied social strategy, solitary nonterritorial (floater) behavior is difficult to study and is thus poorly understood in terms of its causes, consequences, and implications for habitat selection. Floaters are individuals of a primarily stationary population which move over large areas compared to the average home range size. We review evidence of the causes, consequences, and frequency of winter floating behavior, focusing primarily on migrant songbirds. We also address the endocrine basis of alternative behavioral strategies by presenting results from aviary-based dominance trials with Hermit Thrushes (Catharus guttatus). Results from this study and others suggest that the adrenocortical response to social stress plays a key role in both the proximate and ultimate causes of floating behavior. Because winter floating behavior exists in many migrant songbird species, better understanding of winter floaters may be important for population modeling and conservation planning.

Abstract. Understanding the effect of habitat alteration on avian behavior is important for understanding a species' ecology and ensuring its conservation. Therefore, we examined the relationship between Spotted Owl habitat selection and variation in habitat in the Sierra Nevada. We estimated habitat selection by modeling the probability of territory colonization (γ), territory extinction (ε), and breeding dispersal in relation to the amount of mature conifer forest within and among territories. Alteration of ≥20 ha of mature conifer forest (coniferous forest with >70% canopy cover dominated by medium [30.4–60.9 cm dbh] and large [>60.9 cm dbh] trees) within individual territories (n  =  66) was negatively related to territory colonization and positively related to breeding dispersal probability. Although territory extinction was negatively related to the amount of mature conifer forest, it was not clear whether this relationship was due to variation of mature conifer forest within or among territories. Although modeling results for territory colonization and extinction generally supported the hypothesis that individuals are “ideal” when selecting a habitat in the sense that they settle in the highest-quality site available, we did not find a clear benefit in terms of habitat quality for Spotted Owls that exhibited breeding dispersal.

Abstract. The Blue-tailed Bee-eater (Merops philippinus) is a cooperatively breeding and socially monogamous member of the Coraciiformes that displays conspicuous coloration and elongated central rectrices (“streamers”). Humans cannot distinguish males from females; both sexes are brightly colored with a chestnut throat patch, a yellow chin, and green body coloration fading into a turquoise-blue rump and tail. We quantified coloration with ultraviolet- (UV) visible spectrometry and measured morphology to determine the extent of sexual dichromatism and dimorphism. Males displayed more exaggerated coloration, longer tail streamers, and were larger than females. Multiple plumage ornaments (measures of plumage coloration and streamer length) were positively correlated in both sexes. Males in better body condition expressed darker chestnut throats and more chromatic green body plumage. Females in better body condition, however, exhibited more chromatic blue rumps and yellow chins. This study represents the first objective description of plumage ornamentation in the order Coraciiformes.

Abstract. Maternal allocation of resources to eggs and nestlings can potentially have a profound influence on offspring phenotype and fitness. However, it is often unclear how much of the variation in offspring quality is due to maternal or environmental effects. We examined the influence of maternal and environmental effects on egg mass and nestling quality (growth and immune function) in Tree Swallows (Tachycineta bicolor). There was no evidence that the allocation of maternal resources to eggs varied with laying order or offspring sex. Thus, there was no evidence of adaptive maternal allocation to eggs in terms of mass. Instead, egg mass in Tree Swallows appeared to be influenced primarily by ambient temperature during egg formation and differences among females. Nonetheless, female Tree Swallows were likely to realize substantial benefits from producing larger eggs because those young were larger at hatching and grew faster. Furthermore, nestlings that grew faster had a stronger immune function at 11 days of age. Our results suggest that the positive relationship between nestling growth and survival may be due, in part, to the effects of an enhanced immune response.

Abstract. The Henslow's Sparrow (Ammodramus henslowii), a species of conservation concern, winters primarily in longleaf pine (Pinus palustris) forest ecosystems in the southeastern U.S. These pine savannas have been reduced to 5% of their former range, with remaining patches requiring active management with fire to maintain characteristic structure and plant diversity. Wintering Henslow's Sparrow abundance tracks growing-season fires; bird abundance peaks in the winter following burning, then declines in subsequent winters. Fire also determines dominant plant species, suggesting that Henslow's Sparrows may respond to abundance of preferred seeds. To determine diet preferences of Henslow's Sparrows, we tested seeds from eight species of native plants from southeastern Louisiana pine savannas, including species common in the first winter after burning (‘fire grasses’) and species that increase in abundance in the second and subsequent winters after burning. Seed consumption by individual birds differed considerably, suggesting that Henslow's Sparrows forage on a variety of resources in the highly diverse savannas. Henslow's Sparrows preferred fire grasses, especially Muhlenbergia expansa (cutover muhly). They also preferred Dichanthelium angustifolium (needleleaf rosette grass), a species more common in the second year after burning, but consumed relatively little of the sedges Rhynchospora plumosa (plumed beaksedge) and R. gracilenta (slender beaksedge), species common in the second winter after fire. Birds consumed almost none of the ubiquitous grass Schizachyrium scoparium (little bluestem). These results suggest that preferred seeds may include those that are most common in the first winter after burning, but that some suitable seeds are available for at least another winter.

Abstract. Interspecific differences in sexually selected traits may be important for maintaining reproductive isolation among closely related species living in sympatry. We present the first study of plumage color differences among males of partially sympatric species of South American red-breasted meadowlarks—the White-browed Blackbird (Sturnella superciliaris), the Pampas Meadowlark (S. defilippii), and the Long-tailed Meadowlark (S. loyca)—using reflectance spectrophotometry and the avian visual model of Vorobyev and Osorio (1998). Reflectance values of sexually dichromatic red plumage patches were measured on study skins. Total reflectance, reflectance in the short wavelength part of the spectrum, and several measures of spectral shape were extracted directly from the spectra. Our analyses revealed that S. loyca and S. defilippii were brighter and had higher reflectance in the short wavelength part of the spectrum than S. superciliaris. Minimum reflectance was located at higher wavelengths in breeding than in nonbreeding plumage. Interspecific distances in avian visual space obtained from the Vorobyev and Osorio (1998) model were considerably higher than the threshold value for color discrimination, indicating that the differences found are also detectable by birds. Taken together, these results show that the red plumage patches of these three species present significant color differences throughout the year, not only in the visible but also in the UV part of the spectrum.

Abstract. Sequences of the mitochondrial cytochrome b gene (930 base pairs) were used to examine patterns of variation within and between Eastern (Megascops asio) and Western (M. kennicottii) Screech-Owls, and to assess taxonomic affinity of Flammulated Owls (Otus flammeolus) and Whiskered Screech-Owls (M. trichopsis). Analyses support monophyly of the New World Megascops, a sister-group relationship between O. flammeolus and New World forms of Megascops, rather than with Old World Otus, and a closer relationship between the mostly North American M. trichopsis and South American Megascops than between M. trichopsis and North American Megascops. Megascops asio and M. kennicottii formed two distinct monophyletic clades, supporting species-level designations as suggested by morphology and song. Evidence for distinctive subspecies of eastern and western forms of screech-owls was less compelling. In the M. asio group, M. a. mccallii was the only subspecies with a unique haplotype; other subspecies within M. asio were phylogenetically indistinguishable. Subspecies within M. kennicottii were partitioned into three geographic groups, and differences are probably the result of barriers to gene flow (e.g., mountains above 2300 m), which are more pronounced throughout the distribution of M. kennicottii than in the distribution of M. asio.

Abstract. I used an information-theoretic approach to determine support for hypotheses concerning the effects of edge and temporal factors on Golden-cheeked Warbler (Dendroica chrysoparia) nest survival on Fort Hood Military Reservation, Texas, during 2003–2006. I predicted that nest survival would be greater earlier in the breeding season than later in the breeding season, in areas with less forest edge than in areas with more forest edge, and during the laying and incubation stages than during the nestling stage. I used the logistic-exposure method to model nest survival as a function of the explanatory variables and to produce model-based estimates of daily and period survival. The overall daily survival rate was 0.96 (95% CI: 0.94, 0.97) and overall period survival was 0.34 (95% CI: 0.23, 0.46). Forest edge density had the strongest effect on nest survival. Nest survival decreased as forest edge density increased. Period survival tended to decrease as the breeding season progressed, but the 95% confidence intervals of the model-averaged parameter estimates overlapped. These results demonstrate that effective conservation strategies designed to provide high-quality breeding habitat for this endangered species should include measures to reduce predation by edge-adapted predators.

Abstract. Precocial young often experience high mortality prior to achieving flight, especially in the first days after hatching. We quantified relationships between chick age, survival, behavior, and response to natural and anthropogenic danger for the threatened Western Snowy Plover (Charadrius alexandrinus nivosus) in coastal northern California, USA. Plover chicks were most likely to die in the first three days of their 28-day prefledging period, especially on sandy ocean beaches compared to coarser substrates of river habitats. Chick survival in both habitats increased across the ∼120-day chick-rearing period. Improved survival of older chicks coincided with an age-related reduction in brooding and increased distance from the tending parent, which was not related to season. Lower survival of younger chicks was correlated with a tendency to lie motionless when approached by humans; nearly all older chicks responded to human approach by running to evade danger. Chicks of all ages were more likely to lie motionless when potential avian predators flew nearby. Age-dependent survival of Snowy Plover chicks paralleled a trend of increasing development, thermal independence from adults, and capability of evading predators.

Abstract. We evaluated the influence of weather on reproduction of the Red-shouldered Hawk (Buteo lineatus) in an agricultural landscape in south-central Florida where we found relatively high densities of successfully nesting hawks. We used a generalized linear modeling approach within an information-theoretic framework to examine the influence of total rainfall, rainfall frequency, and temperature on the timing of nesting, nesting success, and productivity of hawks during 1995–2000. The best models indicated an influence of rainfall frequency and laying period on hawk reproduction. During years with less frequent rainfall in the summer and fall months prior to the beginning of the breeding season, fewer pairs attempted to nest, and hawks nested later and had smaller clutch sizes and lower productivity. Hawks that nested later in the breeding season had lower hatching success and lower overall nest success. Although Red-shouldered Hawks are generally reported to inhabit forested landscapes throughout their range, a common feature seems to be a dependence on wetlands and riparian habitat for foraging. We propose that the proportion of wetlands throughout the landscape is a unique aspect of south-central Florida that may allow for persistence of unusually high numbers of hawks.

Abstract. We studied song dialects of the Rufous-collared Sparrow (Zonotrichia capensis) in espinal woodland and steppe surrounding Guatraché Lake in Argentina. We recorded 150 individuals and analyzed 14 quantitative song variables. There were significant differences between habitats in song temporal structure. In particular, the internote interval of trills was shorter in the closed compared to the open habitat. This pattern is the reverse of what is usually found in this species, although it has previously been detected in another area of the region in which this study was conducted. This region is characterized by strong winds, which could be related to the reversed pattern found. Song frequencies were lower in the closed woodland habitat than in the open steppe, which also contrasts with the pattern usually found in this species. We also recorded and analyzed the songs in a portion of the area in which the woodland was converted into grassland during the last century. The temporal structure of songs in the converted area was intermediate between song structures in the original habitats and frequencies were lower in the converted area than in both original habitats. Song structure appears to have changed after habitat conversion, but this cannot be confirmed with the data at hand because of the lack of recordings obtained before habitat change.

Abstract. In birds, changing mates generally results in decreased breeding success. Although costs and benefits of pair break-up have been well studied, endocrine mechanisms associated with mate change are poorly known. We measured baseline and stress-induced corticosterone levels in relation to mate change in Black-legged Kittiwakes (Rissa tridactyla). Baseline corticosterone levels were higher in kittiwakes breeding with a new mate than in kittiwakes that did not change mate. Stress-induced corticosterone levels were not influenced by change of mate. Elevated baseline corticosterone levels in birds breeding with a new mate could result from the social stress associated with pair break-up or mirror a higher energetic demand resulting from a lack of coordination between new pair members. Our results emphasize the usefulness of corticosterone levels in elucidating the effects of mate change on the energetic demands of reproduction in free-living birds.

Abstract. Acute corticosterone secretion helps individuals survive perturbation, particularly when faced with unpredictable conditions. However, elevated corticosterone is often incompatible with reproduction. Male Yellow Warblers (Dendroica petechia) breeding at high latitude avoid this conflict by exhibiting a reduced adrenocortical response while provisioning nestlings. To determine if this reduction is cued by nestlings, in June 2000 we switched clutches between nest pairs to manipulate hatching date. We sampled males with early-hatching clutches for the adrenocortical response 24 hr after nestlings hatched, but 24 hr prior to the premanipulated (original) hatching date. Similarly, we sampled males with late-hatching clutches 24 hr after the original hatching date, but prior to nestlings hatching. Males with early-hatching clutches (n  =  6) showed no reduced adrenocortical response, while males with late-hatching clutches (n  =  6) appeared to show a reduced response even without nestlings, suggesting that nestlings do not cue the reduction. An endogenous clock, day-counting mechanism, or hormonal changes may be associated with the reduced adrenocortical response.

Abstract. We provide details on the breeding biology of the Violet-chested Hummingbird (Sternoclyta cyanopectus) based on 67 nests studied in Yacambú National Park, Venezuela, from 2002 through 2006. Clutch size was two white eggs, usually laid every other day. Fresh egg mass (0.95 ± 0.14 g) was 15% of female mass. Incubation and nestling periods were 20.4 ± 0.3 and 26.0 ± 0.4 days, respectively. Nest attentiveness increased from 60% in early incubation to 68% in late incubation. The female spent 50% of her time brooding young nestlings, but ceased brooding by 13 days of age. Only the female fed the young, with a low rate of nest visitation (3.3 trips per hour) that did not increase with age of the young. Growth rate based on nestling mass (K  =  0.28) was slow. Daily predation rates decreased across stages and were 0.064 ± 0.044, 0.033 ± 0.008, and 0.020 ± 0.006 during the egg-laying, incubation, and nestling periods, respectively. Most, but not all, life history traits of the Violet-chested Hummingbird were similar to those reported for other tropical and temperate hummingbirds, providing further evidence that this family shows a relatively narrow range of life history variation.

Abstract. We provide the first detailed analysis of the vocal behavior of Pacific coast Plain Wrens (Thryothorus modestus modestus). Male Plain Wrens sing two categories of song phrase; one phrase is given as a solo song or duet introduction (I-phrases), while the other is sung primarily during duets (M-phrases). Females sing only one category of song phrase, usually during duets (F-phrases). Both sexes have repertoires of their respective song categories. Plain Wren duets show extreme temporal coordination: duets typically begin with male I-phrases, which are followed by the rapid alternation of female F-phrases and male M-phrases. These patterns are congruent with the solo and duet singing behavior of a sister taxon to the Plain Wren, the Canebrake Wren (Thryothorus modestus zeledoni). Our analyses of Plain Wren songs and duets reveal pronounced differences between males and females and contribute to our understanding of the complex duetting behavior of Thryothorus wrens.

Abstract. The recent divergence of Orchard (Icterus spurius spurius) and Fuertes's Orioles (I. s. fuertesi) makes them an ideal system for investigating species boundaries. Orchard and Fuertes's Orioles differ in several respects. They have distinct breeding ranges—Fuertes's Orioles breed in eastern coastal Mexico, whereas Orchard Orioles breed throughout eastern and central North America—and differ in plumage coloration, with adult male Orchard Orioles appearing “chestnut” and Fuertes's Orioles “ochre.” However, no detailed quantitative color analyses have been conducted. To characterize these differences we quantitatively measured plumage color using reflectance spectrometry. The colored plumages of adult male Orchard and Fuertes's Orioles have unique spectral characteristics, with no color overlap between them. Combined with life history differences and previous molecular studies, these findings support the classification of Orchard and Fuertes's Orioles as separate species. Additionally, this study demonstrates the utility of quantitative color measurement as a tool for examining species boundaries.

Abstract. Waterfowl employ a broad array of strategies for acquiring the energy and nutrients needed for egg formation, ranging from storage of endogenous reserves prior to arrival on breeding areas to complete reliance on exogenous food sources available at breeding sites. We used stable isotope analyses (δ¹³C and δ¹⁵N) to quantify the relative nutrient inputs to Harlequin Duck (Histrionicus histrionicus) eggs and, therefore, to identify the strategy of nutrient acquisition and allocation used by females to meet the demands of egg production. Marine-derived endogenous nutrients are isotopically more enriched than freshwater dietary nutrients for Harlequin Ducks that migrate between marine wintering grounds and terrestrial breeding grounds. There was little evidence that endogenous reserves stored on marine wintering areas were allocated to clutch formation. Therefore, Harlequin Ducks relied on food available in streams on breeding grounds for egg formation, and reserves stored on marine areas were likely used during other energetically and nutritionally demanding periods.

Abstract. Numerous investigators have used allometric regression to characterize the relationship between proportional egg composition and egg size, which is a potentially important characterization for assessing maternal investment in reproduction. Herein, we document two important shortcomings of this approach. First, regressing log component mass against log egg mass involves regressing Y on itself, since each component (Y) is necessarily a part of the whole egg (X). This creates correlated errors, which leads to biased estimates of the regression slope. To circumvent this problem, we recommend regressing egg component masses on a relatively inert component like total water mass. Secondly, investigators routinely use ordinary least squares regression to estimate the slope of allometric relationships, which assumes that all error resides in Y. We demonstrate that this assumption is false, but so are the underlying error assumptions of commonly used alternatives such as reduced major axis and major axis regression. Because each egg is unique and determining composition involves destructive sampling, there is no obvious way to assess measurement error in Y versus X. As a solution, we recommend that investigators analyze multiple eggs per clutch whenever possible and fit a reduced major axis based on the among-female component of variability.