In the United States, government-mandated growth in the production of crops dedicated to biofuel (agrofuels) is predicted to increase the demands on existing agricultural lands, potentially threatening the persistence of populations of grassland birds they support. We review recently published literature and datasets to (1) examine the ability of alternative agrofuel crops and their management regimes to provide habitat for grassland birds, (2) determine how crop placement in agricultural landscapes and agrofuel-related land-use change will affect grassland birds, and (3) identify critical research and policy-development needs associated with agrofuel production. We find that native perennial plants proposed as feedstock for agrofuel (switchgrass, Panicum virgatum, and mixed grass—forb prairie) have considerable potential to provide new habitat to a wide range of grassland birds, including rare and threatened species. However, industrialization of agrofuel production that maximizes biomass, homogenizes vegetation structure, and results in the cultivation of small fields within largely forested landscapes is likely to reduce species richness and/or abundance of grassland-dependent birds. Realizing the potential benefits of agrofuel production for grassland birds' conservation will require the development of new policies that encourage agricultural practices specifically targeting the needs of grassland specialists. The broad array of grower-incentive programs in existence may deliver new agrofuel policies effectively but will require coordination at a spatial scale broader than currently practiced, preferably within an adaptive-management framework.

Invasive species pose the second greatest threat to biodiversity after habitat loss. Although invasive plant species negatively affect invaded ecosystems and diminish native biodiversity, they may provide food and other resources for some native birds. As monotypic stands of the invasive Chinese tallow tree (Triadica sebifera) become more common in the southeastern United States, it is important to assess their suitability as habitat for native forest birds. We used point counts to compare habitat use of forest birds in stands of native mixed-species forest, of mature tallow trees, and of young tallow trees on the coast of Texas during 2009 and 2010. The composition of trees in stands of mature and young tallow was more homogeneous than in stands of mixed native species, but mature tallow stands had an understory more complex than that of other habitat types. Mature tallow stands supported significantly fewer species of forest birds than did native forest only during the spring, and birds' population densities were similar in mature tallow and native forest throughout the year. Young tallow stands supported significantly fewer species of forest birds than did native forest in all seasons except for fall and significantly lower population densities during the breeding season (spring and summer). While monotypic stands of Chinese tallow trees provide suitable habitat for some forest birds, especially in winter, to preserve the widest diversity of forest birds we recommend the preservation of native mixed-species forest.

In the arid Southwest, migratory birds are known to use riparian stopover habitats; we know less about how migrants use other habitat types during migratory stopover. Using radar data and satellite land-cover data, we determined the habitats with which birds are associated during migration stopover. Bird densities differed significantly by habitat type at all sites in at least one season. In parts of Arizona and New Mexico upland forest supported high densities of migrants, especially in fall. Developed habitat, in areas with little upland forest, also supported high densities of migrants. Scrub/shrub and grassland habitats supported low to intermediate densities, but because these habitat types dominate the Southwestern landscape, they may provide stopover habitat for larger numbers of migratory birds than previously recognized. These results are complicated by continuing challenges related to target identity (i.e., distinguishing among birds, arthropods and bats). Our results suggest that it is too simplistic to (1) consider the arid West as a largely inhospitable landscape in which there are only relatively small oases of habitat that provide the resources needed by all migrants, (2) think of western riparian and upland forests as supporting the majority of migrants in all cases, and (3) consider a particular habitat unimportant for stopover solely on the basis of low densities of migrants.

Aside from the positive benefits of wind energy, wind farms often bring environmental problems such as noise production and wildlife collision. However, little is known about the effects of wind farms on the ecology of tropical landbirds. In this study, we evaluated changes in bird-community diversity, composition, and structure directly beneath wind turbines, 200 m away from turbines, and in nearby croplands and secondary forests. In general, our results show (1) a gradient of species richness, with values highest in croplands and secondary forests, intermediate values 200 m from turbines, and lowest values beneath turbines, (2) fairly similar bird abundance for all treatments, with values highest in secondary forests in autumn and lowest 200 m from turbines in autumn, (3) bird communities highly similar at each season, but communities at 0 and 200 m from turbines differed strongly in autumn and communities at the rest of the studied sites differed strongly during both spring and autumn, (4) evenness of the bird community greater in secondary forests and croplands and lower at both distances from wind turbines, and (5) the area covered by croplands outside the wind farm played an important role, often related to increases in species richness. Our results also suggest that wind farms have a greater effect on wintering migrants than on residents; however, further studies are required for such a comparison to be tested robustly.

In many birds, egg viability declines the longer that eggs remain unincubated (i.e., incubation delay), possibly because of increased levels of microbial infection. Reuse of nests is common in cavity-nesting species and may also increase exposure of eggs to microbes. Starting incubation before the clutch is complete may help protect early-laid eggs from microbial infection. In this study, we tested the effects of nest cleaning, length of incubation delay, and onset of night incubation on growth of microbes on Wood Duck (Aix sponsa) eggshells and examined the effect of microbial infection on hatching success. We used nest boxes that had been cleaned or not cleaned of previous nesting materials and sampled microbes on eggshells during early and mid egg laying and after the start of night incubation. We detected levels of eggshell microbes (10¹ colony forming units [CFU] egg^-1) lower than at more tropical locations (10^3–4 CFU egg^-1). Levels of heterotrophic bacteria were 2.2 times greater in uncleaned than in cleaned nests but did not vary with length of incubation delay. Levels of Gram-negative bacteria in cleaned and uncleaned nests did not differ but declined by 87% after onset of night incubation. We found no relationship between egg viability and infection of eggshells by either heterotrophic or Gram-negative bacteria. Bacterial infection of eggs was low at our temperate-zone study site and was further reduced with the onset of incubation. We suggest these factors contributed to the negligible effect that bacteria had on egg viability.

Nutrient-allocation models based on stable-isotope analysis are used to determine the nutrient sources birds invest in eggs. This approach is particularly useful for birds that migrate between habitats with distinct stable-isotope compositions. A crucial variable is the difference in stable-isotope values of egg tissues relative to diet, so appropriate adjustments can be used in models comparing nutrients from tissues to putative food sources. We established discrimination factors (Δδ) between the diet and eggs of captive Spectacled Eiders (Somateria fischeri) fed a controlled diet. Relative to diet, values of Δδ¹³C were higher for albumen (2.6‰), yolk protein (2.9‰), eggshell (13.0‰), and shell membrane (3.9‰), and lower for whole yolk (-1.6‰) and yolk lipid (-3.5‰). Values of Δδ¹⁵N of egg components were higher relative to diet (albumen 3.7‰, yolk protein 4.4‰, shell membrane 4.7‰, and whole yolk 3.5‰). Except for egg proteins, these patterns are generally consistent with published values for other birds. We conclude that choice of discrimination factors could markedly affect estimates of source contributions to eggs and so recommend species-specific estimates. We also provide the first reported discrimination factors between the female's diet and embryonic down feathers (Δδ¹³C = 2.1‰ and Δδ¹⁵N = 5.2). Finally, we determined discrimination factors between lipid and protein in diet sources and eggs, thus enabling consideration of these nutrients separately. Our study enhances the framework for nutrient-allocation modeling in eiders and likely other sea ducks.

Many birds, notably colonial nesting seabirds, use public information (the visual, auditory, and olfactory presence of breeding conspecifics) when selecting nesting habitat. When colonies are extirpated, social cues that indicate nesting sites' quality are lost. In the Aleutian Islands, Alaska, storm-petrel populations were destroyed by introduced arctic foxes (Alopex lagopus) and have been slow to return after the foxes' eradication. We tested various social-attraction techniques as a method to encourage recolonization of Leach's (Oceanodroma leucorhoa) and Fork-tailed Storm-Petrels (O. furcata) in Ulva Cove at Amatignak, a former fox-farming island. We tested attraction to playback of each species' calls by broadcasting them in various patterns adjacent to a mist net and attraction to their odors with a T-maze design. We combined these two cues to test whether birds were more likely to enter and inhabit artificial burrows depending on playback and odor treatment. Both species of storm-petrel were attracted strongly to playback of conspecific calls and somewhat to playback of heterospecific calls; Fork-tailed Storm-Petrels were attracted to conspecific odor and entered more artificial burrows when we combined odor and playback cues. We discuss the implications of these results, including the development of protocols to encourage restoration of seabird colonies in the Aleutian Islands after eradication of introduced foxes and Norway rats (Rattus norvegicus).

Group foraging is common among wading birds, and the reasons why individuals forage in groups are of theoretical and practical interest. Although aggregations of foraging wading birds usually form within patches of high-quality habitat, individual wading birds can sometimes increase success by foraging near others. We investigated the hypothesis that individuals derive a benefit from foraging in groups separate from benefits due to habitat quality. We measured the capture rates of birds foraging solitarily and in groups at foraging sites and paired unused sites and used generalized linear mixed models and information-theoretic model selection to access the evidence that individuals increase their foraging success when foraging in groups. The capture rate of Great Egrets (Ardea alba) in groups was higher, supporting the hypothesis that individuals benefit from the presence of other foragers. This rate was highest in intermediate-sized groups but tapered off in larger groups, suggesting that interference competition limits success. Tricolored Herons (Egretta tricolor) showed a similar but less statistically precise pattern. Contrary to expectation, the capture rate of Snowy Egrets (E. thula) foraging alone was higher than that of those in groups. Although fish abundance was greater at used than at unused sites, and at sites with groups than at sites with individuals, we did not detect an effect of prey density on capture rate, probably because prey density was fairly high at all sites studied. Our study adds to the evidence for a direct benefit to individuals due to the presence of other foragers in aggregations.

Foraging-patch selection may be influenced by prey density, prey availability, and the energetic costs associated with searching for and capturing prey. Although prey density influences profitability, prey availability may be influenced by habitat and environmental characteristics. We made in situ observations of Snail Kites (Rostrhamus sociabilis) foraging on their exclusive prey, apple snails (Pomacea paludosa), in selected habitats and environmental conditions. We measured the time to capture a snail at 10 sites on Florida lakes (two sites sampled twice over different years for 12 data points total) and five sites in wetlands of the Everglades. Capture times had an inverse asymptotic relationship to snail density. As snail density increased from ∼0.1 to 0.2m^-2, capture times declined from ∼600 to 200 sec; capture times leveled off to ∼75 sec between 0.4 and 1.4 snails m^-2. Densities of emergent plant stems and water depths, which may influence capture times, were controlled for by site selection; analyses verified these factors had no effects. Snail Kites were significantly more likely to abandon a foraging bout if snail density was <0.2m^-2. We interpret the results in the context of foraging for inconspicuous prey. Our data and other reports indicate ∼0.1–0.2 snails m^-2 as an approximate threshold below which flight costs increase dramatically and fewer kites tend to forage. Our observations contribute to the limited information available on what constitutes prey density sufficient for the Snail Kite, and we discuss associated conservation implications for this endangered dietary specialist.

Natal dispersal is an important, yet often poorly studied, process in population ecology. Natal dispersal can affect population structure and individual fitness. We examined correlates of natal dispersal distances of a population of Swainson's Hawk (Buteo swainsoni) in northern California that has been monitored from 1979 to 2010. We examined the effects of sex, number of offspring fledged, population density, primary productivity around a nest site, and annual trends on dispersal from the natal site to the breeding site of individuals recruited into the breeding population. Female Swainson's Hawks dispersed farther than males. Natal dispersal was negatively correlated with primary productivity and was positively correlated with population density around the nest site. We found no correlation between natal dispersal distance and lifetime reproductive success of those individuals recruited into the breeding population, indicating no fitness costs of increased dispersal. We examined the potential genetic contributions to dispersal distance by examining differences in dispersal distance between full siblings. We found that the dispersal difference between siblings was significantly more similar than would be expected by chance. However, we detected no relationships between paternal or maternal natal dispersal distances and natal dispersal of offspring, although sample sizes were limited. We conclude that observed similarities in siblings' natal dispersal distance were likely a result of similarities in unmeasured environmental variables rather than of a genetic mechanism of natal dispersal. Perinatal conditions seemed to have significant effects on distances offspring dispersed, but increased natal dispersal had no apparent consequences on fitness.

We document here a decline of nearly 30% in the Marbled Murrelet (Brachyramphus marmoratus) population of Washington, Oregon, and northern California between 2000 and 2010. The Northwest Forest Plan is an ecosystem-management plan for federal forest lands in the Pacific Northwest of the United States that incorporates monitoring to determine if species' conservation objectives are met. To evaluate the plan's effectiveness in conserving populations of the murrelet, a species associated with older, late-successional forests, we estimated the murrelet's density in near-shore marine waters of Washington, Oregon, and northern California south to San Francisco Bay. We sampled annually, using line transects and distance estimation. We divided the study area of about 8800 km² into five geographic subareas corresponding to existing murrelet-conservation zones. Annual population estimates for the plan ranged from an estimated 23 700 (95% CI: 18 300 to 29 000) birds in 2002 to a low of 16 700 (95% CI: 13 100 to 20 300) in 2010, representing an average rate of decline of 3.7% annually (95% CI: -4.8 to -2.7%) from 2001 to 2010. This annual rate suggests a total decline of about 29% during this period. We documented downward trends for Washington (conservation zone 1) and for the outer coast of Washington (conservation zone 2). These declines coincide with reductions in the amount of nesting habitat. Further research to evaluate the potential marine and terrestrial factors responsible for the declines is planned.

To avoid competition for parental care, brood-parasitic Common Cuckoo (Cuculus canorus) nestlings evict all of the host's eggs and nestlings within a few days after hatching. Little is known about the physiological effects of eviction behavior on the cuckoo nestling's oxidative balance or about age-related variation in plasma oxidative status and testosterone level of developing birds. We examined whether the cuckoo nestling's plasma oxidative status was related to prior effort in eviction and quantified variation in the level of reactive oxygen metabolites, of nonenzymatic antioxidant capacity, and of testosterone concentration in plasma at various phases of the cuckoo's development. Levels of both reactive oxygen metabolites and antioxidant capacity were greater in older than in younger nestlings, suggesting that younger nestlings effectively counterbalance their increased production of free radicals, whereas, near fledging, levels of reactive oxygen metabolites increase despite improved antioxidant capacity. Possibly, overall energy expenditure increases with age and elevates the production of reactive oxygen species to a rate higher than what the antioxidant system could eliminate. Plasma testosterone level was the highest at nestlings' intermediate phase of growth. High levels of testosterone may be required during the period of fastest growth, and when the growth rate levels off near fledging, testosterone levels may also decline. Cuckoo chicks that evicted more host eggs from steeper nests had higher plasma levels of reactive oxygen metabolites shortly after the eviction period, suggesting that eviction is costly in terms of an increased level of oxidative stress.

Understanding how habitat disturbances such as forest fire affect local extinction and probability of colonization—the processes that determine site occupancy—is critical for developing forest management appropriate to conserving the California Spotted Owl (Strix occidentalis occidentalis), a subspecies of management concern. We used 11 years of breeding-season survey data from 41 California Spotted Owl sites burned in six forest fires and 145 sites in unburned areas throughout the Sierra Nevada, California, to compare probabilities of local extinction and colonization at burned and unburned sites while accounting for annual and site-specific variation in detectability. We found no significant effects of fire on these probabilities, suggesting that fire, even fire that burns on average 32% of suitable habitat at high severity within a California Spotted Owl site, does not threaten the persistence of the subspecies on the landscape. We used simulations to examine how different allocations of survey effort over 3 years affect estimability and bias of parameters and power to detect differences in colonization and local extinction between groups of sites. Simulations suggest that to determine whether and how habitat disturbance affects California Spotted Owl occupancy within 3 years, managers should strive to annually survey ≥200 affected and ≥200 unaffected historical owl sites throughout the Sierra Nevada 5 times per year. Given the low probability of detection in one year, we recommend more than one year of surveys be used to determine site occupancy before management that could be detrimental to the Spotted Owl is undertaken in potentially occupied habitat.

The Bay-capped Wren-Spinetail (Spartonoica maluroides, Furnariidae) is the most specialized and threatened bird inhabiting Spartina-dominated saltmarshes in southeastern South America. These marshes are being modified at an increasing rate, primarily by cattle grazing and fire, which make this spinetail highly vulnerable. To assess how land use affects the Bay-capped Wren-Spinetail, we estimated its density under different intensities of fire and cattle grazing, studied selection of nesting habitat and nest success, and experimented with nest predation to determine predation rates and types of predators. We found similar population densities, nest microhabitats, and rates of nest predation in marshes with a low intensity of or no grazing. In contrast, the species was absent from marshes with a high intensity of grazing and fire. Nest height was the best predictor of daily survival rates. Nest-predation experiments demonstrated that nest height affects predation intensity and predator type. Our results suggest that low-intensity grazing and burning during winter are compatible with the presence and reproduction of the Bay-capped Wren-Spinetail. Thus low-intensity cattle grazing is a viable alternative for the sustainable use of saltmarshes in southeastern South America.

Changes in the structure of woodlands and forests, caused by shifts in management, stand maturity, and composition, have been implicated in the population decline of some bird species in Europe and North America. One such species is the Marsh Tit (Poecile palustris). We investigated relationships between Marsh Tit occupation (derived from territory mapping) and vegetation structure, tree species composition, and proximity to woodland edge in a British woodland, using a combination of 5 years of occupation data and high-resolution (0.5 and 1 m), large-scale (155 ha) habitat models derived from remote sensing. The results demonstrated that Marsh Tit occupation was linked to vegetation characteristics through the woodland's full vertical profile and related significantly and positively with overstory height, tree canopy closure, and the coverage of understory vegetation below the overstory. Marsh Tit occupation was lower within 50 m of the woodland perimeter, where habitat structure was less favorable than in the woodland interior. No preference was shown for areas rich in any particular prevalent tree species. Our results suggest that widespread changes in woodland structure resulting from abandonment by managers are unlikely to be responsible for the decline of the Marsh Tit in Britain and that reintroduction of active management that prevents woodland maturation could be detrimental to remaining populations. The study demonstrates a novel approach to integrating territory maps and remote-sensing data to permit highly detailed analyses of bird—habitat interactions and may have wider implications for woodland management and related bird species.

Variation in habitat quality can influence birds' survival and reproduction profoundly, but many methods for assessing it are logistically difficult. Ptilochronology is a method of determining the nutritional state of a bird by measuring the width of the growth bars on its feathers and may be a useful proxy for assessing habitat quality. Since each growth bar is thought to represent 24 hr of feather growth, the average size of growth bars provides a record of the bird's nutritional state during feather growth. Ptilochronology has been validated in adult birds but may not be reliable when applied to nestlings. We compared the growth rate of remiges and the average width of the growth bars on them in Eastern Bluebird (Sialia sialis) chicks. On average, the width of growth bars was slightly less than 50% of that expected if each bar corresponded to the daily growth of the feather. The average width of the growth bars also failed to correlate with the concurrent growth rates of the wing and the remiges themselves, although growth rates of the latter were significantly related. Growth bars on nestlings' feathers were also narrower than those of adult bluebirds and their size varied considerably. These data suggest that nestlings' feathers undergo daily pulses of growth that are variable in duration and number, and consequently ptilochronology may not be a reliable method for determining the growth rate of the feathers and hence determining the nutritional status of nestling birds.

Variation in aggression among birds on different territories has been hypothesized to be influenced by intrinsic factors such as a bird's age, sex, or size and extrinsic factors such as density of neighbors, abundance of food, or territory size. However, little work has examined how and why aggression may vary within a bird's home range. We examined variation in aggressive behaviors of nonbreeding Black-throated Blue Warblers (Setophaga caerulescens) in Jamaican coffee farms by using experimentally simulated intrusions into home ranges. Our multivariate models confirmed that multiple variables associated with aggression included sex, home range size, proximity of intrusion to home range center, and the presence of another aggressive bird in the area. In addition, we found that birds responded more aggressively to intrusions in shade trees, where insects were more abundant, than in the understory of coffee or of small patches of remnant forest. These results suggest the birds responded to variation in habitat and/or food supply within their home ranges and varied aggressive responses accordingly, revealing a finer scale of behavioral resource matching than previously documented for bird territoriality.

A growing body of evidence indicates that vocalizations of predators and perceived risk of predation can significantly alter avian nesting behavior and reproductive performance. However, it is currently unclear whether birds acoustically discriminate among different types of predators and adjust their short-term behavioral responses accordingly. We investigated this issue via playback experiments in which nests of the Hooded Warbler (Setophaga citrina) were exposed to vocalizations of two nest predators, the Blue Jay (Cyanocitta cristata) and Eastern Chipmunk (Tamias striatus), and a dangerous predator of adults and nestlings, Cooper's Hawk (Accipiter cooperii). We used songs of a common nonpredatory passerine, the Red-eyed Vireo (Vireo olivaceus), as a control. In comparison to responses observed during control trials, adult Hooded Warblers delayed their return to the nest following playback of Cooper's Hawk but returned more quickly after playback of the Eastern Chipmunk, probably because Cooper's Hawks are a threat to adult Hooded Warblers while Eastern Chipmunks pose a risk only to eggs and nestlings. Time of return to the nest following playback of the Blue Jay was nearly identical to that after controls, possibly because of the relative rarity of Blue Jays in our study area. Despite its significant effect on return time, playback had no effect on the number of times adults fed nestlings in the following hour. Overall, our results suggest that nesting Hooded Warblers discriminate among the vocalizations of potential predators and adjust the time of return to their nest according to the nature and degree of perceived risk.

Social information in the form of songs, calls, or other overt behaviors may assist birds in assessing the quality of potential breeding habitat by providing direct public information about a site's quality or by providing indirect cues (e.g., the presence of conspecifics) that may be used to infer its quality. Broadcast of song has been used to demonstrate the influence of conspecific cues on the decisions of migratory passerines to settle; however, there is currently a dearth of information regarding resident species. We tested the influence of conspecific attraction and the effectiveness of song playback on an endangered resident of grassland, the Cape Sable Seaside Sparrow (Ammodramus maritimus mirabilis). We broadcast recorded song over large areas of suitable breeding habitat in the Florida Everglades in an attempt to influence the settlement of male sparrows during territory establishment. Our analyses included the creation of a qualitative sound map that formed a basis for measuring the species' response to playback. Our results suggested that territory establishment by Cape Sable Seaside Sparrows was influenced by these artificial conspecific cues. Our findings have important conservation implications for the species in light of continuing large-scale efforts at habitat restoration in the Everglades ecosystem.

Habitat models that associate organisms with features of their environment can help identify areas for planning conservation strategies. These models, however, should be tested with new data before their conclusions are widely accepted. The Saltmarsh Sparrow (Ammodramus caudacutus) is a species of growing conservation concern along the Atlantic coast of North America. In a previous study, we developed models for Saltmarsh Sparrow presence and nesting habitat. Sparrow occupancy was best predicted from raw reflective properties of marshes derived from remote sensing, whereas sparrow nesting was best predicted from a classification of marsh plant communities. To test these models, we surveyed a stratified random sample of sites for which the probability of sparrow presence had been predicted and compared the observations to the predicted probability of presence and nesting generated from the models. The models' performance, assessed by the area under a receiver-operating characteristic curve and the deviance of observations from the models' predictions, was significantly better than expected by chance alone. Because sparrows are predicted to occur in many places where they are unlikely to nest, monitoring sparrow presence alone will not identify areas important for their nesting.

Why many animals display several different kinds of ornaments has been of recent interest in animal-communication research. Three key hypotheses have been suggested: multiple ornaments can function as (1) multiple messages (each ornament provides different information), (2) redundant messages or back-up signals (all ornaments provide the same information), and (3) unreliable signals (no information). Here we tested the signal properties of ornamental traits of male House Sparrows (Passer domesticus) to better resolve the potential information different ornaments reveal. We repeatedly measured badge size, wingbar area and color, bill color, and leg color in 170 captive males over 3 years. We found that (1) badge size and wingbar area were related to condition (to mass, tarsus length, and the interaction of both), (2) badge size, wingbar area, and bill color were related to testosterone levels, (3) all ornaments except bill color were related to age, and (4) origin had only minor effects. We conclude that badge size, wingbar area, and bill color may function as multiple messages of different testosterone-related behaviors and that badge size, wingbar area, and leg color have the potential to function as back-up signals of age. However, the empirical categorization of ornaments as either multiple messages or back-up signals is not clear, and we suggest instead it is more informative to view multiple ornaments as providers of varying degrees of overlapping types of information.