Stable isotopes of hydrogen have been used as a tool to determine migratory connectivity, or to link breeding and wintering grounds. Because isotopes serve as intrinsic markers, eliminating the need for birds to be relocated after an initial marking, they hold the potential to be an extremely useful tool. From 1998 to 2003 we gathered feathers from juvenile Merlins (Falco columbarius) and Northern Harriers (Circus cyaneus) during their fall migration in the Florida Keys and analyzed them isotopically in an attempt to determine their natal origins. Our results failed to reveal the natal origins of at least one of these two birds of prey.

Conservation-reliant species require continuing management to ensure their long-term persistence. We qualitatively assessed the extent of conservation reliance for 92 California bird taxa listed under federal or California endangered species acts or recognized as California bird species of special concern. Habitat loss and fragmentation are the major threats for over 90% of these taxa, whereas interactions with predators or brood parasites threaten less than half, and human actions imperil roughly 40%. Some form of habitat enhancement is proposed to reduce the threats for most taxa, reinforcing the value of habitat-conservation strategies. Protecting habitat for wetland taxa and restoring habitat for island taxa appear to be particularly costly actions. Importantly, the species of special concern are every bit as conservation reliant as are taxa listed as endangered or threatened; management of these yet-unlisted taxa may be especially effective in preventing them from slipping into a more precarious status. Consideration of the magnitude of threats together with the degree of conservation reliance may help in prioritizing taxa for conservation. The philosophy and practice of conservation and resource management must recognize that continuing actions will be required to maintain the viability of populations of a great many species.

Using mark-recapture, we estimated the age at first return and survival of juveniles of the Sooty Shearwater (Puffinus griseus), a medium-sized burrow-breeding procellariiform, at two New Zealand breeding colonies, Taiaroa Head and The Snares. Sooty Shearwaters were first observed at the colony between 2 and 10 years after being banded as chicks, with a mean of 5.7 years at Taiaroa Head and 5.4 years at The Snares. After accounting for imperfect detectability with a mark-recapture model, we estimate the mean age at first return as 4.8 years. Extrapolation from observed delays between arrival and breeding of the Short-tailed Shearwater (P. tenuirostris) suggests that the mean age at first breeding was 7.7 years. Annualized estimates of survival from 0 to 2 years were 0.54 at Taiaroa Head and 0.41 at The Snares. Overall estimates of the generation time and maximum rate of population growth were 13.1 years and 7.7%, respectively. Many published estimates of age at first return and age at first breeding are biased because they implicitly assume perfect detectability of individuals. Such an assumption in turn leads to bias in estimation of generation time and maximum rate of population growth. A generic population model for the Procellariiformes, in which the parameters are scaled against generation time, could prove to be a useful tool in guiding management of bycatch and sustainable harvest.

From 1997 through 2010, in collaboration with the National Park Service, we released 84 captivereared California Condors (Gymnogyps californianus) to the wild in central California; from 2006 through 2010 we recorded 16 nestings by nine pairs and recovered eggs or eggshell fragments from 12 nests. Mean thickness of shell fragments, without membrane, was 0.46 mm, 34% lower than the average thickness of 0.70 mm of fragments recovered from nine successful nests in interior southern California, 2007–2009. Hatching success in central California was 20–40%, significantly lower than the 70–80% recorded in southern California. The outer crystalline layer was absent or greatly reduced, as in thin-shelled condor eggs laid in southern California in the 1960s. Shell thickness was not related to egg size. Weight/water loss during incubation in the wild averaged three times greater than the normal rate associated with successful hatching; the rate of loss increased significantly with decreasing shell thickness. At least four failures, three from death of the embryo, we attribute to excessive weight/water loss; two other eggs losing substantial weight hatched successfully after artificial incubation at elevated humidities. DDT/DDE from wastes of a DDT factory discharged into the Southern California Bight had previously caused extensive eggshell thinning and reproductive failures of fish-eating and raptorial birds. Feeding on carcasses of California sea lions (Zalophus californianus), reintroduced condors now occupy a higher level of the food web. Like that of other species previously affected, the thickness of condor eggshells should recover as DDE contamination continues to decline.

Stable-isotope ratios of carbon (¹³C/¹²C; δ¹³C) and nitrogen (¹⁵N/¹⁴N; δ¹⁵N) in animal tissues are analyzed to estimate animal foraging ecology because these ratios reflect those of an animal's diet. This reflection is generally indirect, as stable-isotope ratios change with trophic level. These differences, called discrimination factors (reported as Δ), vary considerably by species and tissue. Variations in discrimination factors used in stableisotope mixing models can lead to inaccurate estimates of diets. Therefore, determining accurate discrimination factors specific to species and tissue is important. We established the Δ¹³C and Δ¹⁵N values between diet and blood and feathers from chicks and juveniles of the California Condor (Gymnogyps californianus). Hatchlings were fed rats for 76–119 days, whereas juveniles were fed dairy calves for 64 days. The mean Δ¹³C and Δ¹⁵N values (± SD) between chick feathers and rat muscle were 0.4 ± 0.4‰ and 3.1 ± 0.2‰, respectively; those between chicks' whole blood and rat muscle were -0.7 ± 0.1‰ and 1.7 ± 0.1‰, respectively. The mean Δ¹³C and Δ¹⁵N values between juvenile condors' plasma and calf muscle were 0.9 ± 0.2‰ and 3.3 ± 0.7‰, respectively; those between juveniles' red blood cells and calf muscle were 0.3 ± 0.3‰ and 1.8 ± 0.1‰, respectively; and those between juveniles' whole blood and calf muscle were 0.3 ± 0.3‰ and 1.9 ± 0.2‰, respectively. We report the first discrimination factors for the Cathartidae (New World vultures), and our findings will have important applications in studies of the critically endangered California Condor's foraging ecology.

Sex-related and individually unique vocal features have been demonstrated for many avian and mammalian species. Vocal identity may depend on a call's function and vary within the repertoire of a single species. Vocal features of different call types are very rarely compared in one species. We studied the potential for vocal recognition of sex and individual identity in four types of calls of the White-naped Crane (Grus vipio). We analyzed growls and contact calls (short-range communication) and flight-intention and guard calls (long-range communication). Sexual features were strongly expressed in flight-intention and guard calls; furthermore, the guard call allows reliable sexing of a bird. The sexual distinctiveness of short-range calls was less pronounced. We failed to find significant individual features in any call type. The potential for determining sex from vocalizations may be useful for the monitoring of this endangered species.

The Mountain Plover (Charadrius montanus) has an uncommon parental care system in which males and females tend separate nests. To improve their fitness in this system, females have the opportunity to allocate their reproductive investment to male-tended nests and their own nests differently. To examine investment we measured dimensions of plover eggs in Phillips County, Montana, and calculated their volumes. We modeled possible differences in egg volume in male- and female-incubated nests in relation to the effects of sex of the incubating adult, Julian day of nest initiation, and drought conditions. We measured >1000 eggs from 194 nests tended by 131 females and from 213 nests of 148 males. Male- and female-incubated eggs had similar mean volumes (13.20 versus 13.17 cm³, respectively) but differed significantly across the breeding season. The eggs in female-incubated nests tended to be larger than those in male-incubated nests early in the breeding season but were smaller as the season progressed, while the volume of male-incubated eggs peaked in the middle of the season. Egg volumes were affected by drought conditions, being larger during the driest periods of this study. Volumes within a nest were similar and were not influenced by the age of the incubating adult. Larger eggs tended to produce larger chicks. The similarity in the size of Mountain Plover eggs, even between male- and female-incubated nests and under different environmental conditions, provides evidence for stability of this uncommon system of parental care.

On the western coast of North America, several estuaries provide shorebirds with important winter and stopover habitat. These habitats include not only aquatic estuarine resources but also adjacent upland agricultural lands. The extent to which shorebirds use estuarine vs. upland habitats at these stopover sites is difficult to quantify but crucial to designing strategies for their conservation. We measured stable isotopes (δ¹³C, δ¹⁵N) in whole blood of Dunlins (Calidris alpina) and their prey from two major estuaries in north Puget Sound, Washington, USA, to estimate their relative use of estuarine vs. upland agricultural zones. We identified four isotopically distinct dietary inputs (agriculture high in ¹⁵N, other agriculture, marsh/marine, and freshwater plume). Isotopic sampling and modeling was informed by movements and habitat use derived from radiotelemetry. This isotopic structure allowed us to conclude that these Dunlins obtained about 62% of the protein in their diet from agricultural lands and 38% from the estuary. Our results underline the urgent need to combine management of estuaries and upland agricultural areas in strategies for shorebird conservation.

The Dunlin (Calidris alpina) is a polytypic shorebird with complex patterns of distribution and migration throughout its holarctic range. We analyzed mark-re sighting data obtained between 1977 and 2010 from birds captured at two major staging areas in western Alaska to test the hypothesis that the migration patterns of Alaskan populations are a mixture of parallel and chain, similar to those of Dunlin populations in the western Palearctic. Birds marked on the Yukon—Kuskokwim Delta were found wintering in both Asia and North America, which documented the unexpected mixing of C. a. arcticola from northern Alaska and C. a. pacifica from western Alaska and contradicted our initial prediction of parallel migration pathways for these two subspecies. In its North American winter range C. a. pacifica segregated according to location of marking, confirming our prediction of a chain migration pattern within this population. Individuals of C. a. pacifica marked on the delta were resighted significantly farther north, mostly in southern British Columbia and Washington, than birds marked on the second, more southerly staging area on the Alaska Peninsula, which were resighted primarily in the San Francisco Bay area of northern California. We recommend additional studies use a combination of intrinsic and extrinsic markers to quantify the strength of migratory connectivity between breeding, staging, and wintering areas. Such information is needed to guide conservation efforts because the Dunlin and other waterbirds are losing intertidal habitats at an unprecedented rate and scale, particularly in the Yellow Sea and other parts of Asia.

The basic assumption in the morphology of avian migrants is that longer, more pointed and concave wings are well adapted for long-distance migratory flights. We investigated how wingtip morphology of the Common Snipe was associated with aspects of its migration, such as amount of accumulated fat, refueling rates, length of stay at the stopover site, and timing of post-juvenile molt. Our study took place at the Jeziorsko reservoir, central Poland, where over 400 first-year Common Snipe (Gallinago gallinago) were caught in 2008 and 2009. Wing shape was found to correlate with different components of the species' stopover strategies. After controlling for the date of capture we found that first-year birds with wings adapted for efficient migration (pointed/concave wingtips) refueled more quickly and stayed for shorter times at the stopover site. Pointed/concave wing shape was also associated with accumulation of smaller fat reserves and later initiation of post-juvenile molt during the period of migration. By contrast, snipe having a wing shape adapted for predator evasion (rounded/convex wingtips) refueled more slowly, remained longer, and accumulated greater amounts of fat, which in turn facilitated earlier molting.

In long-lived animals, survival rate is a key demographic parameter affecting population dynamics. The objective of this study was to estimate the apparent survival rate of adult Long-tailed Jaegers (Stercorarius longicaudus) breeding at one of the most northerly sites on earth, northern Ellesmere Island, Canada. Because of the large temporal fluctuations in numbers of lemmings, their primary food source during the summer, we also investigated the effect of annual variations in lemming abundance on survival rate. Analysis was based on 336 individuals marked as adults over 11 years with metal and color bands and capture—mark—recapture techniques. There was strong support for a model with two age classes, as survival of newly marked birds was 18% lower than that of previously marked ones. This difference could be due to a true age effect (if a high proportion of young adults was present in our initial samples) or, perhaps more likely, to the presence of transient individuals in the population. The estimated probability of apparent annual survival of local birds (corrected for color-band loss) was 0.91, comparable to values for other seabirds, and was constant over time. We detected a weak trend for a decrease in apparent survival rate of newly marked birds in years of high lemming abundance but no effect on the survival of previously marked birds. This suggests that conditions at sea during the nonbreeding season may be more important in affecting annual survival. A new longevity record of 22 years was established for the species.

From 2004 to 2006 we studied the spatial activity of Long-eared Owls (Asio otus) breeding in the city of České Budějovice, Czech Republic. Two main factors (size and proportion of home range in developed area) clearly distinguished the owls into two groups: urban (n = 5) and suburban (n = 4). Urban owls occupied home ranges >50% in developed area; home ranges of suburban owls were <50% in developed area. We obtained 866 radio fixes from nine individuals while they were feeding offspring, when both males and females participated in hunting. The home ranges of urban owls were larger than those of suburban owls (95% kernels: 446 vs. 56 ha, respectively). Urban males and females used home ranges of similar sizes that overlapped greatly with those of the opposite sex and with those of other breeding pairs. In contrast, suburban male owls had home ranges larger than those of suburban females. Wooded areas and meadows were the vegetation types preferred by both urban and suburban owls, and both groups avoided vegetation types associated with human activities. Six of nine owls preferred the edges of open habitats, such as meadows and fields bordering wooded areas. Urban owls used remnant green spaces (refugia) within the city area to hunt small mammals. Long-eared Owls use nests made by other species, and may profit in urban environments from increased nest sites due to the relatively recent colonization of urban areas by the Common Magpie (Picapica) and from suitable hunting grounds within the city.

We describe the songs and courtship displays of three closely related hummingbirds, the Peruvian Sheartail (Thaumastura cora), Oasis Hummingbird (Rhodopis vesper), and the endangered Chilean Woodstar (Eulidia yarrellii). The Peruvian Sheartail and Oasis Hummingbird sing complex multisyllabic songs, while the Chilean Woodstar's song is monosyllabic and simple. Like North American “bee” hummingbirds (within the tribe Mellisugini), the Chilean Woodstar and Oasis Hummingbird perform stereotypical close-range shuttle displays and larger display dives. The Peruvian Sheartail also performs shuttle and dive displays but their kinematics are variable and they are less distinct from each other. The greatly elongated tail of the Peruvian Sheartail is split widely during display, such that the five rectrices (R1–R5), including R1 and R2 of each side of the tail, project laterally, apparently as a visual signal. Like most other “bee” hummingbirds, all three species produce sounds during their display dives with their three outer rectrices, R3, R4, and/or R5, although the Peruvian Sheartail's sound is both faint and not produced exclusively during displays. We diagnose an adult male hybrid Peruvian Sheartail × Chilean Woodstar and describe its songs and displays. The hybrid had unique tail morphology with an elongated R2, R3, and R4, and both R1 and R4 were transgressive, longer than those of either parent species. The hybrid was from the Azapa valley, where the Peruvian Sheartail is now common and the Chilean Woodstar rare; hybridization may represent an Allee effect arising from the recent crash of the Chilean Woodstar's population.

Founder effects, new environments, and competition often produce changes in species colonizing islands, although the resulting endemism sometimes requires molecular identification. One method to identify fruitful areas for more detailed genetic study is through comparative morphological analyses. We measured 210 museum specimens to evaluate the potential morphological consequences of colonization across the Philippine archipelago by the Philippine Falconet (Microhierax erythrogenys). Measurements of both males and females differed clearly from island to island. Univariate and multivariate analysis of characteristics showed a latitudinal gradient, with the bill, wing, and tail of southern birds being larger than those of northern birds, forming the pattern of a stepped cline across a succession of islands. The stepped gradient in morphology and extensive differences between islands we observed provide evidence for multiple perspectives on phylogeny, including concordance with aggregate complexes expected on the basis of sea-level fluctuations. However, calculation of diagnosability indices did not support subspecific designations. Sex-specific dominance and dispersal patterns may explain this unusual south-to-north stepped cline, and they also provide a useful format for understanding biogeographical patterns by island. Finally, these morphological data suggest a potentially fruitful area for future genetic studies.

Variation in recruitment patterns and dispersal behavior can have important consequences for population viability, genetic structure, and rates of evolutionary change. From 1992 to 2006 we studied a marked population of American Kestrels (Falco sparverius) nesting in boxes in southwestern Idaho to identify factors that affect local recruitment and natal dispersal distances. A low proportion (4%) of locally produced kestrels (n = 2180) returned to nest in study area boxes. Offspring of locally produced individuals were 3.1 times more likely to return than offspring of parents that did not hatch in study area boxes and, independent of a parent's origin, males were 1.8 times more likely than females to return. Kestrels that hatched earlier in the breeding season and those that hatched immediately prior to a mild winter were more likely to return. Local natal dispersal distances were best explained by sex and parental origin but not by hatching dates. Fifty-four males moved an average of 5.3 km from their natal box to the location of their first breeding in the study area, and 27 females moved an average of 9.8 km. Offspring of locally produced parents dispersed shorter distances within the study area than offspring of other parents, and local natal dispersal distances of locally produced parents correlated with those of their same-sex offspring. Patterns of natal dispersal of American Kestrels in southwestern Idaho appear to be driven by a combination of parental dispersal tendencies and ecological factors. The population consists of a mix of immigrants and philopatric birds.

I examined over 4500 specimens to investigate the evolutionary significance of molt sequences as related to systematic relationships of falcons and parrots. Nodes of molt among the medial primaries (p4–p6) and at s5 among the secondaries, followed by bidirectional replacement from the node within each tract, was confirmed for most or all taxonomic groupings of both falcons and parrots, with the exception of the Kakapo or Owl Parrot (Strigops habroptila), which appears to replace its remiges in sequences similar to those of most other birds. Initiation of primary molt varies between p4 and p5 in falcons and between p5 and p6 in parrots, including within species, suggesting that the node's position may be fluid within a defined area along the alar tract. The coincidence and consistency of these molt sequences in parrots and falcons but in no other order of birds suggests that molt sequence may be a synapomorphic (shared) character state, supporting recent molecular evidence that the Falconiformes and Psittaciformes are sister taxa. A more ancestral molt sequence in Strigops may suggest that it split from other parrots prior to a Falconiformes—Psittaciformes divergence or it may indicate reversion to a primitive character state in response to the species' unique nocturnal and ground-dwelling habits. The results of this analysis further suggest that synapomorphic wing-molt sequences can be used as an indicator of systematic relationships in birds and that molt sequence is perhaps controlled by a neurological process more fixed than timing, extent, and geographic location of molts.

We investigated the reproductive ecology of the Island Scrub-Jay (Aphelocoma insularis), which is endemic to Santa Cruz Island, California, and is a species of conservation concern. We documented daily nest survival, probability of renesting, annual fledging success, and breeding behavior, and determined how these factors varied in relation to the habitat characteristics of the jays' home ranges. Clutch sizes, feeding rates, and nest attendance were not correlated with chaparral cover or with home-range size. In contrast, nest predation, which accounted for 92% of documented nest failures, was less likely at nests that were more concealed, at nests of breeding pairs with home ranges that were smaller and had a greater proportion of taller vegetation, and at nests that were initiated earlier in the breeding season. Researchers' activity near nests increased the likelihood of predation, although the effect diminished with a nest's age. Despite relatively low nest survival, the high frequency of renesting allowed pairs to fledge an average of 1.1 ± 0.1 young per year, or 1.8 ± 0.2 young after the effects of the observer were accounted for. Currently, chaparral habitats on Santa Cruz Island are regenerating following the removal of exotic herbivores. This regeneration could in turn increase and improve breeding habitat for this island endemic.

In western conifer-dominated forests where the abundance of old-growth stands is decreasing, species such as the Brown Creeper (Certhia americana) may be useful as indicator species for monitoring the health of old-growth systems because they are strongly associated with habitat characteristics associated with old growth and are especially sensitive to forest management. Light detection and ranging (lidar) is useful for acquiring fine-resolution, three-dimensional data on vegetation structure across broad areas. We evaluated Brown Creeper occupancy of forested landscapes by using lidar-derived canopy metrics in two coniferous forests in Idaho. Density of the upper canopy was the most important variable for predicting Brown Creeper occupancy, although mean height and height variability were also included in the top models. The upper canopy was twice as dense and the mean height was almost 50% higher at occupied than at unoccupied sites. Previous studies have found indicators of canopy density to be important factors for Brown Creeper habitat; however, this represents the first time that lidar data have been used to examine this relationship empirically through the mapping of the upper canopy density that cannot be continuously quantified by field-based methods or passive remote sensing. Our model's performance was classified as “good” by multiple criteria. We were able to map probabilities of Brown Creeper occupancy in ∼50 000 ha of forest, probabilities that can be used at the local, forest-stand, and landscape scales, and illustrate the potential utility of lidar-derived data for studies of avian distributions in forested landscapes.

The preferences of frugivorous birds for certain colors have been proposed to explain the evolution of fruit color, but evidence supporting this hypothesis is weak. Furthermore, evidence from tropical Asia is lacking, although in tropical Asia birds are more important seed dispersers than are their counterparts in other regions. We experimented with artificial fruit of five colors to evaluate the color preferences of six bird species (33 individuals, 2 families) from tropical Asia. We also checked whether contrast affects color detection by displaying red and green artificial fruits against green and red backgrounds. We found that five species had a consistent and strong preference for red, and one species preferred blue, with a low degree of variation among individuals. Species that preferred a certain color also consumed a small portion of fruit of different colors. Contrast increased the rate of consumption of green and red fruit in comparison with that against a monochrome background. Because all the species in this experiment showed a strong preference for a fruit color, we conclude that the interaction between frugivorous birds and fruit color may be tighter in tropical Asia than in other regions. Although contrast mediated the rate of consumption of red and green fruit to some degree, red was still the most preferred and green the least consumed.

The Robsonius ground-warblers are forest birds endemic to the Luzon Island complex in the Philippine archipelago. Their systematic relationships have long remained ambiguous; until recently they were included in the timaliid genus Napothera. Two Robsonius species are currently recognized on the basis of plumage differences: R. rabori from northern Luzon in the Cordillera Central and the northern Sierra Madre, and R. sorsogonensis from southern Luzon and Catanduanes Island. Recent specimen collections, including the first adult specimen from the Cordillera Central, establish plumage differences between populations of R. rabori in the Cordillera Central and Sierra Madre and reveal a third diagnosable population within Luzon. These differences have gone unnoticed because R. rabori (sensu stricto) had been known only from the juvenile holotype. Molecular phylogenetic data further support the hypothesis that three highly divergent taxa occur across the Luzon Island complex: Robsonius rabori is known only from the northern Cordillera Central in Ilocos Norte; an undescribed taxon (formerly included in R. rabori) occurs in the northern Sierra Madre in Cagayan, Isabela, Aurora, and Nueva Vizcaya provinces; and R. sorsogonensis occurs in southern Luzon (Bulacan and Laguna provinces), the Bicol Peninsula, and on Catanduanes Island. The existence of three putatively allopatric species within the Luzon island complex highlights the role of in situ diversification in island systems, and brings attention to the need for forest conservation to protect geographically restricted populations throughout the Luzon Island complex.

Migratory birds are expected to experience a trade-off between reproductive effort, the timing and pace of molt, and initiation of fall migration. The purpose of this study was to investigate if males and females of a songbird that migrates to the neotropics, the Wood Thrush (Hylocichla mustelina), experience a trade-off between parental care and the timing of molt. We determined the relative contribution of males and females in feeding young, assessed how reproductive effort influenced timing of molt, and tested the prediction that the sex provisioning nestlings at the higher rate should molt later. Males fed significantly more than females throughout the breeding period, and males with higher feeding rates had greater nesting success. Males compensated for females' reduced provisioning rate at late nests, and females initiated molt earlier than males. We found a significant negative relationship between the timing of molt and the number of young fledged per season. Males that began molt earlier than females and late-molting males also increased their pace of molt. For males, the cost of reduced parental care is likely higher than the benefit of earlier molt, but delayed onset of molt is partially mitigated via a faster pace of feather growth.

Large-scale disturbances, by affecting habitat suitability, can influence bird populations and communities. Although we know that disturbances can lead to population decreases and shifts in community structure, our understanding of finer-scaled mechanisms leading to these patterns is limited. We focused on why some territories remain occupied by a species following a large-scale disturbance, while others are abandoned. The Swainson's Warbler (Limnothlypis swainsonii) is especially susceptible to flooding because of its preference for nesting in thick understory vegetation and foraging in leaf litter. In 2008, a major flood inundated a study site where we have studied Swainson's Warblers since 2004. In 2008 and 2009, we resurveyed 42 Swainson's Warbler territories that were occupied between 2005 and 2007 and examined changes in habitat structure and composition and prey abundance influencing territory abandonment. Occupancy declined between the preflood and postflood surveys. Territory abandonment was associated with decreases in leaf litter cover, increases in cover of green vegetation, and decreases in density of total understory stems and giant cane (Arundinaria gigantea) between the preflood and flood periods. Despite the partial recovery of habitat structure and prey abundance after the flood, Swainson's Warbler occupancy did not recover in subsequent years. Multiple habitat variables, including leaflitter cover, density of woody stems, understory density, and prey abundance have been identified as important for Swainson's Warblers, and these results suggest that leaf litter may be especially important in influencing habitat-selection decisions following flood disturbance.

Considered endangered by the U.S. Fish and Wildlife Service, the Golden-cheeked Warbler (Setophaga chrysoparia) breeds exclusively in the juniper-oak (Juniperus ashei-Quercus spp.) woodlands of central Texas. Large-scale, spatially explicit models that predict population density as a function of habitat and landscape variables can provide important insight for its management and recovery. We used distance sampling to model detection probability and to estimate the density and abundance of singing male Golden-cheeked Warblers on Fort Hood Military Reservation. We used an information-theoretic approach to evaluate hypotheses concerning the effects of proportion of forest type and forest cover, forest-edge density, and patch size on density. We fitted generalized linear models with detection probability as an offset term to predict density as a function of the habitat and landscape variables, calculate a model-based density and abundance estimate, and map density across the area sampled. The design-based estimates were 0.39 males ha^-1 and 7557 singing males. The most supported model contained proportion of forest type and forest cover, both of which had a positive effect on density, as well as forest-edge density, which had a negative effect. The model-based estimates of 0.39 males ha^-1 and 7571 singing males were greater than estimates extrapolated from intensive territory monitoring. Knowledge of factors affecting Golden-cheeked Warbler density can be used to inform recovery efforts, and our density model can be used to assess the effects of various activities proposed for military training and of environmental disturbance on warbler densities.

Researchers often attach radio transmitters and other devices to free-living birds without a clear understanding of the possible consequences for their study organisms or their data. Although transmitters may affect parental investment (nest defense and offspring provisioning), this possibility has received little attention. We tested this hypothesis by placing mock radio transmitters on male Northern Cardinals (Cardinalis cardinalis) and comparing their behavior to that of uncaptured birds and procedural controls. Birds with transmitters defended their nests less vigorously than did uncaptured birds but did not modify their provisioning effort. This behavioral modification appears to have ultimately influenced predation rates, as nests of birds with transmitters had lower daily survival rates and were less likely to fledge offspring. Control birds that were captured, handled, and bled had intermediate levels of nest defense and productivity that were statistically indistinguishable from those of birds receiving other treatments, suggesting that capture, restraint, and blood collection may affect birds in ways that are independent of transmitters' effects. Interestingly, we also found limited evidence that females mated to males with transmitters increased their provisioning effort, possibly in compensation for a perceived reduction in their mate's care. Because attachment of a transmitter (and potentially blood sampling) directly affected the behavior and reproduction of birds with transmitters and may have indirectly affected the behavior of their mates, we suggest researchers cautiously balance the benefits of such methods against potential data biases and impairment of reproduction.

Temporal stability in the acoustic structure of dialect songs is a critical variable for studies of cultural evolution in songbirds. Here we report that the structure of the flight whistle song of Brown-headed Cowbirds (Molothrus ater) of the Convict dialect in the eastern Sierra Nevada of California remained essentially stable over almost 30 years. Quantitative measurements showed a minor structural change in one of four syllables and a small but statistically significant increase in duration for whistles recorded in 2006 versus 1990. Audiospectrograms of whistles recorded in the late 1970s revealed no major structural differences from the 1990 and 2006 whistles. These results are consistent with field and ontogenetic studies showing that young cowbirds copy the songs of adult males with very high fidelity, although it may take them up to two years to do so. We contrast the Convict results with those from the adjacent Mammoth dialect, in which the local whistle changed substantially over approximately the same period covered by the current study (O'Loghlen et al. 2011) and discuss a possible explanation for the divergent results based on ecological differences between the two dialects. As far as we are aware, this is the first report of such contrasting results for acoustic stability in the songs of contiguous dialect populations. The stability shown by the Convict whistle is remarkable given the extreme structural variation among whistles from other dialect populations in the area and evidence of dispersal and gene flow among these local populations.