Counts of avian fledglings, nestlings, or clutch size that are bounded below by zero and above by some small integer form a discrete random variable distribution that is not approximated well by conventional parametric count distributions such as the Poisson or negative binomial. We developed a logistic quantile regression model to provide estimates of the empirical conditional distribution of a bounded discrete random variable. The logistic quantile regression model requires that counts are randomly jittered to a continuous random variable, logit transformed to bound them between specified lower and upper values, then estimated in conventional linear quantile regression, repeating the 3 steps and averaging estimates. Back-transformation to the original discrete scale relies on the fact that quantiles are equivariant to monotonic transformations. We demonstrate this statistical procedure by modeling 20 years of California Spotted Owl fledgling production (0−3 per territory) on the Lassen National Forest, California, USA, as related to climate, demographic, and landscape habitat characteristics at territories. Spotted Owl fledgling counts increased nonlinearly with decreasing precipitation in the early nesting period, in the winter prior to nesting, and in the prior growing season; with increasing minimum temperatures in the early nesting period; with adult compared to subadult parents; when there was no fledgling production in the prior year; and when percentage of the landscape surrounding nesting sites (202 ha) with trees ≥25 m height increased. Changes in production were primarily driven by changes in the proportion of territories with 2 or 3 fledglings. Average variances of the discrete cumulative distributions of the estimated fledgling counts indicated that temporal changes in climate and parent age class explained 18% of the annual variance in owl fledgling production, which was 34% of the total variance. Prior fledgling production explained as much of the variance in the fledgling counts as climate, parent age class, and landscape habitat predictors. Our logistic quantile regression model can be used for any discrete response variables with fixed upper and lower bounds.

Birds that nest in tree cavities, despite being relatively safe, are still threatened by predators, which not only can destroy broods but also can kill the parents. Therefore, using safe cavities is crucial for the survival of parents and nestlings. Safe cavities should simultaneously have the smallest entrance (yet large enough to admit the cavity occupant), walls made of hard wood, and depth sufficient for the nest to be placed beyond the reach of predators. I studied nests of Marsh Tits (Poecile palustris) that were attacked by predators and compared cavity features of nests that survived the attack with those that did not. Observations were gathered in pristine conditions (Bial̸owieża National Park, Poland) over 30 yr. There was no direct human interference, and cavities were superabundant. I observed 169 attacks on Marsh Tit broods by a diverse set of predators. Nests survived ∼30% of these attacks. Only 10% survived when a predator entered the cavity, but 29% survived if the predator attempted to pluck the nest from the cavity and 39% survived if the predator tried to remove the cavity wall. The probability of surviving an attack depended on predator identity and cavity attributes; the smallest predators passed through even the narrowest entrances, but small entrances combined with walls of sound wood were effective in stopping larger predators. Nests situated closer to the cavity entrance were less likely to survive attack than nests farther away, but nests were seldom placed beyond the predators' reach, which suggests the existence of some constraints. The probability of surviving an attack strongly increased with the nestlings' age; larger young survived attacks more often, demonstrating that, even in cavity-nesting birds, early development of locomotor capacity might be at premium. I discuss constraints on antipredator adaptations in cavity structure and trade-offs between opposing requirements.

We conducted a comparative study of 2 closely related and sympatric species, Northern Bobwhite (Colinus virginianus) and Scaled Quail (Callipepla squamata), to explore the drivers of interspecific variation in alternative reproductive strategies. Specifically, we used parentage analyses to determine rates of extrapair paternity and intraspecific brood parasitism for each species, and evaluated possible mechanisms for explaining variation in these rates between the 2 species. During 2013–2014, we genotyped hatched (n = 388 for Northern Bobwhite and n = 214 for Scaled Quail) and unhatched (n = 32 for Northern Bobwhite and n = 9 for Scaled Quail) eggshells from 34 Northern Bobwhite and 22 Scaled Quail nests in western Oklahoma, USA. Extrapair paternity occurred in 85% of Northern Bobwhite nests and 9% of Scaled Quail nests. The number of sires for each nest was greater in Northern Bobwhite nests (x̄ = 2.29 ± 0.14 SE sires per nest) than in Scaled Quail nests (x̄ = 1.09 ± 0.06 sires per nest). Intraspecific brood parasitism rates varied considerably between Northern Bobwhite (21%) and Scaled Quail (0%). Northern Bobwhites also parasitized Scaled Quail nests. Variation in extrapair paternity and intraspecific brood parasitism rates between the Northern Bobwhite and Scaled Quail appeared to be related to differences in their life history strategies (r-selected vs. K-selected strategy) as well as differences in breeding population densities.

In birds, the preen gland produces a waxy secretion that protects the plumage from environmental conditions and ectoparasites and is, therefore, important for feather maintenance, individual quality, and health. Several studies have demonstrated that the size of the preen gland and the composition of its secretion change throughout the course of reproduction, thus indicating an increased requirement for preen gland secretion during the breeding season. However, the exact timing of the gland's size increase is unknown. To get information about this temporal pattern, we repeatedly measured the preen glands in 22 breeding pairs of adult domesticated Zebra Finches (Taeniopygia guttata) during the course of reproduction (prior to the breeding season, on the day the first egg was laid, on the day the first chick hatched, on the day the first chick fledged, and after the chicks became independent). Zebra Finches have biparental care, in which both sexes are involved in incubation and food provisioning. As expected, our data reveal that the preen gland increases in size during the breeding season and decreases afterward. Moreover, we found sex-specific differences in the temporal pattern of gland enlargement. Females reached a maximum gland size when the first chick hatched, whereas male gland size peaked earlier, when the first egg was laid.

Birds are physiologically polyspermic, with normal embryonic development following penetration of the inner perivitelline layer (PVL) of the ovum by multiple sperm. The PVL traps dozens to thousands of the sperm present at the time of fertilization, providing information about the number of sperm that traversed the females' reproductive tract and reached the ovum. Broadly, across avian species, the number of sperm detected on the PVL is positively related to ovum size and female body mass. However, relatively few studies have characterized the amount of variation that occurs within and between closely related species that are similar in size. We characterized variation in the average number of sperm trapped by the PVL, across species and between and within breeding pairs, in 3 similar-sized species of estrildid finch. The average number of PVL sperm changed significantly across the laying order in 2 of the species, and there was significant variation between breeding pairs in all 3 species. The variation in PVL sperm number was not always consistent within a pair across multiple breeding attempts (2 species examined). Our data highlight the need to better understand the patterns and processes of selection that optimize and alter the number of sperm reaching the ovum within and across species.

The breeding seasons of Nearctic–Neotropical migratory songbirds are thought to be constrained by the temporal demands of subsequent seasons. In any population, a subset of adults may fail to fledge young despite repeated attempts, which may prolong reproductive effort. We used geolocators to track the single-brooded Veery (Catharus fuscescens) from a Delaware, USA, breeding site to South America and back, to examine how reproductive failure and the timing of reproduction affected the spatiotemporal aspects of the nonbreeding season. We also considered the effect of additional variables: age, sex, time in population (i.e. experience), productivity, and nutrition via ptilochronology. We found that clutch completion date and reproductive success were significant predictors of South American entry and arrival date at first winter sites, respectively. Reproductive success also influenced the latitude of winter sites. For females only, there was a consequence of reproductive failure: unsuccessful females entered South America later, and arrived at first winter sites later, than successful females. Although there was a trend for unsuccessful females to exit South America earlier the following spring, they did not arrive in Delaware earlier, nor were they more reproductively successful the following year. We found an effect of age and sex on the timing of South American exit. Females differed from males in the timing of fall migration in South America, the timing of South American exit, and the timing of arrival back in Delaware. Our finding that arrival and settlement of adult Veeries in South America is primarily determined by former reproductive outcomes, rather than age, sex, or life history experience, is consistent with the contention that the breeding season of single-brooded species is constrained by subsequent seasons. Our results have implications for conservation, considering that the effect of reproductive failure goes beyond reduced fecundity.

Geographically isolated populations of birds often differ in song. Because birds often choose mates on the basis of their song, song differentiation between isolated populations constitutes a behavioral barrier to reproduction. If this barrier is judged to be sufficiently strong, then isolated populations with divergent songs may merit classification as distinct species under the biological species concept. We used a dataset of 72 pairs of related but allopatric Neotropical passerines (“taxon pairs”) to compare 2 methods for measuring song divergence between isolated populations: statistical analysis of 7 acoustic traits measured from spectrograms, and field playback experiments that “ask the birds themselves” if they perceive foreign song as conspecific or not. We report 4 main findings: (1) Behavioral discrimination (defined as failure to approach the speaker in response to allopatric song) is nonlinearly related to divergence in acoustic traits; discrimination is variable at low to moderate levels of acoustic divergence, but nearly uniformly high at high levels. (2) The same nonlinear relationship held for both song learners (oscines) and nonlearners (suboscines). (3) Song discrimination is not greater in taxon pairs ranked as species compared to taxon pairs ranked as subspecies. (4) Behavioral responses to allopatric song are symmetric within a taxon pair. We conclude (1) that playback experiments provide a stronger measure of species recognition relevant to premating reproductive isolation than do acoustic trait analyses, at least when divergence in acoustic traits is low to moderate; and (2) that playback experiments are useful for defining species limits and can help address the latitudinal gradient in taxonomy, which arises because species are defined more broadly in the tropics than in the temperate zone. To this end, we suggest that 21 Neotropical taxon pairs that are currently ranked as subspecies, but that show strong behavioral discrimination in response to allopatric song, merit classification as distinct biological species.

Climate change can affect the distribution, abundance, and phenology of organisms globally. Variations in the timing of passage during autumn and spring migration can have consequences at individual and population levels. We assessed whether global climatic indexes and increasing air temperature over a 28 yr period were concurrent with shifts in the autumn migration phenology of 16 eastern North American raptor species. We used count data from 7 eastern North American raptor-migration watch sites and examined whether key species-specific traits such as migration strategy (complete vs. partial and trans-equatorial vs. not), diet specialization, body mass, flight strategy (soaring vs. flapping), and latitude of the northern limit of breeding distribution were associated with a shift in the timing of autumn migration. Our results suggest an overall delay across species in autumn migration passage date of ∼1 day decade⁻¹, which coincided with an increase in temperature across eastern North America. This shift in average autumn passage date was more pronounced in short-distance migrants ( 1.03 days decade⁻¹); no shift was detected in trans-equatorial migrants. Although we did not detect clear links between annual climatic indexes and the other life-history traits studied, the results nonetheless indicate that the autumn migration phenology of eastern North American raptors may be delayed by ongoing climate change. However, the amplitude of these effects varies on a species-by-species basis. Our results—combined with new evidence of an earlier passage during spring migration for the same species in the same area—suggest that, since 1985, most raptors spent ∼2 additional days decade⁻¹ north of our study's migration sites. Such an increase in time spent at northern latitudes in a large number of terrestrial avian predators over a wide geographic area may have profound impacts on population and ecosystem dynamics. Unraveling such impacts will require linking interspecific and intraspecific variations in phenological adjustments to ongoing climate change.

Ducks are an excellent group to study avian genital evolution. Penis morphology of ducks is diverse, and penis length and elaboration are positively correlated with levels of male competition resulting from forced extra-pair copulations, and with female genital elaboration resulting from sexual conflict. Here we examined whether penis morphology is affected by social environment. We found experimental evidence that in a male-biased social environment, consisting of several males and fewer females, the penis in Lesser Scaup (Aythya affinis) grew longer in 2 separate years, than in males housed in pairs, as predicted if male–male competition influences penis morphology. In Ruddy Ducks (Oxyura jamaicensis), males instead showed evidence of reproductive delays that were explained both by a male's size and his social environment: most males in social groups exhibited shorter penises, variable onset and duration of genital maturation, and faster penis growth rate. These 2 species have different levels of post-copulatory competition in nature, with Ruddy Ducks having more extreme penis sizes and more promiscuity than Lesser Scaup. The results suggest that waterfowl can exhibit complex, socially dependent phenotypic plasticity and reproductive maturation that can generate intraspecific variation in their genitalia.

Geographic variation in bird vocalizations is common and has been associated with genetic differences and speciation, as well as with short-term changes in response to anthropogenic noise. Because vocalizations are used for individual recognition in many species, geographic variation in these traits may affect mate choice, pair bonding, and territory defense. Anecdotal evidence suggests the existence of geographic variation in vocalizations between isolated populations of Gentoo Penguins (Pygoscelis papua), but there have been no comprehensive studies of Gentoo Penguin vocalizations across a broad geographic range. We used acoustic recordings of ambient colony sound at 22 breeding colonies in the Antarctic Peninsula and South Shetland Islands, South Georgia, the Falkland Islands, and Argentina to address 2 main questions regarding Gentoo Penguin vocalizations: (1) How do ecstatic display calls vary both within and between individuals, colonies, and regions? (2) Can ecstatic display calls be used to distinguish subspecies? We found high levels of variation between individuals and between colonies, but little additional variation between regions or subspecies. We found no trends to suggest a latitudinal gradient in vocal characteristics, although we did find that some measures varied with relative distance between colonies. Although we found significant differences at the colony level, unknown calls could not easily be categorized to colony or region by machine learning. We conclude that the vocal soundscape of each colony is driven by variation between individuals within a colony and, developing independently from neighboring colonies, becomes differentiated from other colonies through a process of drift. Although individual calls could, in most cases, be identified to subspecies by machine learning, our analysis suggests that subspecies differences may be driven by variation among colonies and that subspecies identification may be unreliable using acoustics alone.

The balance of abiotic and biotic stressors experienced by a species likely varies across its range, resulting in spatially heterogeneous limitations on the species' demographic rates. Support for spatial variation in stressors (often latitudinal gradients) has been found in many species, usually with physiological or correlative occupancy data, but it has rarely been estimated directly with demographic data. We collected demographic data from 23 sites spanning the majority of the Saltmarsh Sparrow (Ammodramus caudacutus) breeding range. Using data from 837 nests, we quantified the abiotic and biotic variables most important to nest survival, which is the dominant driver of both fecundity and population growth rate in this species. We separately estimated daily nest failure probability due to nest depredation (biotic stressor) and nest flooding (abiotic stressor), which collectively account for almost all nest failure in the species. Nest depredation decreased with latitude, whereas nest flooding was not related to latitude. Instead, nest flooding was best predicted by a combination of maximum high tide, extremity of rare flooding events, and date. For a single vital rate, we observed predictable variation in competing biotic and abiotic stressors across this species range. We observed that biotic and abiotic stressors were geographically independent, both on a large spatial scale and locally. Our results suggest that stressors on the fecundity of Saltmarsh Sparrow vary systematically across its range, but independently. The observed patterns of biotic and abiotic stress provide information for efforts to conserve the Saltmarsh Sparrow, which is considered threatened. Further, understanding the effects that different stressors, and their interactions, have on demographic rates is necessary to unravel the processes that govern species distributions and to effectively conserve biodiversity in the face of global change.