T. Scott Sillett

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.

Multiple density-dependence mechanisms regulate a migratory bird population during the breeding season.

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

EXPERIMENTALLY REDUCING NEIGHBOR DENSITY AFFECTS REPRODUCTION AND BEHAVIOR OF A MIGRATORY SONGBIRD

Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic–Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that i...

Full-annual-cycle population models for migratory birds

ABSTRACT Full-annual-cycle (FAC) models integrate seasonal demographic and environmental processes to elucidate the factors that limit and regulate animal populations. Unlike traditional, breeding-season-focused models of migratory populations, FAC population models include the effects on population dynamics of events in both the breeding and the nonbreeding season (i.e. winter and migration). Given that migratory birds can spend most of the year away from the breeding grounds and face seasonally specific threats and limitation, FAC models can provide critical and unique insights about their population dynamics. We review existing FAC population model types, including demographic network models, seasonal matrix models, and individual-based models, with examples of each type. We also suggest some approaches new to FAC population modeling—integrated population models and integral projection models—and make recommendations for the development and implementation of these models. Incorporating model components such as density dependence, migratory connectivity (the demographic linkages between breeding and nonbreeding areas), and seasonal interactions can be critical for model realism but can also increase model complexity and development time. Much of the development of FAC population models has been more theoretical than applied. The main limitation to the application of the developed models is availability of empirical data for all annual stages, particularly knowledge of migratory connectivity and density-dependent seasonal survival. As these data become more available, the models outlined here should find additional uses.

MINIMUM ESTIMATES OF SURVIVAL AND POPULATION GROWTH FOR CERULEAN WARBLERS (DENDROICA CERULEA) BREEDING IN ONTARIO, CANADA

Abstract The Cerulean Warbler (Dendroica cerulea) tops many lists of species of conservation concern because of severe population declines and habitat loss. Here we present the first robust estimates of annual survival and population growth rates for this species. We used capture—mark—recapture models to estimate survival of adult male Cerulean Warblers in an eastern Ontario population that has been studied since 1994. Adult male survival probability (ϕ) was constant over time in our best-supported model. Our second-best-supported model indicated a negative effect of a 1998 ice storm on survival. The third-best-supported model indicated a significant year effect on survival. On the basis of those results and previously published estimates of annual fecundity, we calculated a population growth rate using a two-stage Leslie matrix. Population growth rate (λ) was 0.73, using the estimate for constant survival. Model elasticities imply that adult mortality had a stronger effect on λ than did seasonal fecundity. Oversummer survival estimates suggest that events during migration or on wintering grounds are responsible for most adult male mortality. It appears that our study population, thought to be one of the healthiest known for this species, may not be currently reproducing at a high enough rate to accommodate adult mortality. However, caution must be used when interpreting those results, given the possibility of underestimating survival and fecundity of this species.

Hierarchical distance-sampling models to estimate population size and habitat-specific abundance of an island endemic

Population size and habitat-specific abundance estimates are essential for conservation management. A major impediment to obtaining such estimates is that few statistical models are able to simultaneously account for both spatial variation in abundance and heterogeneity in detection probability, and still be amenable to large-scale applications. The hierarchical distance-sampling model of J. A. Royle, D. K. Dawson, and S. Bates provides a practical solution. Here, we extend this model to estimate habitat-specific abundance and rangewide population size of a bird species of management concern, the Island Scrub-Jay (Aphelocoma insularis), which occurs solely on Santa Cruz Island, California, USA. We surveyed 307 randomly selected, 300 m diameter, point locations throughout the 250-km2 island during October 2008 and April 2009. Population size was estimated to be 2267 (95% CI 1613-3007) and 1705 (1212-2369) during the fall and spring respectively, considerably lower than a previously published but statistically problematic estimate of 12 500. This large discrepancy emphasizes the importance of proper survey design and analysis for obtaining reliable information for management decisions. Jays were most abundant in low-elevation chaparral habitat; the detection function depended primarily on the percent cover of chaparral and forest within count circles. Vegetation change on the island has been dramatic in recent decades, due to release from herbivory following the eradication of feral sheep (Ovis aries) from the majority of the island in the mid-1980s. We applied best-fit fall and spring models of habitat-specific jay abundance to a vegetation map from 1985, and estimated the population size of A. insularis was 1400-1500 at that time. The 20-30% increase in the jay population suggests that the species has benefited from the recovery of native vegetation since sheep removal. Nevertheless, this jays tiny range and small population size make it vulnerable to natural disasters and to habitat alteration related to climate change. Our results demonstrate that hierarchical distance-sampling models hold promise for estimating population size and spatial density variation at large scales. Our statistical methods have been incorporated into the R package unmarked to facilitate their use by animal ecologists, and we provide annotated code in the Supplement.

Adaptive divergence despite strong genetic drift: genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis)

The evolutionary mechanisms generating the tremendous biodiversity of islands have long fascinated evolutionary biologists. Genetic drift and divergent selection are predicted to be strong on islands and both could drive population divergence and speciation. Alternatively, strong genetic drift may preclude adaptation. We conducted a genomic analysis to test the roles of genetic drift and divergent selection in causing genetic differentiation among populations of the island fox (Urocyon littoralis). This species consists of six subspecies, each of which occupies a different California Channel Island. Analysis of 5293 SNP loci generated using Restriction‐site Associated DNA (RAD) sequencing found support for genetic drift as the dominant evolutionary mechanism driving population divergence among island fox populations. In particular, populations had exceptionally low genetic variation, small Ne (range = 2.1–89.7; median = 19.4), and significant genetic signatures of bottlenecks. Moreover, islands with the lowest genetic variation (and, by inference, the strongest historical genetic drift) were most genetically differentiated from mainland grey foxes, and vice versa, indicating genetic drift drives genome‐wide divergence. Nonetheless, outlier tests identified 3.6–6.6% of loci as high FST outliers, suggesting that despite strong genetic drift, divergent selection contributes to population divergence. Patterns of similarity among populations based on high FST outliers mirrored patterns based on morphology, providing additional evidence that outliers reflect adaptive divergence. Extremely low genetic variation and small Ne in some island fox populations, particularly on San Nicolas Island, suggest that they may be vulnerable to fixation of deleterious alleles, decreased fitness and reduced adaptive potential.

Warm Springs, Early Lay Dates, and Double Brooding in a North American Migratory Songbird, the Black-Throated Blue Warbler

Numerous studies have correlated the advancement of lay date in birds with warming climate trends, yet the fitness effects associated with this phenological response have been examined in only a small number of species. Most of these species–primarily insectivorous cavity nesters in Europe–exhibit fitness declines associated with increasing asynchrony with prey. Here, we use 25 years of demographic data, collected from 1986 to 2010, to examine the effects of spring temperature on breeding initiation date, double brooding, and annual fecundity in a Nearctic - Neotropical migratory songbird, the black-throated blue warbler (Setophaga caerulescens). Data were collected from birds breeding at the Hubbard Brook Experimental Forest, New Hampshire, USA, where long-term trends toward warmer springs have been recorded. We found that black-throated blue warblers initiated breeding earlier in warmer springs, that early breeders were more likely to attempt a second brood than those starting later in the season, and that double brooding and lay date were linked to higher annual fecundity. Accordingly, we found selection favored earlier breeding in most years. However, in contrast to studies of several other long-distance migratory species in Europe, this selection pressure was not stronger in warmer springs, indicating that these warblers were able to adjust mean lay date appropriately to substantial inter-annual variation in spring temperature. Our results suggest that this North American migratory songbird might not experience the same fecundity declines as songbirds that are unable to adjust their timing of breeding in pace with spring temperatures.

Seasonal and population variation in male testosterone levels in breeding orange-crowned warblers (Vermivora celata).

Comparative hormone studies can reveal how physiology underlies life history variation. Here, we examined seasonal variation in plasma testosterone concentration between populations of male orange-crowned warblers (Vermivora celata) breeding in Fairbanks, Alaska (V. c. celata) and on Santa Catalina Island, California (V. c. sordida). These populations face different ecological constraints and exhibit different life histories. Alaska birds have a short breeding season, low annual adult survival, and high reproductive rates. In contrast, Catalina Island birds exhibit high adult survival and low reproductive rates despite having a long breeding season. We examined seasonal variation in male testosterone concentrations as a potential mechanism underlying differences in male reproductive strategies between populations. From 2006 to 2008, we sampled males during the pre-incubation, incubation, and nestling stages. Alaska males exhibited a seasonal testosterone pattern typical of northern passerines: testosterone levels were high during pre-incubation and declined during incubation to low levels during nestling provisioning. Testosterone concentrations in Catalina Island males, however, did not vary consistently with breeding stage, remained elevated throughout the breeding season, and were higher than in Alaska males during the nestling stage. We hypothesize that in Alaska, where short seasons and high adult mortality limit breeding opportunities, the seasonal testosterone pattern facilitates high mating effort prior to incubation, but high parental investment during the nestling stage. On Catalina Island, elevated testosterone levels may reflect the extended mating opportunities and high population density facing males in this population. Our results suggest that population variation in seasonal testosterone patterns in orange-crowned warblers may be a function of differences in life history strategy and the social environment.

AN EXPERIMENTAL STUDY OF HABITAT SELECTION BY BIRDS IN A COFFEE PLANTATION

Unique components of tropical habitats, such as abundant vascular epiphytes, influence the distribution of species and can contribute to the high diversity of many animal groups in the tropics. However, the role of such features in habitat selection and demography of individual species has not been established. Understanding the mechanisms of habitat selection requires both experimental manipulation of habitat structure and detailed estimation of the behavioral and demographic response of animals, e.g., changes in movement patterns and survival probabilities. Such studies have not been conducted in natural tropical forest, perhaps because of high habitat heterogeneity, high species diversity, and low abundances of potential target species. Agroforestry systems support a less diverse flora, with greater spatial homogeneity which, in turn, harbors lower overall species diversity with greater numerical dominance of common species, than natural forests. Furthermore, agroforestry systems are already extensively managed and lend themselves easily to larger scale habitat manipulations than protected natural forest. Thus, agroforestry systems provide a good model environment for beginning to understand processes underlying habitat selection in tropical forest animals. Here, we use multistate, capture-recapture models to investigate how the experimental removal of epiphytes affected monthly movement and survival probabilities of two resident bird species (Common Bush-Tanager [Chlorospingus ophthalmicus] and Golden-crowned Warbler [Basileuterus culicivorus]) in a Mexican shade coffee plantation. We established two paired plots of epiphyte removal and control. We found that Bush-Tanagers were at least five times more likely to emigrate from plots where epiphytes were removed compared to control plots. Habitat-specific movement patterns were not detected in the warbler. However, unlike the Golden-crowned Warbler, Common Bush-Tanagers depend upon epiphytes for nest sites and (seasonally) for foraging. These dispersal patterns imply that active habitat selection based on the presence or absence of epiphytes occurs in C. ophthalmicus on our study area. Survival rates did not vary with habitat in either species. Interestingly, in both species, survival was higher in the nonbreeding season, when birds were in mixed-species flocks. Movement by Common Bush-Tanagers into areas with epiphytes occurred mostly during the breeding season, when mortality-driven opportunity was greatest.