Timothy J. Hayden

Habitat fragmentation and genetic diversity of an endangered, migratory songbird, the golden‐cheeked warbler (Dendroica chrysoparia)

Landscape genetic approaches offer the promise of increasing our understanding of the influence of habitat features on genetic structure. We assessed the genetic diversity of the endangered golden‐cheeked warbler (Dendroica chrysoparia) across their breeding range in central Texas and evaluated the role of habitat loss and fragmentation in shaping the population structure of the species. We determined genotypes across nine microsatellite loci of 109 individuals from seven sites representing the major breeding concentrations of the species. No evidence of a recent population bottleneck was found. Differences in allele frequencies were highly significant among sites. The sampled sites do not appear to represent isolated lineages requiring protection as separate management units, although the amount of current gene flow is insufficient to prevent genetic differentiation. Measures of genetic differentiation were negatively associated with habitat connectivity and the percentage of forest cover between sites, and positively associated with geographic distance and the percentage of agricultural land between sites. The northernmost site was the most genetically differentiated and was isolated from other sites by agricultural lands. Fragmentation of breeding habitat may represent barriers to dispersal of birds which would pose no barrier to movement during other activities such as migration.

No energetic cost of anthropogenic disturbance in a songbird

Anthropogenic or natural disturbances can have a significant impact on wild animals. Therefore, understanding when, how and what type of human and natural events disturb animals is a central problem in wildlife conservation. However, it can be difficult to identify which particular environmental stressor affects an individual most. We use heart rate telemetry to quantify the energy expenditure associated with different types of human-mediated and natural disturbances in a breeding passerine, the white-eyed vireo (Vireo griseus). We fitted 0.5 g heart rate transmitters to 14 male vireos and continuously recorded heart rate and activity for two days and three nights on a military installation. We calibrated heart rate to energy expenditure for five additional males using an open-flow, push-through respirometry system showing that heart rate predicted 74 per cent of energy expenditure. We conducted standardized disturbance trials in the field to experimentally simulate a natural stressor (predator presence) and two anthropogenic stressors. Although birds initially showed behavioural and heart rate reactions to some disturbances, we could not detect an overall increase in energy expenditure during 1- or 4-hours disturbances. Similarly, overall activity rates were unaltered between control and experimental periods, and birds continued to perform parental duties despite the experimental disturbances. We suggest that vireos quickly determined that disturbances were non-threatening and thus showed no (costly) physiological response. We hypothesize that the lack of a significant response to disturbance in vireos is adaptive and may be representative of animals with fast life histories (e.g. short lifespan, high reproductive output) so as to maximize energy allocation to reproduction. Conversely, we predict that energetic cost of human-mediated disturbances will be significant in slow-living animals.

Population structure in an endangered songbird: maintenance of genetic differentiation despite high vagility and significant population recovery

Black‐capped vireos (Vireo atricapilla), an endangered, migratory species dependent upon early successional habitat, have experienced significant recovery since its protection. In light of its vagility and known increase in population size and range, limited genetic differentiation would be expected in the species. Using 15 microsatellite loci and an extensive sampling regime, we detected significant overall genetic differentiation (FST = 0.021) and high interpopulation differentiation compared to other migratory birds. Although proximate sites (separated by < 20 km) tended to be genetically similar, there was no apparent association of either geographical distance or landscape attributes with differentiation between sites. Evidence of a population bottleneck was also detected in a site located near other large concentrations of birds. Although black‐capped vireos are capable of large‐scale movements and the population has experienced a recent expansion, dispersal appears too insufficient to eliminate the genetic differentiation resulting from restricted colonization of ephemeral habitats.

Adrenocortical responses to offspring-directed threats in two open-nesting birds

Dependent young are often easy targets for predators, so for many parent vertebrates, responding to offspring-directed threats is a fundamental part of reproduction. We tested the parental adrenocortical response of the endangered black-capped vireo (Vireo atricapilla) and the common white-eyed vireo (V. griseus) to acute and chronic threats to their offspring. Like many open-nesting birds, our study species experience high offspring mortality. Parents responded behaviorally to a predator decoy or human 1-2m from their nests, but, in contrast to similar studies of cavity-nesting birds, neither these acute threats nor chronic offspring-directed threats altered plasma corticosterone concentrations of parents. Although parents in this study showed no corticosterone response to offspring-directed threats, they always increased corticosterone concentrations in response to capture. To explain these results, we propose that parents perceive their risk of nest-associated death differently depending on nest type, with cavity-nesting adults perceiving greater risk to themselves than open-nesters that can readily detect and escape from offspring-directed threats. Our results agree with previous studies suggesting that the hypothalamic-pituitary-adrenal axis, a major physiological mechanism for coping with threats to survival, probably plays no role in coping with threats to offspring when risks to parents and offspring are not correlated. We extend that paradigm by demonstrating that nest style may influence how adults perceive the correlation between offspring-directed and self-directed threats.

Prebasic Molt of Black-Capped and White-Eyed Vireos: Effects of Breeding Site and the El Niño–Southern Oscillation

Abstract Ecological factors on the breeding grounds are expected to have a relatively large effect on the timing of molt in migratory birds breeding at middle latitudes (e.g., southern North America), because constraints on the time available to molt are probably weak. We investigated the timing of prebasic molt in adults of two migratory passerines, the endangered Black-capped Vireo (Vireo atricapilla) and the common White-eyed Vireo (V. griseus), on their breeding grounds in Texas. We compared the onset and rate of molt between sites differing in habitat and between years differing greatly in precipitation due to the El Niño–Southern Oscillation (ENSO). Black-capped Vireos began molting 13 days later (8 July) and completed molt 12 days faster (50 days) in low quality, clumped habitat than in more typical, contiguous shrubby habitat (62-day molt starting 25 June). Long-term estimates of onset and duration of molt based on study skins were intermediate. White-eyed Vireos in the site with clumped habitat also started molting significantly later (13 July), but did not molt significantly faster (56 days), than those in the shrubby site (63-day molt starting 2 July). Both species started molting significantly later in the wet La Niña year than in the year with dry El Niño conditions. Body condition did not differ between sites or years. Our molt duration estimates of ~55 days for Black-capped Vireos and ~60 days for White-eyed Vireos suggest weak time constraints on molt in these populations. This study highlights the sensitivity of molt dynamics to events and conditions during the preceding breeding season.