Luke K. Butler

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.

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 AND MOLT-RELATED MOVEMENTS IN ASH-THROATED FLYCATCHERS

Abstract We describe the timing and location of the prebasic molt in the Ash-throated Flycatcher (Myiarchus cinerascens), a hawking insectivore with resident populations in Mexico and migrant populations that breed in the western United States. The timing of fall molt with respect to migration is of particular interest for Ash-throated Flycatchers from the United States because they breed in arid lowland habitats that are probably unproductive in late summer, and because time constraints on molt might force northern populations to leave their breeding grounds before molting. Adults and juveniles depart their breeding grounds to arrive and molt in the region of the Mexican monsoon shortly after the monsoon rains begin. Diminishing food on the breeding grounds and increasing food in the monsoon region seem more important than time constraints for explaining molt-related movements by northern populations, because more southerly breeding birds east and west of the monsoon region also move to the monsoon region to molt. We found age class differences in the timing and duration of molt. In adults the primary molt starts approximately 14 July and requires 76 days to complete, whereas in juveniles, primary molt starts approximately 1 August (18 days after adults) but requires only 50 days to complete. We found no evidence that juveniles molt more primaries simultaneously than adults, so we conclude that the daily growth rate of individual feathers must be higher in juveniles than adults.

Documenting Molt-Migration in Western Kingbird (Tyrannus verticalis) Using Two Measures of Collecting Effort

ABSTRACT. We used museum specimens to describe the timing and location of the postbreeding molt in Western Kingbird (Tyrannus verticalis), an insectivore that breeds in arid lowlands of western North America, where late summer conditions are exceedingly dry. Like many other western migrants, adult Western Kingbirds depart their breeding grounds and move to molt in the Mexican monsoon region. By contrast, juveniles stay on the breeding grounds in late summer, delaying their eccentric primary molt and body molt until after undertaking part of their fall migration. The high number of juvenile specimens collected on the breeding grounds in late summer confirms that the decrease in adults, measured as the percentage of all Western Kingbirds that are adults, is not an artifact of inactive collectors. We also demonstrate adult departure using museum databases to calculate the percentage of all passerines collected in breeding areas outside the southwestern molting grounds that were adult Western Kingbirds. The close correlation between these indices validates the use of total number of passerines as an index of the collecting effort targeting a specific passerine. Our results provide another example of the importance of the Mexican monsoon region for molting passerines, highlighting the need to preserve habitat in this region.

Intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds: an experimental approach

We conducted experiments on the intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds, Agelaius phoeniceus . In the female choice experiment, we gave captive females a choice between males with normal or dulled epaulets, and between males with normal or reddened epaulets. Females tended to associate more with normal males than with dulled males, but were equally likely to perform precopulatory displays to the two types of males. Females associated significantly more with redder-than-normal males and were more likely to perform precopulatory displays to reddened males. In the male–male competition experiment, we presented free-living territorial males with a male model to which we could attach wings with dulled, normal or reddened epaulets. Presence of the model with reddened epaulets resulted in significant increases in display rates by territory owners, but the increase in display rates was probably in response to males other than the territory owner flying through and trespassing rather than to the reddened epaulet model per se. Normal epaulets produced a significant decrease in song rates and approach distances of owners. Responses of territorial and other males did not differ significantly during normal and dulled epaulet presentations. These results support the hypothesis that epaulets of male red-winged blackbirds are salient inter- and intrasexual signals: unusually red epaulets were attractive to females and attracted more attention and aggression from male conspecifics than normal epaulets. Epaulet colour may thus be an example of intrasexual selection opposing intersexual selection.

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.

The Grass is Always Greener: Do Monsoon Rains Matter for Molt of the Vermilion Flycatcher (Pyrocephalus rubinus)?

ABSTRACT. Millions of migratory and resident passerines molt in southwestern North America each summer, when ecological productivity spikes in response to the North American Monsoon. Although a monsoon-region molt is a critical event in the annual cycle of many species, the benefits of molting in the monsoon region are unclear. I used museum specimens to compare molt dynamics of the Vermilion Flycatcher (Pyrocephalus rubinus) in relation to the timing and location of the monsoon rains, and between northern and southern monsoon latitudes. In the north and the south, populations did not vacate arid non-monsoon areas to molt in adjacent monsoon areas, unlike several other lowland western passerines with more northern distributions. Onset, rate, and intensity of molt were similar between populations in and out of the monsoon region at the same latitude. By contrast, molt started significantly later and was significantly faster at northern latitudes than in the south, suggesting a strong, latitude-based temporal influence on molt dynamics, which was correlated with later nesting in the north. Thus, for this insectivorous passerine, the monsoon region may simply provide adequate habitat for molting at a low latitude, rather than abundant resources important for molt, as has often been proposed to explain the evolution of molt-migration to the monsoon region by western migrants. Similarities in foraging ecology of Vermilion Flycatchers and other western passerines may make these results applicable to other species, helping to clarify the role of the monsoon region in the evolution of molt schedules in North American passerines.