Individual variation in natural or manipulated corticosterone does not covary with circulating glucose in a wild bird.

Animals respond to sudden challenges with a coordinated set of physiological and behavioral responses that enhance the ability to cope with stressors. While general characteristics of the vertebrate stress response are well described, it is not as clear how individual components covary between or within individuals. A rapid increase in glucocorticoids coordinates the stress response and one of the primary downstream results is an increase in glucose availability via reduced glucose utilization. Here, we asked whether between- and within-individual variation in corticosterone directly predict variation in glucose. We collected 2673 paired glucose and corticosterone measures from 776 tree swallows (Tachycineta bicolor) from four populations spanning the species range. In adults, glucose and corticosterone both increased during a standardized restraint protocol in all four populations. Moreover, in one population experimentally increasing a precursor that stimulates corticosterone release resulted in a further increase in both measures. In contrast, nestlings did not show a robust glucose response to handling or manipulation. Despite this group-level variation, there was very little evidence in any population that between-individual variation in corticosterone predicted between-individual variation in glucose regulation. Glucose was moderately repeatable within individuals, but within-individual variation in glucose and corticosterone were unrelated. Our results highlight the fact that a strong response in one aspect of the coordinated acute stress response (corticosterone) does not necessarily indicate that specific downstream components, such as glucose, will show similarly strong responses. These results have implications for understanding the evolution of integrated stress response systems.

Wind and rain are the primary climate factors driving changing phenology of an aerial insectivore.

While the ecological effects of climate change have been widely observed, most efforts to document these impacts in terrestrial systems have concentrated on the impacts of temperature. We used tree swallow (Tachycineta bicolor) nest observations from two widely separated sites in central Alaska to examine the aspects of climate affecting breeding phenology at the northern extent of this species' range. We found that two measures of breeding phenology, annual lay and hatch dates, are more strongly predicted by windiness and precipitation than by temperature. At our longest-monitored site, breeding phenology has advanced at nearly twice the rate seen in more southern populations, and these changes correspond to long-term declines in windiness. Overall, adverse spring climate conditions known to negatively impact foraging success of swallows (wet, windy weather) appear to influence breeding phenology more than variation in temperature. Separate analyses show that short windy periods significantly delay initiation of individual clutches within years. While past reviews have emphasized that increasing variability in climate conditions may create physiological and ecological challenges for natural populations, we find that long-term reductions in inclement weather corresponded to earlier reproduction in one of our study populations. To better predict climate change impacts, ecologists need to more carefully test effects of multiple climate variables, including some, like windiness, that may be of paramount importance to some species, but have rarely been considered as strong drivers of ecological responses to climate alteration.

Day length, reproductive effort, and the avian latitudinal clutch size gradient.

Explaining latitudinal patterns in life history traits remains a challenge for ecologists and evolutionary biologists. One such prominent pattern is the latitudinal gradient in clutch size in birds: the number of eggs laid in a reproductive bout increases with latitude in many species. One intuitive hypothesis proposes that the longer days at high latitudes during the breeding season allow parents to spend more time foraging each day, which results in greater total food delivery to the brood each day, and hence more offspring produced. This day length hypothesis is virtually untested, although it was proposed nearly 100 years ago. We developed a conceptual framework for distinguishing between the day length hypothesis and the widely accepted alternative hypothesis that attributes the latitudinal gradient in clutch size to increased per capita food resources at higher latitudes. Using this framework to contrast components of reproductive effort and life history patterns in a mid- and high-latitude Tree Swallow (Tachycineta bicolor) population provided clear evidence for the day length hypothesis, but little evidence for the alternative. Our findings suggest that the length of an animal's workday may be an important, but unappreciated, component of reproductive effort.

Environmental unpredictability shapes glucocorticoid regulation across populations of tree swallows.

The ability to respond appropriately to challenges is an important contributor to fitness. Variation in the regulation of glucocorticoid hormones, which mediate the phenotypic response to challenges, can therefore influence the ability to persist in a given environment. We compared stress responsiveness in four populations of tree swallows (Tachycineta bicolor) breeding under different environmental conditions to evaluate support for different selective pressures in driving the evolution of glucocorticoid regulation. In accordance with the environmental unpredictability hypothesis, stronger stress responses were seen in more unpredictable environments. Contrary to the reproductive value hypothesis, the stress response was not lower in populations engaging in more valuable reproductive attempts. Populations with stronger stress responses also had stronger negative feedback, which supports a "mitigating" rather than a "magnifying" effect of negative feedback on stress responses. These results suggest that combining a robust stress response with strong negative feedback may be important for persisting in unpredictable or rapidly changing environments.

Temporal and individual variation in offspring provisioning by tree swallows: a new method of automated nest attendance monitoring.

Studies of the ecology and evolution of avian nesting behavior have been limited by the difficulty and expense of sampling nest attendance behavior across entire days or throughout a substantial portion of the nestling period. Direct observation of nesting birds using human observers and most automated devices requires sub-sampling of the nestling period, which does not allow for the quantification of the duration of chick-feeding by parents within a day, and may also inadequately capture temporal variation in the rate at which chicks are fed. Here I describe an inexpensive device, the Automated Perch Recorder (APR) system, which collects accurate, long-term data on hourly rates of nest visitation, the duration of a pair's workday, and the total number of visits the pair makes to their nest across the entire period for which it is deployed. I also describe methods for verifying the accuracy of the system in the field, and several examples of how these data can be used to explore the causes of variation in and tradeoffs between the rate at which birds feed their chicks and the total length of time birds spend feeding chicks in a day.