Climate change projected for northeastern North America in the 21st century is likely to influence the distribution and abundance of migratory songbirds by altering the amount and quality of habitat available to them. Long-term studies of Black-throated Blue Warblers (Dendroica caerulescens) have revealed that climate strongly influences habitat quality in this species. The focus of the present proposal is on how a changing climate will affect the key choices that birds make: where to breed (territory selection), whether to return to a territory in a subsequent year (site fidelity), and how to allocate limited resources into mating and parental care (reproductive effort).
The long-term research proposed complements multiple other studies within the Hubbard Brook Ecosystem Study that focus on ecosystem responses to environmental change. This research will provide numerous opportunities for undergraduate training and education of the general public through lectures, educational materials, international partnerships (e.g., the Smithsonian Migratory Bird Center's Bridging the Americas program) and web-based information. Results will be of critical interest to policy makers and decision makers because of their multiple connections to biodiversity, the health of public lands and the broader interest of the public in birds, especially migratory songbirds.
and the effects of those responses on their reproductive success and behavior. Hence, we studied the choices that birds made and that had the potential to affect where to breed (territory selection), whether to stay on a territory or shift between seasons (site fidelity), and how to allocate limited resources into mating and parental care (reproductive effort). Our goals were to determine whether observed behavioral and demographic adjustments of individuals in both space (along a natural environmental gradient) and time (over a series of years) were adequate to compensate for climate-induced changes in territory quality. We worked in hardwood forests at the Hubbard Brook Experimental Forest in New Hampshire. Below we summarize our findings. 1. Territory selection. We hypothesized that territory site selection by birds across an elevational gradient would represent a flexible response to spring weather conditions. Low elevation hardwood sites at Hubbard Brook generally leaf out earlier, have less food, and more predators than high elevation sites. Climate predictions indicate that warming will reduce habitat quality at higher elevation sites. We quantified arrival times, territory quality, annual reproductive success, nestling provisioning, fledgling mass, and fledgling survival of black-throated blue warblers (Setophaga caerulescens), or BTBW, on low, middle and high elevation study plots. We found that annual variation in temperature affects BTBW reproductive output, in large part due to settlement patterns: in warm springs BTBW settled early in the best habitats (i.e. at higher elevations), initiated breeding earlier, and thus achieved higher reproductive output (Fig. 1). Although this suggests that BTBW populations may benefit from a warming climate, populations thus far have remained relatively stable, and our results indicate that this may be because increased reproductive output is balanced by increased adult mortality in warmer years. 2. Site fidelity. We hypothesized that climate change would affect spatial and/or temporal variability in territory quality, and thereby, the benefits to birds of being faithful to a territory location. To test this hypothesis, territory quality, annual reproductive success, nestling provisioning, fledgling mass, and annual survival of marked individual BTBWs have been measured since 1997 on territories across the elevational gradient. The primary factors influencing dispersal of BTBW were habitat quality and reproductive success, with males and females pursuing different dispersal strategies (Fig. 2). 3. Reproductive effort. We hypothesized that climate change would affect the relative allocation of effort between reproduction (including both parental and mating effort) and self-maintenance, and that these responses would differ for males and females. We measured BTBW parental care and mating effort (extrapair paternity, or EPP, rates) in years of low food availability versus years of high food availability across the elevation gradient. We found that BTBW adjust their mating and paternal effort in response to differences in ecological conditions across the elevation gradient (Fig. 3). At higher elevations, the mean proportion of extra-pair young sired per nest was greater than at lower elevations and males sang at a higher rate. Male provisioning rates were slightly reduced at the higher elevation, but varied more than expected at each elevation. To understand these patterns, we quantified the relationship between male testosterone and individual investments in mating and paternal effort. Song rate was positively associated with testosterone levels, suggesting that testosterone affects reproductive effort. During the fertile stage, BTBW males at higher elevations, where EPP rates are highest, had substantially elevated testosterone levels compared to those at low elevation where EPP rates were low. Males with elevated testosterone levels typically suffered reduced body condition. Resource allocation to mating and paternal behaviors differed across the elevation gradient, and these differences were reflected in circulating testosterone levels. Conclusion. We plan to continue our research program, emphasizing tests of hypotheses about the bird community structure and dynamics, territory selection, site fidelity and the allocation of reproductive effort in relation to ongoing climate change. Both our research and that of others show that individuals and populations of at least some bird species can response flexibly and adaptively to a changing climate, although the magnitude of these avian responses varies and might not compensate for rapid changes in climate conditions. This conclusion may be misleading, however, because studies to date have focused on only a few climate indicators, specifically temperature or food, whereas birds likely are responding to multiple complex changes occurring in their environments, particularly shifts in the phenology, distribution, and dynamics of their physical environments, habitats, and foods across a complex landscape. Our research, focusing on the key choices that individuals make and the fitness consequences of those choices over a broad range of specific environmental conditions, provides for a unique understanding of bird responses to climate change. This work will permit a more complete assessment of whether those responses are adaptive and compensatory.
