Annual schedules, environmental cues and stress physiology in an irruptive, nomadic songbird, the red crossbill (Loxia curvirostra).

摘要

Appropriate scheduling of life cycle stages has enormous fitness consequences for animals. In the temperate zone, resource availability typically changes seasonally and is tracked by life cycle stages in many organisms. Seasonally predictable environmental changes allow animals to prepare for impending challenges. Opportunistic species, however, exploit food resources that are temporally dissociated from season. Historically, these species have been described as being infinitely flexible in timing of life cycle stages. I demonstrate that the red crossbill (Loxia curvirostra), a quintessential, north-temperate opportunist, actually is not infinitely flexible. Specifically, they show seasonal cycles of reproductive, molt and migratory physiology. Several hypotheses could explain why temporal flexibility is limited despite aseasonal food resources, including fundamental physiological constraints, phylogenetic constraints, and conflicting selective pressures.;Opportunists do, however, show high degrees of behavioral flexibility compared to seasonal migrants, presumably due to increased resource unpredictability. One mechanism that may be important for coping with unpredictable resources is an endocrine-mediated stress response that redirects behavior and physiology towards survival. In support of this hypothesis, I found that reduced food availability resulted in increased baseline corticosterone and activity levels in captive red crossbills, and that this effect was attenuated by allowing visual and acoustic access to a neighbor with ad libitum food. These results suggest that the use of public information can alter the hormonal response to changes in food availability and may affect migratory decisions.;Finally, I present field data on stress physiology during breeding and molt in nomadic opportunists. In contrast to seasonal migrants, zebra finches (Taeniopygia guttata) and red crossbills do not suppress the stress response during feather molt. These data reveal a potentially significant relationship between molt duration and degree of stress suppression, which I discuss in light of several hypotheses. Dramatic variation in environmental conditions during the winter and summer breeding seasons of red crossbills allows for a within-species evaluation of interactions between environmental conditions, stress physiology, and breeding. Winter breeders showed greater stress suppression than summer breeders, but overall there was little seasonal variation in stress physiology in red crossbills. This may be reflective of a nomadic and opportunistic lifestyle.