Physiological ecology of the stress response in free-living sparrows
Although a large literature details the mechanisms of glucocorticoid secretion and action in the laboratory, few studies have investigated the interaction of stress physiology with behavioral, physiological and environmental factors in free living animals. I currently collaborate with Dr. Tom Hahn (U.C. Davis), to investigate the role of glucocorticoids in irruptive migration during the breeding season in the Mountain white-crowned sparrow (Zonotrichia leucophrys oriantha), breeding on Tioga meadow just East of Yosemite Park.
Tom carrying traps.
This population of white-crowned sparrows has been studied continuously since 1968, primarily by Martin L. Morton of Occidental College. Dr. Morton handed the field site over to Dr. Hahn in 1995, and I joined Dr. Hahn in 1997. Hence, for the last 35 years, data on timing of migration, nesting, nest success, as well as how each parameter varies with changing environmental conditions have been collected on this population. In addition, every year all nestlings and juveniles are banded, so that we have a population of known age individuals, usually ranging from 1-9 years of age. I have been collecting measures of stress physiology for the last 7 years. Together, Tom Hahn and I are coordinating a long-term project that will continue the data base while pursuing new questions about stress physiology and fitness; we have just received a 5-year NSF-LTREB grant for this work (I am lead PI, Dr. Hahn is co-PI).
When mountain white-crowned sparrows return to the breeding grounds in May, weather conditions are extremely unpredictable; fresh snow may temporarily cover all available foraging areas. The birds’ behavior can change rapidly and dramatically in response to these storms. They often abandon their breeding territories and temporarily move several kilometers to lower elevations, where weather conditions are less severe and foraging sites are available. The decision to wait out the storm or to abandon a territory can have major repercussions for the breeding success of that animal.
Lee Vining Creek, June 8th, 1997
Lee Vining Creek, June 8th, 1998
We are currently investigating environmental and physiological factors that regulate this irruptive migration (e.g. food availability, body condition, stress physiology), as well as the fitness consequences of variation in these mechanisms and behaviors. Using radio-telemetry, we obtain behavioral measures of the birds responses to storms, including time of departure, time of return after the storm, and low elevation location during territory abandonment. From each individual we also collect measures of stress reactivity, including total hormone levels in response to a standardized stressor, and corticosteroid binding globulin levels to determine free (unbound) hormone levels reaching tissues. Environmental assessments include temperature, relative humidity, % snow cover, and food availability. In addition to the behavioral, physiological, and environmental data collected, we obtain blood samples from every adult and nestling on the study site, allowing for determination of genetic reproductive success of every individual in the study. Given the long-term nature of this study, and the short life span of the white-crowned sparrow (3 years on average), we are able to determine lifetime reproductive success of most of the individuals in the study. Altogether, we are working to clarify how animals adjust to unpredictable events in their environment and to ascertain the adaptive significance of those adjustments.