Meeting Abstract
Birds use both photoperiodic and supplemental cues to time reproduction. For migrants, however, reproductive development must be temporarily halted until migration has been completed. Final transition into the breeding life history stage may be modified by the endocrine stress response, as the hypothalamic-pituitary-adrenal (HPA) axis has well-known suppressive effects on the hypothalamic-pituitary-gonadal (HPG) axis. We hypothesized that greater HPA-axis activity in migrants may act as a “brake” on the development of the HPG-axis. We predicted that prior to migratory departure, migrant birds would have elevated HPA-axis activity (also associated with hyperphagia and fattening) compared to resident birds. To test this prediction, we sampled baseline corticosterone (CORT), stress-induced CORT, and negative feedback efficacy of Dark-eyed Juncos (Junco hyemalis) in an overwintering population that included both migratory (J.h. hyemalis) and resident (J.h. carolinensis) individuals. Juncos were sampled in western Virginia in early March, which was about 2-4 weeks before peak migratory departure for migrants and 4-6 weeks before first clutch initiation for residents. Contrary to our prediction, we found that migrants had lower baseline and stress-induced CORT compared to residents. These results suggest that delayed breeding in migrants is affected by other physiological mechanisms. Additionally, our findings also suggest baseline CORT does not permissively enhance pre-migratory fattening, as migrants had lower baseline CORT and were fatter than residents.
