Does mating system affect the physiological stress of provisioning birds? A comparative study of polygynandrous Bicknell's thrush (Catharus bicknell]) and socially monogamous Swainson's thrush (Catharus ustulatus) breeding sympatrically.

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University of New Brunswick


Bicknell's Thrush is one of two migratory bird species in North America with a polygynandrous mating system. Having multiple male feeders at the nest, and a skewed sex ratio in adults, suggests that Bicknell' s Thrush adults partition the feeding effort. Since nestling provisioning is physiologically stressful, I predicted that Bicknell' s Thrush would be less stressed than a monogamous counterpart during the nestling provisioning period. I compared Bicknell's Thrush with Swainson's Thrush, a close relative and concurrent monogamous breeder in the study area. Heterophil to lymphocyte ratio (H:L) was used as a surrogate for corticosterone in the analysis of physiological stress. To test the sensitivity of H:L to physiological stress, I compared breeding and overwintering birds. Overwintering birds were less physiologically stressed than breeding birds, and had no circulating hematozoa. Both species of Thrush were found to be parasitized during the breeding season, but hematozoa infections and infestation with trombiculid mites had no effect on body condition or H:L. Most Bicknell's Thrush females provided at rates similar to either sex of Swainson's Thrush. Most Bicknell' s Thrush males provided less than 27% of the feeds at a nest, but total rates for males feeding two broods were similar to female Bicknell' s Thrush. I hypothesized that male Bicknell' s Thrush would have the lowest H:L, but could not conclude that they were the least stressed during the provisioning period. The results were most likely confounded by Swainson's Thrush whose nests had failed. I see the potential of H:L as a tool for deducing nest success in demographic studies where catching adults is easier than finding nests: provisioning birds, of either species, had significantly higher H:L ratios than those sampled after their nest had failed.