Redstart Stress Response Differs by Habitat
By Gregory Gough
The American redstart, a small songbird that nests in North America and winters in the tropics, gets stressed out just like the rest of us. But does the stress response have any long-term consequences?
Scientists studied redstarts on their wintering grounds in Jamaica to find out. Scientists captured the birds, held them for half an hour (enough time for the stress response to kick in) and took a blood sample.
Researchers analyzed the blood samples for corticosteroids, a hormone that is elevated when the stress response is high. It was thought that birds with a healthy stress response would be birds that would be more likely to survive—think of the "fight or flight" response.
The redstarts were marked with a uniquely-numbered metal leg band and recaptured in subsequent years. Comparing how long the birds lived with their stress response would give clues as to the role stress plays in a bird's life.
Data analysis showed that birds that wintered in scrubby habitats, and had high-stress responses (high corticosteroid levels), were more likely to return in later years. Interestingly, birds that wintered in mangrove habitats did not show this pattern; high-stress levels were not found in returning birds.
Why the discrepancy? Mangroves are a better habitat for redstarts to overwinter in than scrub. Mangrove habitats offer more food and water, so the birds maintain their weight better throughout the winter, and can leave earlier on spring migration, get better breeding habitats, and raise more young.
Mangroves, therefore, appear to be pretty easy on the stress levels. Scrub, on the other hand, is a high-stress environment. Birds that have a high-stress response appear to do better in a high-stress environment.
This article summarizes the information in this publication:
Angelier, Frederic, Holberton, R. L. and Marra, Peter P. 2009. Does Stress Response Predict Return Rate in a Migratory Bird Species? A Study of American Redstarts and their Non-Breeding Habitat. Proceedings of the Royal Society (London) B, 276(1672): 3545-3551.
In vertebrates, the adrenocortical stress response activates an emergency life-history stage, which is thought to promote survival by helping individuals escape life-threatening situations. Although the adrenocortical stress response promotes many behavioural and physiological changes, it remains unclear whether this stress response actually translates into higher survival in wild vertebrates. We measured the adrenocortical stress response of non-breeding American redstarts (Setophaga ruticilla), a migratory bird that wintered in habitats of either high (mangroves) or low suitability (scrubs), and subsequently monitored their return rate during the following non-breeding seasons. The intensity of the adrenocortical stress response was consistent within individuals across the non-breeding season and was positively correlated with return rates in redstarts that wintered in scrubs, but not in redstarts that wintered in mangroves. Thus, in a context-dependent manner, the ability of an individual to physiologically react to stress determines its ability of returning to its non-breeding territory the following winters. For an individual, the ability to mount an important adrenocortical stress response probably benefits to survival. However, this beneficial effect probably depends on an individual’s environment and phenotypic characteristics because these two variables are likely to affect its probability of being confronted with life-threatening stressors during its annual life cycle.
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