My project will build on previous studies of American Kestrel nestlings’ corticosterone response to repeated stressors and will attempt to clarify some currently debatable questions. Whitman et al found that nestlings that were gently handled for 15 minutes every day displayed lower total corticosterone levels than unhandled birds when subjected to a capture and handling stress series (i.e., birds were put in a bag for one hour and repeatedly bled).
Earlier work by Love et al found that nestlings that had been previously subjected to at least one stress test had lower corticosterone levels during subsequent stress tests compared to unhandled nestlings. While both studies demonstrate that repeated stress correlates with a depressed HPA response in nestlings, it is still unclear whether this is due a true modification in HPA function or mere adaptation to handling stress. It is also not known whether this is a developmental phenomenon, affecting only very young nestlings, or if repeated stressors at any point during the nestling stage would alter HPA response.
In order to shed light on these questions, I will gently handle American Kestrel nestlings for 15 minutes/day for seven consecutive days either early, late, or not at all during the nestling stage. Prior to fledging, nestlings will have corticosterone levels measured during a traditional stress test and after being subjected to a noise stressor. My hypothesis is that nestlings handled early during development will have lower corticosterone levels during both the stress test and exposure to the noise stressor. This would indicate that early handling of American Kestrel nestlings affects the development of the HPA axis and subsequent stress responses. The results of this project could have implications for bird research and conservation efforts, especially those involving nestlings.
Thesis Abstract: Early exposure to stressors affects subsequent stress responses in both mammalian and avian species, with the likelihood for lasting effects depending, in part, on the magnitude of the stressor. However, it is unclear whether lasting effects are the result of developmental changes to the hypothalamic-pituitary-adrenal (HPA) axis or habituation to the stressor. I investigated the effects of human handling, a known stressor, in free-living American kestrel (Falco sparverius) nestlings to determine if this brief, non-invasive stressor causes lasting changes in the stress response of handled birds, and whether alterations in the stress response were the result of developmental changes (early-handled birds) or habituation (later-handled birds). Broods were randomly assigned to one of three treatment groups: 1) early-handled nestlings were held daily for 15 min for 7 days following hatching, 2) late-handled nestlings were held for the same time and duration beginning on the 18th day post-hatching, and 3) control birds were not handled before sampling for circulating corticosterone (CORT). When nestlings were 25 or 26 days old, they were exposed to either a handling restraint protocol (similar to the treatment of handling) or a novel noise stressor. The stress test not applied on the first day was conducted the next day. During the hour-long restraint protocol, I collected five 75 ul samples of blood through venipuncture of the alar vein. HPA response to a novel stressor was measured by sampling CORT immediately after exposing birds to 10 min of 96 dB(A) (±1 dB) white noise. I analyzed serum CORT levels using enzyme-linked immunoabsorbent assays (ELISA). There was no effect of handling treatment on CORT patterns sampled over the 1 hr restraint test (P = 0.092), though a priori contrasts showed a significant difference between CORT levels of late-handled and control birds at 60 min (P = 0.01). Late-handled birds had lower CORT compared to control birds. In addition, there was no significant differences among handling treatments on noise-induced CORT (P = 0.94). Taken together, these results suggest that the HPA axis was unaffected by human handling and that the changes in CORT at 60 min in late-handled birds is likely the result of habituation to human handling rather than to developmental changes. Further, results indicate that mild and brief stress associated with human handling is unlikely to have negative lasting effects on the stress physiology of young birds.
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