Chronic Noise Exposure has Context-dependent Effects on Nestling Stress Physiology in Tree Swallows (Tachycineta bicolor)
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Anthropogenic noise is increasing in intensity and scope, resulting in changes to acoustic landscapes and largely negative effects on a range of species. In birds, noise can mask acoustic signals used in a variety of communication systems, including parent-offspring communication. As a result, nestling birds raised in noise may have challenges soliciting food from parents and avoiding predators around the nest. Given that passerine nestlings are confined to a nest and therefore cannot escape these challenges, noise may act as a chronic stressor and alter the stress physiology of nestlings. If noise acts as a chronic stressor during development, nestlings may exhibit negative effects such as reduced immune function and alterations to the development of the HPA axis. Negative effects may also persist into adulthood and ultimately reduce reproductive success. In my study, Tree Swallow (Tachycineta bicolor) nestlings were raised in either continuous, white noise or in the absence of this added noise. I tested whether noise exposure affected baseline, stress-induced, and integrated feather corticosterone (the main glucocorticoid in birds) levels, as well as cell-mediated immune responses and the ratio of heterophils to lymphocytes (H/L ratio), as measures of immune function. Given that stress and immune responses may vary with the competitive environment, I also examined how nestling responses to noise varied with nestling size. I found that, overall, exposure to noise did not alter nestling stress physiology or immune responses. However, lighter nestlings raised in noise exhibited lower baseline, higher stress-induced, and lower integrated CORT than their heavier counterparts. Lighter nestlings raised in noise did not, however, exhibit compromised cell-mediated immune responses or increased H/L ratios. Overall, my findings suggest that noise can affect the stress physiology of developing birds; however, these effects may depend on developmental conditions and the presence of other environmental stressors, such as competition. My findings suggest that competitively disadvantaged nestlings may be more vulnerable to the effects of noise and highlight why populations are not uniformly affected by noise. Further research is needed to understand how noise-induced alterations to the HPA axis during development affect fitness and reproductive success over time.