Human disturbance may be an influential environmental stressor that affects birds across life stages. I examined whether external and endogenous factors including habitat type, habitat quality and individual quality (hereafter quality), or human disturbance affect American Kestrel reproductive success in southwestern Idaho. Specifically, I was interested in how these factors lead to nest failure or abandonment. I also investigated whether elevated corticosterone (CORT) concentrations mediate the relationships among explanatory variables and nest failure. As nestling kestrels may respond to stressors differently from adults, I examined whether conditions experienced during the nestling stage affected nestling American Kestrel (Falco sparverius) hypothalamic pituitary adrenal (HPA) axis activity. I predicted that nestlings in high disturbance areas and those exposed to experimental stress would be sensitized to acute handling stress (i.e. would exhibit high baseline and stress induced CORT levels).
To examine these relationships I monitored 89 nest boxes posted along roads with varying amounts of traffic and surrounding development in Southwest Idaho during the 2008 and 2009 breeding seasons. I captured adult birds during the incubation stages and then followed nest fates by checking the boxes at expected hatch dates, and when nestlings were 10 and 25 days old. Nestling kestrels, raised in high and low human disturbance areas were exposed to a chronic stress protocol (CSP). At 25 days of age nestlings were sampled for baseline and stress induced plasma CORT. Twenty six (36%) of 73 nests failed and most (n = 23, 88%) nests failed during incubation. Human disturbance, but not habitat type or quality measures, was negatively associated with kestrel reproductive success. A disturbance score based on traffic patterns and the distance of the nest from the road best predicted kestrel reproductive success (disturbance: β = -1.08, 95% CI: -1.87 – -0.28; distance from road: β = 2.44, 95% CI: 0.54 – 5.42). Adult female kestrels nesting near interstates and busy roads had elevated CORT (χ2 = 6.07, P = 0.01; β = 0.10, 95% CI: 0.02 – 0.19) and nests of females with elevated CORT were less likely to be successful (χ2 = 4.37, P = 0.04; β = -1.09, 95% CI: -2.20 – 0). There was no relationship between adult male CORT and disturbance or success. Nestling CORT did not vary with human disturbance levels (F1, 27 = 1.97, P = 0.17), CSP treatment (F1, 27 = 0.00, P = 0.97), or with an interaction between human disturbance and CSP (F1, 27 = 0.00, P = 0.97) suggesting that nestling kestrels do not sense or perceive external conditions related to human disturbance as stressful. Because kestrels are cavity nesters, nestlings may be buffered from external conditions and instead may respond more powerfully to stressors within the nest such as brood size. Brood size at fledging best explained baseline (β = 0.10, 95% CI: -0.03 – 0.28) and stress induced CORT (β = 0.06, 95% CI: -0.06 – 0.19).
This study provides evidence that adult but not nestling kestrels respond to human activity with a physiological stress response and this can lead to changes in individual behavior resulting in reproductive failure via abandonment. At a larger scale, this study highlights the adverse affects of human disturbance on seemingly “urban-adapted” organisms such as American Kestrels.