Effects of transient human disturbance on avian species are of particular concern on Department of Defense (DoD) installations that support populations of federally listed (threatened and endangered [TES]) birds. These activities are often conducted within habitats that support such bird species, thus exposing individuals of these species to “harassment” as defined under the Endangered Species Act (ESA) of 1973, as amended. If military training activities elicit a stress response in individuals of a TES, this would constitute harassment and is considered “take” as defined under the ESA. The regulatory consequences of take are significant and can result in widespread restrictions on training activity in habitats of TES species. Understanding avian response to transient human activity and whether individuals are capable of modulating their response to repeated disturbance is important in mitigating potential effects of military training activities and reducing potential restrictions on training activities.

The objectives of this project were to determine: (1) chronic and acute stress response in TES and non-endangered passerines in response to non-lethal human disturbance as measured by adrenocortical response and energy expenditure in free-flying individuals, (2) whether individuals modulate their stress response to multiple exposure to human disturbance, and (3) whether species differ in stress response as a function of life history traits. Secondary tasks in support of these objectives included studies to evaluate other factors affecting endocrine response and energy expenditure including habitat disturbance, site characteristics, and transmitter effects. Analysis programs were developed to improve data reduction and analysis capabilities for heart-rate telemetry data.

Physiological responses were evaluated in free-living TES and common passerine species to human disturbance indicated by endocrine stress response as measured by plasma corticosterone and energy expenditure as measured by telemetered heart-rate. A series of experiments was conducted on endangered black-capped vireos (Vireo atricapilla) and golden-cheeked warblers (Dendroica chrysoparia) and non-endangered white-eyed vireos (V. griseus) on Fort Hood, Texas to evaluate chronic and acute response to human disturbance trials. These experiments utilized state-of-the-art hormone assays and remote telemetry technologies to evaluate adrenocortical response, habituation, and energy expenditure in response to military training disturbance. Final research questions used data from these experiments to test hypotheses related to disturbance response based on life history characteristics.

Overall, only weak and mixed evidence was found that physiological response measures in songbird species in this study are acutely or chronically sensitive to human activity at exposure levels in this study, though adults exhibited strong behavioral responses. A decline in baseline corticosterone occurred in the habitat specialist golden-cheeked warbler in habitats with high road densities that was not observed in the habitat generalist white-eyed vireo. Comparison of these results with studies from other non-passerine avian taxa suggests potential differences in disturbance response in species with significantly different life-history characteristics.

The results of this study are generally hopeful from a conservation perspective. They suggest that mild forms of human activity for a short period of time are not likely to cause acute stress in most songbirds. It is not known, however, if thresholds exist for more continuous longer-term disturbances than were conducted in this study that would elicit acute or chronic stress in songbirds. Furthermore, although physiological responses were relatively small in these studies, birds nevertheless altered their behavior, including abandoning normal mating and parenting activities. This suggests that behavioral response to disturbance may provide a more direct measure of fitness consequences of human disturbance.

This is the first study that integrates these two physiological measures of response—an endocrine stress response as measured by plasma corticosterone and energy expenditure as measured by telemetered heart-rate—to human disturbance in free-flying passerine species. It addresses key knowledge gaps in how wild animals respond and adapt to potential disturbance from human activities. This research also provided the rare opportunity to directly measure these stress responses in two federally listed avian species.