Nest Success of Bald Eagles Exposed to Anthropogenic Activities in the United States

Abstract

Anthropogenic pressures on the global landscape are rapidly increasing, with well-documented negative impacts on avian populations. As an encouraging counterexample to general declines, the bald eagle Haliaeetus leucocephalus population in the United States has continued to grow dramatically since its 20th century decline, with breeding pairs now colonizing areas with high levels of human activity. Evidence of the impact of human activity on nesting bald eagles is mixed, with some studies reporting declines in reproduction, while others suggest reproduction is comparatively unaffected. We assessed the effects of anthropogenic activities on bald eagle nest success by compiling data from bald eagle incidental take permits issued by the U.S. Fish and Wildlife Service for unintentional disturbance of breeding bald eagles. We used generalized linear logistic regression models in a Bayesian framework to evaluate the relationship between types of human activity (n = 6), levels of human development in the environment around nests (n = 5), and whether or not the activity resulted in a significant alteration of the surrounding habitat. There were more permits issued for nests in suburban (40%) than in natural (12%) or industrial (9%) environments and nearly half (47%) of the permits were for building activities; there was a similar number of permits where the habitat was altered (46%) or unaltered (54%). Overall mean nest success during authorized activities from 103 nest-seasons was 84% (95% credible interval: 76–90%) and nest success rates were similarly high (77–100%) for all categories within covariates (p > 0.6). The top model was without fixed effects, accounting for 47% of the model set weight, and the next three models, the only other models with widely applicable information criterion weight, included the activity type and habitat alteration covariates. The only parameters with 95% credible intervals encompassing zero were infrastructure and landscape modification activities, for which all nests exposed to these activities were successful; however, these estimates also had very high uncertainty. This indicates that the covariates we tested were weak predictors of nest success. Some permitted nests were monitored prior to or following years of authorized activity, and there was no significant difference in nest success between activity and non-activity years. We provide further evidence that the growing contingent of bald eagles nesting in human-developed environments tolerate anthropogenic activities to a degree that they are able to nest successfully at rates comparable to the general U.S. population. This study improves our understanding of bald eagle reproductive performance when exposed to human activities and will provide better guidance for managing this species.