Abstract

Return to Conservation Biology Research on Birds

Grier, J.W. and J.E. Guinn. 2003. Bald eagle habitats and responses to human disturbances in Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 48 pp.

Abstract:

Removal of the bald eagle (Haliaeetus leucocephalus) from the U.S. endangered and threatened species list has been proposed but delayed, pending consideration of habitat needs and the development of a population monitoring plan for the species. This project was conducted to evaluate the species' habitat use in the state of Minnesota where a large population of bald eagles nests across several different ecoregions and in the presence of varying levels of human activity.

A total of 24 habitat and human-presence variables were measured at a sample of 120 active nest sites and 162 random sites across the state. Variables were chosen a priori based on a review of previously published studies and the biology of the species. Variables included characteristics of the nest tree and surrounding vegetation, several physical habitat characteristics, and neighboring human presence, including buildings, roads, and land uses. Measurements within 100 m were conducted on site and within 1,000 m via remote sensing/aerial photography.

Variables were considered individually, that is, on a univariate basis, from a descriptive standpoint. However, numerous correlations exist among variables and, in some cases, the range of values for a given variable was so small or skewed as to not provide explanatory value. Hence, the number of variables (parameters) was reduced for proper inferential analyses. As recently recommended by others, "significance testing" for sampling-based field studies of this nature is no longer considered appropriate and was not used here (although it is likely that such techniques on a multivariate basis would have yielded similar outcomes). Rather, we used discriminant function analysis to compare nest sites versus random sites and information-theoretic model selection to compare nest productivity with nest site characteristics.

Discriminant analysis separated nest sites from random sites primarily on the basis of nest tree diameter and distance from shoreline. Productivity was not explained well by any of the variables we analyzed, that is, variation in productivity did not appear to depend on the observed variation among the independent variables. Thus, within the broad range of basic requirements (proximity to water bodies, substantial trees for nest support, and an adequate prey base), eagle habitat is highly variable and not specialized. We did not find either the habitat characteristics or the physical presence of humans per se to be very explanatory or limiting for the presence of bald eagles in Minnesota. As a consequence, we have few recommendations for habitat management beyond insuring the continued existence of large-diameter trees.

The rebound of the bald eagle population did not happen with concurrent changes (increases) in habitat. Rather, it appears that both the former population decline and the recent population increases resulted from demographic (reproduction and survival) factors that were probably not related to habitat or human presence per se. As long as the public is sympathetic toward eagles and their needs, and not harassing the birds or impacting eagle reproduction and survival, nesting bald eagles and humans appear to coexist satisfactorily in close proximity. Thus, it appears that the continued welfare of bald eagles depends most importantly on protection of the birds themselves, via continuing education of the public and enforcement of existing regulations. While eagle habitat should not be ignored, we find little evidence that it is a major concern based on these data. At least in the state of Minnesota, changes of habitat that would be sufficient to impact nesting bald eagles would probably alter the very nature of the state itself!

Full document (874 KB) This is a PDF file. You will need Adobe Acrobat Reader to download it.