Bald eagles (Haliaeetus leucocephalus) wintering in northern Arizona select perches based on food availability, visibility and cover

Abstract

I reviewed and summarized literature on bald eagles and habitat use, particularly with respect to the several hundred bald eagles that winter in northern Arizona. Since food is scarce in the winter, perches are critical to effective utilization of limited food sources. I have described perches used by bald eagles wintering in northern Arizona. Arizona Game and Fish Department (AZGFD) and Army National Guard Camp Navajo (Camp Navajo) attached Platform Transmitter Terminals (PTTs) to 13 bald eagles during the winters of 2005, 2007, and 2009. These were the bald eagles that I used in my study. I hypothesized that proximity to prey, visibility, and protected microclimate are the factors that influenced eagles in their choice of perch sites. To evaluate proximity to prey, I measured distance to road-killed large animals (such as ungulates), lakes, roads, rivers, and other water features. To ascertain visibility, I used measures of variation in vegetation height, topographic position index (TPI), and percent tree cover. To identify areas where there might be a protected microclimate, I considered northness (position relative to north-south aspect), eastness (position relative to east-west aspect), and slope. I found that the majority of perches (55%) selected by the eagles were in Petran montane conifer forest (PMCF) followed by Great Basin Conifer Woodland (GBCW; 29%). I used PMCF forest type as a habitat envelope and analyzed what within PMCF the eagles selected. To compare perches to randomly-selected points, I used binary logistic regression and an all-possible subsets program with generalized linear mixed modeling. I used Akaike's Information Criterion (AIC) to assess all possible models and determine the "best" model form. Bald eagles selected day perches that were closer to road kill sites, lakes, roads, and prey sources (combined) than random. They selected north-facing sites with greater diversity in vegetation structure. They also selected hilltops (positive TPI) more than random. The two most selected forest types, PMCF and GBCW occur mostly near Flagstaff and along the Mogollon Rim. I believe this habitat is selected most often because of the tendency of prey to concentrate there and availability of perch sites. Eagles are opportunistic and will utilize whatever prey is available. These forest types provide taller perch trees surrounded by lower vegetation providing easy flight access and a panoramic view of the area. Thinning of small and medium sized trees as well as use of prescribed fire could maintain and increase these types of habitat.