Assessing Human Disturbance of Breeding Bald Eagles with Classification Tree Models

Abstract

We recorded 4,188 events of human activity and associated bald eagle (Haliaeetus leucocephalus) response in the vicinity of 13 central Arizona nest sites during 1983-85. A hierarchical classification of 9 dependent and 3 independent parameters was developed to quantify pedestrian, aquatic, vehicle, noise (gunshot/sonic boom), and aircraft disturbance groups. Type and frequency of response varied inversely with the distance from an eagle to the disturbance. Bald eagles were more often flushed from perches than nests and were most easily disturbed when foraging. Pedestrian was the most disturbing human activity, whereas aircraft was the least. We developed classification tree (CART) models for pooled and group disturbances to evaluate response severity and to formulate disturbance-specific management criteria. Response frequencies and critical distances for pooled disturbance were 64% at 583 m. Frequencies, distances, and the influence of secondary characteristics varied among disturbance groups. The CART models ranked distance to disturbance as the most important classifier of eagle response, followed in decreasing order of discriminatory value by duration of disturbance, visibility, number of units per event, position relative to affected eagle, and sound. This procedure offers improved specificity in human disturbance assessment. J. WILDL. MANAGE. 55(3):500-511 The variable effects of human activity on the reproductive performance of bald eagles (Grier 1969, Fraser 1985) imply a threshold for detrimental impact between pristine isolation and outright destruction. Because that threshold can be highly variable and often subtle, its determination is difficult and requires controlled experimentation (Grier and Fyfe 1987). Yet, experimental disturbance research on bald eagles is limited, with major emphasis on flushing distances and frequencies (Stalmaster and Newman 1978, Knight and Knight 1984, Fraser et al. 1985). Distance has been the primary mechanism of protection in traditional breeding bald eagle management, with distance guidelines typically applied unilaterally to all disturbance types. Management concepts to protect nesting bald eagles have evolved from concentric-circle buffer zones at nest sites (Mathisen 1968), through single territory zonation (Mathisen et al. 1977, U.S. Fish Wildl. Serv. 1981), to a multiple territory, regional approach (e.g., F. B. Isaacs and G. Silovsky, Bald Eagle Manage. on the Fremont Natl. For., U.S. For. Serv., Lakeview, Oreg., 1981). These distance-oriented zonation approaches also have included temporal control and audio/visual buffering guidelines. Because of the complex, multivariate nature of disturbance thresholds, site-specific management guidelines have been consistently recommended. Various aspects of disturbance (i.e., distance, noise, timing, position, visibility) have been mentioned or specifically addressed in previous bald eagle research (Grubb 1980, Hansen et al. 1980, Andrew and Mosher 1982). Knight and Knight (1984) postulated that flight distances licited by different types of disturbances during different bald eagle behaviors could be used to develop zones for restricting human activities. However, simultaneous identification and analysis of an inclusive range of dependent and independent factors inherent in disturbance management are lacking. We present a hierarchical scheme for evaluating potentially disturbing human activity. With data collected following that scheme, we develop disturbance-specific, quantitative models for predicting bald eagle response frequencies and for formulating management criteria. Although we do not present a disturbance threshold for detrimental impacts on reproductive performance, we intend these techniques to be a step toward that goal by being more specific in assessing disturbance. We thank the 71 Forest Service volunteers and the many professional personnel of the Tonto, Prescott, and Coconino national forests whose dedication and field assistance during 1983-85