Mate and Nest Site Fidelity in a Resident Population of Bald Eagles

Abstract

The faithfulness of birds to their mates and nesting places has long been of interest to ornithologists (Darley et al. 1971, Lenington and Mace 1975, Greenwood and Harvey 1976, Harvey et al. 1979, Ollason and Dunnet 1978). Rowley (1983) listed advantages for birds breeding in the same place with the same mate, including physiological and behavioral characteristics associated with the age of the partners, the best breeding sites, and efficiency of mating with a familiar partner. Improved reproductive success is often associated with age and experience, and remated pairs often produce more and superior young than first-time nesters (Greenwood 1980). Successfully nesting birds frequently have greater mate retention rates than birds that failed in a previous nesting attempt. Harvey et al. (1979) suggested that long-lived birds, and birds living in stable environments (e.g., Bald Eagles, Haliaeetus leucocephalus) may be more faithful to sites and mates than other birds. Few data on mate and nest site fidelity of birds of prey are available, although Newton (1979) cites several examples of raptors that show a high degree of nest site fidelity. Newton and Marquiss (1982) reported strong site fidelity for a population of the Sparrowhawk (Accipiter nisus) in south Scotland; additional reports have been presented for the Flammulated Owl (Otus flammeolus) in Colorado (Reynolds and Linkhart 1987); the Ural (Strix uralensis) and Tawney (Strix aluco) Owl in Finland (Saurola 1987); urban-breeding Merlins (Falco columbarius) in Saskatchewan, Canada (Warkentin et al. 1991); and Ospreys (Pandion haliaetus) in the eastern U.S. (Poole 1989). Stalmaster (1987) suggested that Bald Eagles are generally assumed to mate for life. Gerrard et al. (1992) reported high site fidelity for four adult Bald Eagles in Saskatchewan, with one female on her same territory for 13 years. Based upon nest defense behavior and egg size, Gerrard et al. (1992) also inferred mate fidelity at several sites. Other notes and comments on nest site and mate fidelity in Saskatchewan Bald Eagles appear in Gerrard et al. (1983) and Bortolotti and Honeyman (1985). In this paper, we report mate and nest site fidelity of 20 banded or color-banded resident Bald Eagles in California, monitored between May 1983 to May 1993. We also report seven instances of breeding adult replacement observed over this period. All 20 banded Bald Eagles occurred as adults in one of 10 traditional nesting territories in the Pit River ' Received 12 April 1993. Accepted 16 June 1993. SHORT COMMUNICATIONS 1053