Repeatability in migration of Ferruginous Hawks (Buteo regalis) and implications for nomadism

• Published in: The Wilson journal of ornithology Vol. 131; no. 3; pp. 561 - 570
• Main Authors: Watson, James W., Keren, Ilai N.
• Format: Journal Article
• Language: English
• Published: Fort Collins Wilson Ornithological Society 01-09-2019
• Subjects: Accuracy; Animal behavior; Annual range; Bird migration; Birds; Breeding; Breeding seasons; Buteo regalis; Coefficients; Distribution; Documentation; Ferruginous Hawk; fidelidad a rango de distribución; MAJOR ARTICLES; migración; migration; Migratory species; nomadism; nomadismo; Nomads; Occupancy; Ornithology; Population; Population status; Prairie dogs; Prey; range fidelity; repeatability; repetibilidad; Reproducibility; Reproduction; Seasonal variations; Seasons; Summer; Telemetry; Territory; Trends; Wildlife management; Winter; United States > US; North America
• ISSN: 1559-4491; 1938-5447
• DOI: 10.1676/18-171

Nomadism in birds is characterized by inconsistent and unpredictable migrations to breeding or nonbreeding areas often related to changing prey conditions. The Ferruginous Hawk (Buteo regalis) has been described as nomadic in studies that documented fluctuations in breeding territory occupancy and synchronous changes in prey abundance. However, documentation of tracked individuals engaged in nomadic movements is lacking and is important for evaluating population status. Between 1999 and 2014 we studied repeatability in patterns of interannual range occupancy and migration of radio-tagged, breeding adult Ferruginous Hawks (n = 33). We computed repeatability coefficients to test individual fidelity to seasonal ranges and migration paths, and timing of migrations among range-wide populations over multiple years (range = 2–6 yr). Hawks exhibited dramatic seasonal differences in annual repeatability of individual movements. During the post-breeding period in late summer, repeatability coefficients for annual range occupancy and migration path fidelity were low (r = 0.37), only 50% of hawks showed range fidelity, and hawks occupied multiple ranges (x̄ = 1.5 ± 0.2 ranges/yr). In contrast, repeatability was moderate to high (r ≥ 0.65) in the breeding season and winter, 83% and 87% of hawks showed range fidelity to respective ranges, and hawks occupied an average of one range each season. All hawks returned to the same breeding region each year. Variation attributed to differences between individuals was significant for most migration characteristics, providing evidence for flexibility within the species for individuals to adapt to different situations and conditions, but with each individual tending to follow the same patterns over multiple years. Seasonal nomadism in Ferruginous Hawks was limited to the post-breeding period in late summer, and we found no evidence for systematic nomadism in spring migration of Ferruginous Hawks that would confound interpretation of breeding population trends. However, we recommend further telemetry studies to simultaneously document migration repeatability for this species prior to and during dramatic prey declines.