Several lines of evidence support the hypothesis that abiotic and biotic factors directly or indirectly control species' distributions. Despite the importance of assessing the environmental factors governing species' distributions, it is not clear how such factors influence migratory species. We evaluated environmental factors related to the nonbreeding distribution (September through March) for Harlan's Red-tailed Hawks (Buteo jamaicensis harlani), using a maximum entropy (Maxent) approach. We evaluated records spanning the nonbreeding season from 1 September to 31 March, gathered from four different sources including online data and published literature; we used maximum and minimum temperatures, precipitation, solar radiation, wind speed, and cloud cover as abiotic factors. Based on the most influential environmental factors (determined using percent contribution, a measure representing the increase in likelihood associated with each environmental factor), we delineated the current winter distribution (December through February) and evaluated niche overlap. From December through February, we found that minimum temperature showed the highest percent contribution (62.2%, 55.9%, and 66.9% in each month, respectively). Response curves for minimum temperature were quadratic in shape in September, non-monotonic in shape in October, and sigmoid in shape in November, December, January, and February, with high suitability values for temperature ranging from 0–20°C and low suitability for temperature,0°C. However, habitat suitability decreased at higher levels of solar radiation (.6000 kJ m–2 d–1) during fall migration (September, October) and spring migration (March). Our revised winter distribution map, based on ecological niche modeling, includes areas of ecoregions not previously included in Harlan's Hawk range maps in the western USA, and northeastern Mexico.
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Vol. 55 • No. 1