The relationship between productivity and habitat quality in gray hawks

Abstract

I studied Gray Hawks (Asturina nitida) nesting along the upper San Pedro River in southeastern Arizona from 1995 to 1997. I identified 27 territories within the study area with a mean of 24.3 territories occupied per year. Productivity averaged 1.32 young per occupied nest during the study. Years did not differ in number of successful nests or in number of young produced. Mean size of home ranges (n = 10 males), based on the 90% adaptive kernel method, was 59.2 ha (range 21.4-91.2). Gray Hawk diet was composed of 68.6% reptiles, the majority being whiptail lizards ( Cnemidophorus spp). Daily foraging patterns were bimodal with peaks during mid-morning and late afternoon. Almost all Gray Hawk nests were located in cottonwoods (Populus fremontii). Nest trees tended to be dominant trees in the area. Gray Hawks did not use vegetation randomly within their home ranges. Mesquite (Prosopis juliflora) and cottonwood were used more than expected based on availability. Nearly 80% of all foraging locations were in mesquite. Within mesquite, Gray Hawks used areas of taller trees and lower horizontal cover than found at random sites. Trees used as foraging perches were taller than the surrounding canopy. Foraging and random sites did not differ in prey abundance indicating that prey availability is more important than prey abundance in determining where Gray Hawks forage. I developed a method of predicting home ranges for all 27 territories utilizing telemetry information from a sample of male hawks, a geographic information system, and logistic regression. This model produced estimates of vegetative composition within home ranges that were correlated strongly with vegetation composition in home ranges based on telemetry. The models I developed performed better than circular buffers around nest sites. Using the predicted home ranges, I developed an ordinal logistic model to identify what vegetative components influence Gray Hawk productivity. Area of mesquite was the primary determinant of habitat quality in Gray Hawks. Cottonwood-willow (Salix gooddingii) vegetation also can increase quality of home ranges when mesquite is present in low amounts.