Influence of Nestling Number on Foraging Habits of Ferruginous Hawks in Idaho

Reproductive output of wildlife is often influenced by diet; as such, it is important to quantify to further understand its influence on demography and ecology. Ferruginous Hawks ( Buteo regalis ) are a large raptor associated with grasslands and colonial rodents. Clutch size of these birds is generally up to six nestlings; although, there is substantial inter-nest and inter-annual variation in reproductive output. We used motion sensitive digital videography to measure diet and reproductive output of Ferruginous Hawks in southwestern Idaho. Preliminary analysis suggests that the most common prey species brought to nests were Piute ground squirrels ( Urocitellus mollis ) and kangaroo rats ( Dipodomys spp. ). Other taxa brought to nests included snakes, lizards, birds, and unidentified small mammals. Total biomass of prey delivered differed by number of nestlings; however, biomass per nestling did not vary. There was also evidence that the diet of these hawks changed over the course of the nesting season by species and abundance of prey. Next steps for this project are to identify characteristics of diet associated with variation in reproductive output of Ferruginous Hawks. This study will provide insight in how this declining species may be influenced by the rapid changes occurring within grassland and shrub-steppe habitats.