Accipiter Hawk Occupancy In Metropolitan Areas Driven By Avian Prey Biomass Across The United States

Abstract

Many raptor species declined during the mid-20th century due to the proliferation of organochlorine pesticides, habitat destruction, and persecution. Yet thanks to protection and conservation efforts, raptor populations have repatriated former habitats and simultaneously colonized metropolitan areas. Our goal was to determine the factors that allowed accipiter hawks, a woodland raptor, to occupy metropolitan areas throughout the United States. To do so, we used observations of accipiter hawks amassed by thousands of volunteers as part of Project FeederWatch. We combined information of hawk occurrence and prey availability with satellite-based observations of urban land cover, tree canopy cover, imperviousness, and climate data, and identified the factors associated with hawk occupancy in 30 metropolitan areas across North America during the winters of 2015 to 2018. We hypothesized that patterns of hawk occupancy varied due to inter-city differences in land cover, prey availability and climate. We predicted that accipiter hawks were more likely to occupy metropolitan areas with lower imperviousness, higher tree canopy cover, colder climate, and higher prey biomass. Across the top thirty metropolitan areas participating in Project FeederWatch, accipiter hawk overall occupancy rates varied from 23.7% - 61.6%. Based on model selection, we found that the best model of accipiter occupancy included prey biomass and open agriculture area. Accipiter hawks were more likely to occupy sites within cities that supported a higher prey biomass and more agriculture than cities that supported lower prey biomass and little-to-no agriculture. Among the two factors, available prey biomass was by far the best predictor of accipiter hawk occupancy. In contrast, accipiter hawks occupied major metropolitan areas irrespective of climate, imperviousness, tree canopy cover, and other land cover features so long as there was sufficient prey. Our findings advance the understanding of how recovering raptor species occupy metropolitan areas as a further evaluation of whether human-modified landscapes can provide critical habitat for species of conservation concern.