Congeneric predators fill discrete niches created by the relative abundances of their prey species

To what degree is niche partitioning driven by underlying patterns in resources such as food, rather than by competition itself? Do discrete niches exist? We address these questions in the context of Cooper's and sharp-shinned hawks, two broadly sympatric, North American, bird-eating raptors in the genus Accipiter. We find that the resource base, as quantified by body masses of birds at bird feeders, is approximately lognormal (smallest birds are most abundant), with lesser modes (peaks) in abundance at larger body mass. The predators appear to exploit peaks in the resource base, with sharp-shinned hawks focusing on small prey items (median of 26.5 g), and Cooper's hawks taking prey from the two most abundant peaks (both the small body mass peak and a lesser peak at medium body mass ~90 g). We tested the ability of citizen scientists to distinguish these notoriously similar species, and we determined the influence of potential false positive detections on our conclusions. We find that citizen scientists struggle to distinguish these predators from one another, and 18% of Cooper's hawks were identified as sharp-shinned hawks, while 27% of sharp-shinned hawks were identified as Cooper's hawks. Yet, simulations show that this uncertainty did not jeopardize our qualitative conclusions.