What’s For Dinner? Sharp-shinned and Cooper’s Hawks Pick Different Foods

By Kathi Borgmann

Watching chickadees and finches jostle at your feeder provides countless hours of entertainment. And every once in a while, a Cooper’s Hawk or a Sharp-shinned Hawk might fly in and stir things up. These two bird-eating raptors co-occur throughout much of North America, but given their similarities, a common question often emerges—how do they coexist? A central tenet in ecology suggests that when two ecologically similar species co-occur, competition should drive one or both to take advantage of different niches, for example by eating different prey.

New research in the Journal of Avian Biology set out to better understand if these similar raptor species evolved to select different prey and to compare how the availability of prey might shape their choices.

Eliot Miller, collections manager at the Macaulay Library at the Cornell Lab of Ornithology, and colleagues took advantage of data collected by citizen scientists for Project FeederWatch to answer this question. “A few years ago,” Miller said, “we created an option for FeederWatchers to tell us not only which birds were at their feeders, but how those birds were interacting with one another. That meant we could figure out the preferred prey of Cooper’s and Sharp-shinned Hawks, because we had data on who was eating who and we had data on prey availability.”  

From October 2015 to April 2022, FeederWatchers observed 1,186 incidences of Cooper’s Hawks eating a bird at their feeders and 677 incidences of a Sharp-shinned Hawk eating a bird. Prey available for these raptors consisted of greater numbers of small-bodied birds, such as Pine Siskins, with fewer medium-sized birds, such as jays, and still fewer larger-bodied birds, such as quails. 

Miller and colleagues found that Sharp-shinned Hawks took smaller-bodied birds, such as juncos and Pine Siskins most often. Cooper’s Hawks also took smaller-bodied birds, but they mainly took medium and larger-bodied birds, such as European Starlings and pigeons. Miller explains that a focus on different size prey may be one component that allows these species to coexist. “Cooper’s Hawks are increasingly common in urban areas and they are bigger, so their focus on larger prey, such as pigeons, isn’t necessarily surprising. Still, no one has studied predator preferences at this spatial scale before, so to see such a strong result speaks to the power of citizen scientists to document these patterns,” said Miller. Interestingly, Miller and colleagues also found that what they eat is reflected by their habitat preferences. “Although Sharp-shinned certainly can be found in urban areas, particularly in the winter, they are often found in forests,” said Miller, “and the cool thing is that we were able to see that in their preferences for prey that also frequently occur in forests, such as Evening Grosbeaks and Purple Finches.” 

Even though Cooper’s and Sharp-shinned Hawks are notoriously difficult to distinguish, Miller and colleagues found that their results held up even if some observers may have made an error in identifying the hawks. Miller used photos from the Macaulay Library to quiz FeederWatch participants and expert birders to see how frequently each group might misidentify raptors. Respondents identified Cooper’s Hawks as Sharp-shinned Hawks 18% of the time and Sharp-shinned Hawks as Cooper’s Hawks 27% of the time. The researchers used this information to re-run their analyses and found that even with these misidentification rates, the overall pattern with Sharp-shinned focusing on smaller birds and Cooper’s Hawks on both smaller and medium to larger birds stayed the same. “In some ways our quiz was testing a worst-case scenario,” said Miller. “Observers frequently use size and behavioral characteristics to distinguish these species, features that are lost in a photo-based quiz. So, the fact that observers got some of these quiz questions wrong is not surprising. What is more important to us is that even misidentification rates as high as what we observed in the quiz didn’t jeopardize our conclusions—yet another example of the power of partnering with citizen scientists to understand the world around us.”

Miller said this study brought even more questions to mind and he hopes others might also want to dig into FeederWatch data. For example, Miller said their results suggest that ground-foraging species might be at greater predation risk, but he said, “we didn’t directly test that question, although it would be a cool question to examine.” FeederWatch data are freely available and Miller hopes “other researchers will dig in and investigate this and other fascinating questions.”

Miller, E. T., O. M. Aodha, E. I. Greig, D. N. Bonter, and W. M. Hochachka. (2022). Congeneric predators fill niches created by the relative abundance of their prey species. Journal of Avian Biology: e02934 doi/full/10.1111/jav.02934