Proximity of hummingbirds to Accipiter cooperii nests in Tucson, AZ
Smaller avian species such as hummingbirds have been found to cluster their nests near the nests of larger raptor species, benefiting because intermediate predators avoid those areas. The purpose of this study was to determine whether the abundance of hummingbirds was related to the proximity of Cooper’s Hawks nests in urban Tucson, Arizona. I visited 15 active Cooper’s Hawk nest sites and at each nest I walked in a randomly selected direction and counted hummingbirds (all species combined) for eight minutes at 5 different stations: 50 m, 100 m, 200 m, 300 m, and 400 m from the nest.
I counted twice as many hummingbirds within 100 m from Cooper’s hawk nests (8 hummingbirds in total) as I did past 100 m up to 400 m (14 hummingbirds total). On average, 2.07 hummingbirds were counted per nest within 100 m, versus 0.93 hummingbirds between 100 m to 400 m from the nest. The counting stations of 50 m and 100 m from nests were within the average core use area of Cooper’s hawks in Tucson. Core use areas are where the hawks spend the majority of their time and it is possible that their presence deterred intermediate predators that would otherwise have depredated hummingbird nests. Their influence could be an example of a trophic cascade in an urban setting. An alternative explanation is that the presence of large trees used by Cooper’s hawks as nests influences the abundance of humming birds. Because the majority of species that predate hummingbird nests (e.g., jays) are not common in Tucson, further investigation should explore if and how the presence of Cooper’s hawks benefit hummingbirds in urban areas.