Pauline Gaines captured this amazing shot of a tiny and furious kingbird aggressively attacking a red tail hawk at Bonny Lake park in Colorado.. The hawk got close enough to the smaller bird's nest that it became territorially outraged to the point of essentially piggybacking on the much larger predator.
This phenomenon where much smaller, and often seed-eating, songbirds will attack a bird of prey that ventures too close to a next is known as "mobbing." Kingbirds are particularly known for their fearless response to aggressive hawks; see these amazing images by photographer Rob McKay. Crows and jays are also particularly given to mobbing—see these images of a much smaller single crow aggressively attacking a White-bellied Sea Eagle . Reed Warblers are particularly known for aggressively mobbing cuckoos, who favor Reed Warbler nests as targets for laying their own eggs, and then abandoning their young to be reared by the smaller birds—at the cost of their own brood. Mobbing is particularly common during breeding season, but it isn't restricted. It depends on the species involved. Crows in particular seem to attack hawks and owls at any time of year. Even Seagulls, apparently, engage in mobbing; see this photograph by Hans Nydahl of a juvenile Golden Eagle being mobbed by a gull.