The Cooper's hawk is usually a top predator in the daytime but is not immune from attack by other predators. The worst predator of this species is almost certainly the great horned owl ("Bubo virginianus"). This rather large owl (averaging more than three times heavier than a Cooper's hawk) is known to regularly track down fledglings and adults as well as raid the nests of other birds of prey. Many records show great horned owls will visit the nests of birds of prey and pick off the young nightly until the prey resource is exhausted (i.e. all young or sometimes adult birds of prey are killed). Furthermore, given the opportunity, great horned owls readily expropriate the nests built by the raptors they kill as their own. Given its preference for secluded wooded spots near woodland openings, Cooper's hawks are frequently the subject of unwanted attention from horned owls. Both the young, especially around the time they leave the nest or are recently fledged, and adult Cooper's hawks are vulnerable to these owls. While little data has been collected on the overall effect great horned owls have on Cooper's hawk populations, it is known that for the larger, more formidable goshawk that as many as 40% of radiotagged juveniles within a study appeared to meet their demise via horned owls. Due to their threat level, the calls of great horned owls provoke a strong response from Cooper's hawks and banders and researchers usually utilize stimuli of great horned owls to attract a mobbing Cooper's hawks. Reportedly, Cooper's hawks will temporarily tolerate and possibly even cooperate with crows when one or the other spots a great horned owl in the daytime, both species appearing to join forces to mob the threatening owl out of the vicinity. In one case, after a great horned owl pair failed to successfully breed in a nest built by other Cooper's hawks, a pair of Cooper's who tried to nest was supplanted by horned owls, possibly of the same pair who had previously failed.