Extra Adults at Harris' Hawk Nests

Abstract

Nests of Harris'' hawks in Arizonia [USA] were found with 3 adults. Threesomes may be more successful than twosomes since an extra adult could probably increase prey supplies at a nest, thereby insuring greater fledgling success in low prey years; a 3rd hunting adult would leave more time for the primary nest-attending hawk to guard the nest from predators; and, if 1 adult perished, the other 2 could probably shift their nest duties and avoid loss of the brood. Twosomes and threesomes of Harris'' hawks laid approximately the same number of eggs per nest, supporting the idea that most if not all threesomes consisted of 2 males and 1 female. Nest-helping may be an adaptation that increases nesting success due to better nest attentiveness and prey procurement in a desert environment where food resources are scattered and subject to seasonal or yearly fluctuations.