Hawks (Order Falconiformes) and owls (Order Strigiformes) are long-lived, territorial, and often show delayed maturation, traits common in cooperative breeders. Yet, of over 250 species of hawks and nearly 140 species of owls, fewer than a dozen species possess some form of group breeding. This low level of sociality may reflect characteristics of food limitation or aggressiveness distinctive to raptors (see below), or it may indicate our limited knowledge of reproductive behavior in this group. In particular, little is known about the nesting behavior of tropical raptors, yet it is there that one might expect cooperative breeding to occur. The observation that a third of the known group-breeding hawks are tropical or subtropical in occurrence suggests strongly that generalizations about raptor sociality should be made carefully.

Those hawks where more than two breeders have been recorded at a nest generally fit into three categories:

1. Unusual, probably aberrant threesomes that occur in typically monogamous species, as occurs rarely in the Red-tailed Hawk (Buteo jamaicensis: Santana et al. 1986).

2. Helper-at-the-nest systems typified by the regular occurrence of juvenile or young-adult helpers, as occurs in the Mississippi Kite (Ictinia mississippiensis; Parker and Ports 1982) and Merlin (Falco columbarius; James and Oliphant 1986).

3. Multiple-adult breeding groups that are maintained between breeding attempts.

In this paper, we focus our attention on two members of the latter categories, the Galápagos Hawk (Buteo galapagoensis), which apparently mates polyandrously, and Harris' Hawk (Parabuteo unicinctus), which may have helpers at the nest and multiple-adult breeding groups.