The benefits and drawbacks for birds of nesting in groups seem to be highly situational. Although scientists have hypothesized about the advantages of group nesting in terms of enabling group defensive behavior, escape from predation by being surrounded by neighbors (called the selfish herd hypothesis), as well as escaping predators through sheer numbers, in reality, each of these functions evidently depends on a number of factors. Clearly, there can be safety in numbers, but there is some doubt about whether it balances out against the tendency for conspicuous breeding colonies to "attract" predators, and some suggest that colonial breeding can actually make birds more vulnerable. At a common tern colony in Minnesota, a study of spotted sandpipers observed to nest near the tern colony showed that the sandpipers that nested nearest the colony seemed to gain some protection from mammalian predators, but avian predators were apparently attracted to the colony and the sandpipers nesting there were actually more vulnerable. In a study of a least tern colony in Connecticut, nocturnal avian predators in the form of black-crowned night herons and great horned owls were observed to repeatedly invade a colony, flying into the middle of the colony and meeting no resistance.