Why do some birds, but not others, nest in tightly packed colonies? About one avian species in eight is a colonial nester, either with its own kind or in mixed-species aggregations. The habit is widespread taxonomically; African weavers, relatives of House Sparrows, do it, and so do penguins, Brewer's Blackbirds, and sea gulls. As with other forms of gregariousness, whether colonial nesting evolves in a species depends on the balance between the advantages and disadvantages of the behavior from the viewpoint of the individuals involved. Social interactions related to foraging seem to be one major reason for the maintenance of coloniality in species with unpredictable food supplies that are patchy but locally abundant.

In eastern Washington, Brewer's Blackbirds forage in short vegetation around ponds and streams, mostly close to the colony site, but sometimes a mile or more away. The birds appear to dine primarily on emerging damselflies and other insects, and seem to concentrate their feeding in areas where prey are most abundant. Since the blackbirds exploit resources that are variable in space and time, natural selection does not favor individual territories for resource control. Instead coloniality apparently developed because it allows less-adept birds to follow more-successful foragers when they leave the colony to feed, and perhaps also because it provides some protection against predation.

The Brewer's Blackbird is not the only species that learns about food sources from other foragers. Cliff Swallows that have been unsuccessful in finding food, return to the colony and follow a successful forager to a food source. Most seabirds that nest together also forage together, suggesting that they too, can benefit from each others' good fortune on the hunt.

Some birds, however, nest colonially and forage alone; others forage in flocks and nest alone. Do these make the "information-center" hypothesis less likely? Not necessarily. Herons, which rely on stealth and must forage alone, still apparently learn a great deal about the productivity of remote feeding sites from other birds in their breeding colonies. Flock-feeding ducks do not breed colonially, presumably because their ground nests would be highly vulnerable to predators if grouped in colonies. But the ducks do come together daily in communal "loafing areas -- and information can be exchanged there, rather than at a nesting colony.

In the "information-center" hypothesis, reduced danger of predation is only a secondary benefit of coloniality. While increased numbers do increase chances of detecting the approach of a predator, beyond a colony size of a few hundred individuals the advantage of adding more sentries is vanishingly small. If predator detection were the major advantage of colonial breeding, why should some colonies have thousands of individuals? One possibility is that it leads to "predator saturation." Eggs and nestlings represent a large food resource, but they are present for only a short time. That time may be too brief for some predators to build or maintain populations large enough to take full advantage of the resource. On the other hand, large colonies sometimes expose their members to predation -- since some predators will be attracted to vulnerable colonies and kill many, or even all, of the individuals in the colony.

The "information-center" hypothesis gains plausibility when we consider the occurrence of colonial nesting in species that appear to have little need for mutual predator defense, such as many Old World vultures and (before its decline) the California Condor. Closely related pairs of species, in which one is colonial and one is not, offer further support. The Eurasian Lesser Kestrel breeds in colonies, while the closely related Common Kestrel, whose range partly overlaps the Lesser's, is usually a solitary breeder. It seems unlikely that the Lesser Kestrel cannot find safe places to nest where it feeds, and instead travels considerable distances to gain further security by becoming a member of a colony. The reason for the difference is probably related to diets rather than predator pressures. The Lesser Kestrel feeds mostly on insects, and presumably can learn from other colony members where locusts or other suitable prey are abundant. The Common Kestrel, on the other hand, preys largely on vertebrates -- and its success may well be predicated on intimate knowledge of a limited territory.

The avoidance of predation does seem to be the major reason for coloniality in at least some species whose prey is more uniformly or predictably distributed. For example, gaining information about food supplies seems unlikely to be an important reason for the formation of Bank Swallow aggregations. These birds do not forage in groups, 10-day-old young weigh less in large colonies than in small, and in times of hunger, survival of young is lower in large than in small colonies. There is, however, evidence for the antipredator hypothesis in this case. Predators are sometimes deterred by mobs of swallows, and mobs are bigger in the larger colonies. Also, central nests suffer less from predation than do peripheral nests, and larger colonies have proportionately more central nests.

The predator hypothesis is also given some support by the demise of the bird that was the all-time champion colonial nester. The Passenger Pigeon once nested in colonies of billions of birds covering many square miles. When its numbers were reduced to the point at which large colonies could no longer be formed, it declined to extinction in spite of the presence of abundant habitat and food and the absence of further human molestation. In large colonies, the birds presumably saturated local predators; nests of scattered survivors simply may have been too vulnerable to predation. Therefore the Passenger Pigeon may have evolved the need for the presence of large numbers before it would be stimulated to breed.

Perhaps the resolution of the information vs. predation dispute lies in the suggestion that young birds in a colony benefit most from acquiring information, while older birds gain more through protection from predation. Older, more experienced birds are almost always better able to find food than inexperienced individuals. They, however, would suffer more competition for the food they find if they are followed by colony mates to the feeding grounds. So why do they join colonies and accept that competition rather than nest alone? The answer may be that older birds are also more dominant birds, and can acquire the safest nesting sites in the center of the colony. Younger birds, in contrast, may accept a higher risk of losing their eggs and young at the periphery of the colony, but their feeding success is enhanced by the presence of more experienced birds for them to follow.

SEE: Flock Defense; Parasitic Swallows; Communal Roosting; Mixed-Species Flocking; Mobbing; The Passenger Pigeon; Disease and Parasitism.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.