Tree swallows are social creatures. Breeding together at close quarters, raising their young within sight of each other, they live together not quite like the members of one family, more like the inhabitants of a row of tenements on the same block. There are social problems. The adults tend to cooperate when predators turn up, ganging up noisily when any alien bird comes by to threaten a nest, but when left to themselves they live the disorderly lives of city folk, each one trying to get ahead of the rest. They compete for nesting space, and, since some living quarters are better than others, there is a tendency towards grabbiness. They know each other as though by name; they are genuine individuals. But they don’t necessarily like each other, and they are not what you’d call real friends.

Somehow, despite the internal squabbles and constant competitions, the tree swallow societies manage to get by and survive, year after year. At first glance, it is a puzzle that they can do this, for the communities have the look of instability. All day long, the non-breeder birds, out of season for the time being, are a menace to the successful householders, darting towards their nests, threatening the new nestlings, trying various techniques of eviction. The parents are always on the watch and always under strain, compelled by circumstance to forage out for food and, at the same time, obliged to stay close enough at hand to protect their offspring against vandalism. The non-breeders, driven by the selfishness of their own genes, live in wild anxiety for the housing they will need when their turn at reproduction comes along next season. And, since they are at the moment free of family responsibilities, they can try a break-in whenever they feel like it. They have time on their hands.

But the non-breeders have another reason for getting close to a successful nest: plain curiosity. Having never surveyed such real estate or constructed a household of their own, they need to learn how. Or, at any rate, this seems to be an important part of their intrusive behaviour: they like to come close and look the place over.