Barn Swallows tolerate extra adults at their nests. These extras may help at the nest for an entire breeding season and are not necessarily related to the nesting pair. This situation sometimes leads to polygyny. Other helpers may replace a deceased pair member. The extra adults do not help much in feeding the young, but do assist in building the nest, incubation, and brooding. A nest with helpers is often “owned” by an older female, and the hypothesis is that male attendants are trying to secure high-quaility mates. Juveniles from first broods also help at the nest. In this case, the helpers do supply substantial amounts of food. Occasionally unrelated juveniles serve as helpers.
Sometimes extra adults are chased off. Curiously, reproductive success is not affected by the presence of adult helpers. The extra adults are most often found when swallow populations have a skewed sex ratio, and not all individuals can find a mate or a nesting site. Related juvenile helpers benefit from the experience of “baby sitting,” and assuring the survival of their parents’ genes. Why unrelated juveniles would be tolerated is less clear (Brown and Brown 1999).