Data from: North American barn swallows pair, mate, and interact assortatively

While female mate choice is well established, mutual choice may play a
larger role in mate selection than currently recognized. Assortative
mating is a common form of non-random mating in animals that can result
from mutual choice. However, few studies address assortative patterns
beyond the social pair, potentially overlooking assortativity in the
mating pair and in the social environment that shapes reproductive
decisions. We asked whether North American barn swallows (Hirundo rustica
erythrogaster) breeding in a large colony form pairs, mate (through both
within-pair and extra-pair fertilizations), and interact assortatively by
ventral plumage color, wing length, and age. Social interactions were
tracked using proximity loggers, which recorded close contact between
tagged individuals when birds were mating and laying eggs. Barn swallows
paired and mated assortatively by their ventral plumage color; however,
the assortative patterns in mating pairs were not as strong as they were
in social pairs. Barn swallows also interacted assortatively, associating
more often with individuals of both sexes who had similar phenotypes
relative to the other birds in the colony. Finally, older males and
females with darker ventral plumage achieved the highest reproductive
success. Investigation of assortative behavior beyond the level of the
social pair provides a more complete understanding of mate choice and
suggests a mechanism that may maintain the large variation in ventral
plumage color in North American barn swallows.