Egg size decreases with increasing female tail fork depth in family Hirundinidae

Female ornamentation evolves as a nonfunctional copy of functional male ornamentation or through direct selection on female ornamentation. Because females invest many resources in both reproduction and ornamentation, an evolutionary tradeoff between female ornamentation and maternal investment can be predicted, but it has rarely been demonstrated. Using phylogenetic comparative analysis, we studied egg size in relation to female fork depth in the family Hirundinidae, which has a wide range of fork depth (from 0 to >50 mm). Because deeply forked tails are aerodynamically costly, we predicted that species in which females have deeply forked tails suffer reduced egg size due to inefficient foraging during the egg formation period. Egg length was significantly related to nest type but not to female or male tail fork depth, whereas we found a significant negative relationship between egg breadth and female, but not male, tail fork depth. As a consequence, a significant negative relationship was found between egg volume and female, but not male, tail fork depth. Because female and male fork depths were not significantly related to clutch size, clutch size would not compensate the relationship between egg size and fork depth. The current finding supports the hypothesis that female ornamentation trades off with maternal investment.