DNA fingerprinting reveals relation between tail ornaments and cuckoldry in barn swallows, Hirundo rustica

In an experimental study in Denmark, it was previously found that male barn swallows (Hirundo rustica) with elongated tail streamers obtained an apparent fitness advantage through earlier pairing, an increased frequency of second clutches, and higher total reproductive success per season. In a parallel study of five barn swallow colonies in Ontario, Canada, we also found that elongated males paired earlier and thus were apparently preferred by females. Now, using DNA fingerprinting on families from two of those Ontario colonies, we show that five elongated males fathered only 59% of the offspring in their nests, whereas six shortened males fathered 96% of their nestlings. Thus, although elongated males were clearly preferred by females at the time of pair formation, tail elongation may have hampered the ability of a male to guard his mate, resulting in an increase in extrapair fertilizations (EPFs). A significant negative correlation between the number of EPFs and natural tail length in this experimental study also suggests that tail streamer length may reflect male quality. (Behav Ecol 1990; 2: 90–98)