Illegitimate offspring are a better investment for female house wrens

Birds have a reputation for monogamy, but it is often only an appearance. In fact, males and females alike engage in extra-pair copulations in many cases. A recent study set out to find out what’s in it for unfaithful female house wrens.

The benefit of cheating is easy to see for males: their illegitimate offspring gets reared at no expense by another pair in another nest. The rewards aren’t so obvious for the other sex. Females always provide care, and a cuckolded father might even invest less energy in the chicks; not to mention the risk of contracting STDs that comes with mating. Despite this inequality, female birds still cheat on their social mates.

House wrens are small, short-lived, passerine birds that are common throughout North America. Each year, pairs form for the breeding season, the female picking the males she desires. Males and females look alike, so she cannot assess a potential mate’s quality through showy displays or colorful attributes.

The oldest males get cuckolded the most, found the researchers. This is surprising: in the absence of other information, choosy females should consider older males better quality. This is because, well, they’re survived so far. Consistent with this, younger males also get cheated on more than intermediate aged ones.

Older birds produce lesser quality sperm. They make less of it, and it can be less motile and more mutation ridden. The clutches they produce contain fewer chicks with weaker immune systems. So why pair up with old males at all?

It turns out that female house wrens may not be as picky as we think they are. House wrens nest in cavities, and a good territory might be the most important quality for a male (the ones that have one rarely get cheated on regardless of age). However, because she won’t encounter very many mates before the breeding season, a female might just settle for the first one with a decent nesting location and start making chicks. She would then mate with the other males she encounters.

This settling and cheating strategy is a great one. The researchers found that chicks born of female extra pair mating were better quality! They were the most likely to return breeding, making more chicks themselves. Mating is a complex game, but apparently cheating pays off for female birds too.


Source: Bowers et al. 2015. Increased extra-pair paternity in broods of aging males and enhanced recruitment of extra-pair young in a migratory bird. Evolution.

Photo: Wikipedia commons