…but I’ve just become a big fan of house wren chicks. Here’s why.
Despite being laid over the course of several days, house wren chicks in the same brood typically fledge within hours of each other. This means that some chicks are considerably less developed than their older siblings when older chicks are ready to fledge. Researchers at Illinois State University have shown, however, that older siblings deliberately postpone their fledging to increase the survival chances of younger broodmates.
House wren chicks fledged when they were around 15 days old – but in asynchronously hatched broods (where the chicks had been laid over a longer period), the age at fledging was significantly higher. Bowers et al. also conducted a cross-fostering experiment, artificially altering the age variation within the brood by introducing chicks from other broods. In these experimental nests, they found the same result – where there was high variation in age among the chicks, fledging was delayed.
The study confirmed that delayed fledging was beneficial for younger siblings – they were more likely to survive to the next year – whereas the delay had no significant effect on the larger, older chicks. Although overall delay was beneficial for the current brood, it also meant that house wren mothers were less likely to produce a second brood in the same season.
So, why did older chicks postpone their own fledging for the sake of their younger siblings if it didn’t affect their own chances of survival, and negatively affected their mother’s chances of reproductive success?
Bowers et al. suggest that the older chicks are acting on kin selection. Kin selection, a theory developed by W.D. Hamilton, states that if an organism performs an action that helps its close relatives to breed, the costs of helping are overcome by the benefits of having your genes passed on in the offspring of your kin. In this case, delaying fledging increases the chances that a house wren’s younger brother or sister will survive to produce its own chicks – chicks that will carry on parts of the same genetic code.
In this case, it pays to help your relatives. So, why then are future siblings disregarded? House wrens live in monogamous pairs, but females may mate with other males during the breeding season (Forsman et al. 2008) – two broods produced by one female in the same year, then, may have different fathers. According to kin selection, therefore, chicks should favour the survival of their own brood over the survival of the next. This is an example of parent-offspring conflict: it is in the chicks’ best interests to remain in the nest until they are all ready to fledge, but it is in the mother’s best interest that the chicks fledge early so that she may reconceive. Perhaps not such cute offspring after all…
lovely photo from Yalonda_Maneval’s photostream