Blue Tits Lay Extra-Pair Eggs Earlier

Blue tit (Cyanistes caeruleus) mating systems are a point of interest for ornithologists because they combine exclusive-seeming pair bonds with extra-pair mating. Thus their social systems have been the focus of ongoing research. A recent paper found that the chicks resulting from extra-pair mates may actually have an advantage over their siblings.

But the new findings in blue tits suggest that the superiority of extra-pair offspring might have little to do with their genes. They found that eggs fertilized by males other than the mating partner tend to be laid and to hatch earlier. Indeed, they report, nearly 75 percent of extra-pair offspring were produced in the first half of the clutch.

Hatching first helps these chicks compete against the other nestlings for food, making it more likely that they will survive to fledge and reach adulthood.

The team offers some hypotheses for why extra-pair offspring would occur early in the laying process:

And plenty of ideas have been put forth. Females may elect to mate with extra-pair males to guard against the possibility that their social partner is infertile. They might also engage in sex with other males primarily to avoid being harassed by them. In other words, it might simply be easier and less risky just to give in.

Although the new findings don't rule out any of the possibilities, the researchers said they are "perhaps most consistent with the fertility insurance hypothesis," which predicts that females should seek extra-pair copulations before laying starts so that all eggs could still be fertilized in the event of pair-male infertility....

Alternatively, he added, if extra-pair young are indeed genetically superior, placing them early in the laying order may be the mother's strategy to promote their chances of survival through the most risky period of their life.

It is not immediately clear whether any of these possibilities have been tested.