Everyone knows that females are programmed to be monogamous and males to be promiscuous, since a female is limited in how many offspring she can have in any period of time but males, by spreading their seed far and wide, have practically limitless opportunities for paternity. There is therefore little adaptive advantage to a female’s promiscuity, but lots of evolutionary pressure for males to mate with anything that will stand still, goes the dogma. Except that someone forgot to tell the blue tits.

These female birds often mate with males other than their regular partners, scientists have long known, but the reason has been a bit of a mystery. A new study suggests that the old favorite explanation—that these “extra-pair” copulations (as in, outside the pair bond) produce genetically-superior offspring—might not be the right explanation. Instead, scientists report online in Current Biology, when female blue tits stray, producing broods of chicks that have different fathers, the offspring of fathers other than mom’s regular mate may get a head start in life, suggests Michael Magrath of University of Groningen, The Netherlands.

Blue-tit eggs fertilized by males other than the regular mate tend to be laid before eggs fathered by the mate, they report, and to hatch earlier, too: almost 75 percent of the extra-pair eggs were laid in the first half of the clutch. As a general rule, “earlier hatching chicks perform better than later hatching siblings,” notes Magrath, because by hatching first they get an edge in competition for food. In fact, this early emergence accounts for almost all the differences between the offspring of mom’s regular mate and those of her extracurricular friend, he says, “indicating that non-genetic laying order effects largely accounted for the observed superiority of extra-pair offspring.”

What remains unknown is why Lothario’s eggs hatch first. But if extra-pair matings produce chicks that are first out of the gate, it may be a reflection of the ultimate reason for female promiscuity: as insurance against the possibility that her regular mate is infertile. By seeking other fathers for her chicks, the female blue tit can make sure that all her eggs are fertilized even if her mate is shooting blanks. “Because birds can store sperm for an extended period in specialized storage tubules, females may have little need to continue engaging in extra-pair copulation after laying starts, and this would rather neatly explain the decline in extra-pair offspring that we observed with laying order,” Magrath said.