You know the story of mammalian fertilisation: millions of sperm enter the vagina, only one fertilises the egg, more than one messes up the embryo, yadda yadda yadda. Turns out that's not the only way it can work.
Birds, for example, always have multiple sperm penetrating each egg. Only one of those sperm actually fertilises the ovum, but the rest hang around inside the egg's cytoplasm. This quirk of bird biology was first observed in 1904, but no one knows what they're doing in there.
A study in the latest Proceedings of the Royal Society B has taken some baby steps toward understanding their function. Nicola Hemmings and Tim Birkhead from the University of Sheffield artificially inseminated zebra finches and chickens with low numbers of sperm and compared the result to either (in the case of zebra finches) natural copulations or (in the case of chickens) artificial insemination with large numbers of sperm.
They found that if bird ova aren't penetrated by multiple sperm, they won't develop into chicks. No one knows exactly why embryonic development in birds needs the extra sperm. It may be related to the size of the bird egg: sperm carry proteins that activate embryonic development, but experiments with Japanese quail suggest that their eggs need more of those proteins than a single sperm can carry.
Hemmings and Birkhead also found that a larger proportion of sperm than expected made it to the egg in the low-sperm experiments, suggesting that the mother bird is somehow controlling how many sperm survive the trip through their oviducts.