Do you see the pair of tits in the above photo? Pretty nice, right? Well, they're not Japanese great tits (Parus major), which use syntax. They're Eurasian blue tits (Cyanistes caeruleus), which scientists find fascinating for other reasons.
After carefully observing these tits for a long time, researchers at the Max Planck Institute for Ornithology concluded that the females among normally monogamous birds sometimes stray and mate with male birds other than their social mates. ITV reports:
Scientists found that birds which often foraged together in the colder months were more likely to end up as breeding pairs or partners outside of their couple.
They say many socially monogamous bird species engage in sexual behaviour outside their pair bond, often resulting in chicks. [...]
They studied blue tits that typically form monogamous pairs, but frequently mate outside their pair.
First-author of the study, Kristina Beck, said: “As most extra-pair sires are close neighbours, one could think that extra-pair paternity in blue tits might simply be the result of coincidental meetings between neighbours, and not a social preference for specific mating partners.”
About half of all nests contain at least one young with a genetic father other than the social one, and up to 15% of all offspring are sired by extra-pair males.