Little yellow birds called great tits are at war with smaller birds called blue tits. Great tits are stealing the nests of blue tits, and to retaliate, blue tits are sneaking their eggs into the nests of great tits, forcing them to raise chicks that aren’t their own. The two tits resort to these tactics when faced with nest shortages, according to new findings published in Behavioral Ecology and Sociobiology.

Interspecific brood parasitism is when eggs from two species are incubated by a single female. To detail this phenomenon in blue tits (Cyanistes caeruleus, pictured above) and great tits (Parus major, pictured below), a team led by Rafael Barrientos from Universidad de Castilla-La Mancha monitored 38 forest plots over a span of three years. Both species nest in the cavities of trees as well as in artificial nest boxes set out by the researchers.

The team found a total of 39 mixed-species clutches in 1,285 nests. That’s only 3% overall, but the proportion was as high as 7.2% in small woodlands. The higher the nest-box occupation rate, the greater the prevalence of mixed-species clutches.

The two species behaved very differently when they were faced with nest-hole shortages. In 17 of the mixed nests, blue tits had slipped one or two eggs into clutches incubated by great tits. In 17 other nests, great tits had invaded and laid eggs in blue tit nests. These hostile (and sometimes bloody) nest takeovers happened more often at the end of the season. These differences in strategies are likely due to one simple factor: Great tits are bigger, so they’re the usurpers.

“The blue tits are probably thinking: ‘If you take my holes to breed, at least you will rear my chicks,’” Barrientos told New Scientist. Although, he’s not sure if the sneaky blue tits are the same ones who were robbed of their nests in the first place. And while blue tits raised by great tits think of themselves as great tits for a while, they learn to recognize their own species’ songs after they leave the nest.

Their size difference might also explain why great tit chicks have higher hatching and fledging rates than their blue tit broodmates raised in the same nest. Blue tit chicks from pure broods had higher hatching and fledging rates as well. However, great tit chicks raised in pure broods didn’t seem to have any advantage over those raised in mixed ones.

According to the team, this is the first study to detail the egg-sneaking behavior of wild blue tits and the nest-robbing behavior of great tits. Mixed-species clutches appear to be a response to nest-hole shortage, a concept the researchers call the “last resort hypothesis.”



Image in text: Great tit (Parus major) in autumn. Victor Tyakht/shutterstock