Take wing, this box is mine (Image: Bernd von Jutrczenka/dpa/Corbis)
Two species of tiny tits have been fighting a quiet war, invading each other鈥檚 nests and laying eggs that hatch alongside the enemy鈥檚 young. The confused chicks are likely to grow up thinking they are a different species, with potentially far-reaching evolutionary consequences. This all seems to be driven by the limited number of adequate nesting sites.
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When great tits, Parus major, can鈥檛 find a good place to lay their eggs, they simply invade the nests of the smaller blue tit species Cyanistes caeruleus. The blue tits are half the size of the invaders so usually abandon their nests and fly away. That may be a smart move, those that stick around to defend their homes risk being killed in a bloody takeover.
But that鈥檚 not the end of the story. In some cases the small birds get their revenge by sneaking their own eggs into great tits鈥 nests.
at the University of Castilla-La Mancha set up nearly 3000 nesting boxes in the forests of central Spain. He used these to monitor the nesting behaviour of great and blue tits over a three year period.
In 17 instances, Barrientos recorded great tits stealing the nests of blue tits and raising their chicks alongside those that hatched from blue tit eggs already in the nest. Two of these nests still contained the dead body of the previous owner.
But in 17 other cases, blue tits sneaked an egg or two into great tits鈥 nests. Overall, mixed species nests represented three per cent of all clutches, but this was as high as seven per cent in small woodlands.
鈥淭he blue tits are probably thinking: 鈥業f you take my holes to breed, at least you will rear my chicks鈥,鈥 says Barrientos. Although he is not sure if the same individuals that had their nests stolen went on to sneak their eggs into great tit nests.
Behind enemy lines
The study is the first to detail the egg-sneaking behaviour of wild blue tits, a type of so-called brood parasitism, and it is the first to provide clear evidence of nest-robbing behaviour in great tits, according to the team.
So what does growing up in an enemy鈥檚 nest do to one鈥檚 self-image? Previous research, , has shown that adoptive blue tit chicks grow up thinking they are great tits, recognising great tit songs as their own. This is commonly known as sexual misimprinting.
But these effects last only for a while, says Barrientos. Once they leave the nest, they somehow learn to recognise their own species鈥 songs. In contrast, great tits raised by blue tits spend their whole lives thinking they are blue tits, singing like their adoptive parents, says Barrientos. Sometimes they even try to mate with blue tits.
Why blue tits have the ability to correct their behaviour but great tits don鈥檛 is a mystery. 鈥淥ne hypothesis suggest that blue tits may have more to risk than great tits,鈥 says at the University of Oslo, Norway. 鈥淎doptive blue tits, thinking they are great tits, try to compete with them for food, nest sites and mates, resulting in their injury or even death, as they are much smaller.鈥
But there may be some advantages. 鈥淏eing raised by foster parents of another species may also cause learning of new songs and use of new feeding niches,鈥 says Slagsvold. And eventually, such mixed species broods may bring about important evolutionary consequences, leading to hybridisation of the two species, he says.
Journal reference: Behavioral Ecology and Sociobiology, DOI:
