Cheryl Cronnie, who has been monitoring a blue tit nest hole in one of the the reserve’s veteran oaks, has discovered and photographed an interloper, a possible nest thief: a great tit.

Blue tits (Cyanistes caeruleus) and great tits (Parus major) have always competed for nest sites: they are both small woodland species that nest in holes in trees. However, in the last 20 years the UK has lost 14% of its tree cover, the number of adequate nest sites has therefore fallen and the competition between the two species grows fiercer.

The original claimants: a pair of blue tits.

A study reported in New Scientist has found that if great tits are unable to find an appropriate nest site, they will invade a blue tit nest. The blue tits, half the size of a great tit, usually abandon the nest and leave their clutch of eggs. But the great tits will sometimes hatch the blue tit eggs as well their own. In return, blue tits ousted from a nest, will sometimes come back and sneak their own eggs into what is now a great tit’s nest.

In a study of 300 woodland nest boxes over a period of three years, scientists found 17 instances in which great tits raised blue tit chicks after the blue tit parents had been ousted from the nest and another 17 instances in which blue tits smuggled eggs into great tit nests: a form of brood parasitism.

At the end of the study, it was found that a minimum of 3% of all nests were mixed species nests, rising to 7% in smaller areas of woodland where there were fewer nest sites.

The interlopers: a pair of great tits.

The outcome is that blue tit chicks raised as great tits will revert, a few weeks after fledging, to blue tit behaviour but that great tits, reared by blue tits, will think of themselves as blue tits for the whole of their lives.

Such drama in our oak trees!

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