Blue Tits Delay Nesting

We have been watching pairs of blue tits house hunting in our oak trees since February and we predicted an early nesting season for the species. But this period of cold weather with frosty nights may have slowed things up.

Oak trees are a favoured nesting site for blue tits because, if all goes well, they feed their young on the caterpillars of the winter moth (Operophtera brumata) that lays its eggs only in oak trees and times their hatch to the emergence of the new oak leaves in spring.

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[1] flightless winter moth female; [2] winter moth caterpillar

The birds seem to be able to time the hatching of their eggs to coincide with the hatching of the moths’ eggs. This mechanism is not well understood, but it appears there are variables in bluetits’ nesting behaviour that they can use to match local conditions that affect the caterpillar hatch. They can, for instance, vary the time from the laying of their first egg until incubation begins, the size of their clutch, and the intensity at which they incubate.

This year’s cold snap may delay the oak trees coming into leaf, which can delay the winter moth hatch; the blue tits, which have already found nest sites and built nests, will then delay egg-laying or incubation.

The signals between tree, moth and blue tit are still the subject of research but it appears that climate change is happening so fast now that none of these three species has had time to adapt their behaviour. Local conditions are subject to new and more frequent extremes, the signals that enable the relationship between the three species are therefore less clear, the inter-relationship is becoming a less dependable survival tactic, particularly for the blue tits .

Blue tits are an adaptable species but their dependence on the winter moth hatch for their own breeding success might well be a danger in an unpredictable future.

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A series of photographs by DKG from a 2019 blue tit’s nest in one of the oak trees between Cornfield and Sleepers Field