Earlier springs driven by climate change are creating a “mismatch” between when caterpillars hatch and baby birds are feeding, scientists have warned.
Data collected from “citizen scientists” across the UK has helped researchers compare the emergence of oak tree leaves and caterpillars and the timing of nesting by blue tits, great tits and pied flycatchers.
With spring coming earlier due to rising temperatures, leaves and caterpillars emerge earlier in the year, and forest birds which feed on them have to breed sooner to avoid missing out on food sources for their hungry chicks.
The earlier the spring, the less able the birds are to do this, and the peak in caterpillars is more out of sync with the peak in chicks demanding food, the study published in the journal Nature Ecology and Evolution found.
With continued spring warming expected due to climate change, the scientists warned the hatching of forest birds will be “increasingly mismatched” with peaks in caterpillar numbers.
The biggest disparity was for pied flycatchers, a migratory species which are not in the UK in winter to react to earlier warm weather, though they feed their chicks more winged insects so may be less dependent on the caterpillar peak.
The research team – led by the RSPB and universities of Exeter and Edinburgh, as well as Durham, Sheffield, Glasgow, Stirling and Cardiff – found no evidence to support the theory that it is worse in southern Britain than the north, where birds might be “buffered” from climate change.
Population declines of birds which feed on insects in southern Britain do not seem to be the result of species facing a greater disconnect between caterpillars emerging and when they nest, the researchers said.
Dr Malcolm Burgess, of the University of Exeter and RSPB, said: “Forests have a short peak in caterpillar abundance, and some forest birds time their breeding so this coincides with the time when their chicks are hungriest.
“With spring coming earlier due to climate change, leaves and caterpillars emerge earlier and birds need to breed earlier to avoid being mismatched.
“We found that the earlier the spring, the less able birds are to do this.”
Dr Ally Phillimore, from the University of Edinburgh, added: “We found no evidence of north-south variation in caterpillar-bird mismatch for any of the bird species.
“Therefore, population declines of insectivorous birds in southern Britain do not appear to be caused by greater mismatch in the south than the north.”
The first leafing dates of oak trees were collected by members of the public through the Woodland Trust’s Nature’s Calendar scheme and caterpillar abundance was monitored by collecting droppings under oaks.
And the timing of egg laying by blue tits, great tits and pied flycatchers were recorded by volunteers in the British Trust for Ornithology’s nest record scheme.
Martha Boalch, from the Woodland Trust, said: “As seasons and the weather continue to fluctuate, we need to understand our native flora and fauna is coping.”
She said citizen scientists who collected data for the Nature’s Calendar project, which records the changing seasons, helped provide scientific data on a scale that could not otherwise be achieved.
