Early springs mean less food for hungry nestlings

For millennia, the oak forests of Britain have had a tightly timed spring-summer cycle – the oak leaves unfurl, the caterpillars feast on them, and the forest birds feed the caterpillars to their chicks. Now, with a warming climate, the leaves and caterpillars are emerging earlier, creating a mismatch between peak food resources and hungry chicks. 

New research published in the journal Nature Ecology and Evolution suggests that as spring warming continues due to climate change, the hatching of forest birds will be “increasingly mismatched” with optimal caterpillar numbers.

“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,” says lead author Malcolm Burgess, of the University of Exeter and the Royal Society for the Protection of Birds (RSPB).  

“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.”

Burgess and colleagues used citizen scientists to gather data on the oak-caterpillar-bird system across Britain, including the spring emergence of leaves, the amount of caterpillar droppings, known as frass, under the trees, and finally the timing of nesting of three forest bird species; blue tits, great tits and pied flycatchers. 

Bird life cycles were calculated using first egg dates (FED) from across the region covering the period 1960 to 2016, using nests from 36,839 blue tits (Cyanistes caeruleus), 24,427 great tits (Parus major) and 23,813 pied flycatchers (Ficedula hypoleuca).

The researchers found substantial trophic mismatch in the oak forests across the United Kingdom.  

The researchers found that in recent times maximum demand from nestlings and peak caterpillar numbers were out of synch on average by 3.39 days in blue tits, 2.01 days in great tits and a substantial 12.87 days for the pied flycatcher.  

Burgess suggests that the greater mismatch in the pied flycatchers was due to the fact that “as migratory birds, they are not in the UK in winter and therefore are much less able to respond to earlier spring weather.”{%recommended 6908%}

The mismatch between caterpillar abundance and great tit nestlings has been determined previously. However, this study presents the first assessment of whether the effect is greater in southern Britain than in the north. Surprisingly, early spring had the same effect in both regions.

Co-author Ally Phillimore, from the University of Edinburgh in Scotland, says, “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.”

Oaks and winter moths (the main caterpillar food of the birds) have more plasticity than vertebrates in the timing of their spring cycles – and respond rapidly to warming temperatures. 

It seems the only way the birds will catch up is to evolve.  

“Our work suggests that as springs warm in the future, less food is likely to be available for the chicks of insectivorous woodland birds unless evolution changes their timing of breeding,” says another of the researchers, Karl Evans, from the University of Sheffield’s Department of Animal and Plant Sciences.