Why hungry chicks are missing out on caterpillars

WARMER springs are leaving birds ‘out of sync’ with the arrival of the juicy caterpillars they feed their young on.

Birds lay their eggs so they will hatch when the food supply their offspring depends on is at its most abundant. But with spring now arriving around two weeks earlier than 50 years ago, caterpillars are getting ahead of the game. They now emerge considerably earlier than the chicks that eat them.

The worst affected bird is the pied flycatcher, which arrives from Africa in the spring – only to find its chicks are 13 days too late for prime caterpillar feeding season, a study led by the RSPB found. Blue tits are also missing out by 3.4 days, as well as great tits, whose chicks were emerging two days later than ‘peak caterpillar’ time.

With continued spring warming expected due to climate change, the study published in the journal Nature Ecology and Evolution warned the hatching of forest birds will be ‘increasingly mismatched’ with peaks in caterpillar numbers.

However, the research team, which included the universities of Exeter and Edinburgh, found the imbalance in caterpillar supply and demand was not large enough to blame for the decline in some bird populations.