Per the Beeb:
The chicks hatch and are fully grown within two weeks, so they need something that’s really abundant – that’s why they synchonise their breeding so hatching co-incides with the emergence of the caterpillars.”
The caterpillars’ appearance is triggered by ambient temperature – that has been shown in the laboratory – and it is believed that great tits also begin their breeding cycle in response to temperatures.
On the other hand, the caterpillars aren’t coping as well.
