I bought my summer bedding today, and while I was digging the old compost out of a pot I came across this chap:
Many birds time their breeding season around the emergence of caterpillars. They are wet, soft and protein-rich, and make ideal food for their young chicks, so it makes sense for their eggs to hatch when there are lots of them about.
In turn, the peak time for caterpillars is determined by there being enough soft young growth for them to eat – which depends on the weather. How birds are able to make this fine calculation – which can differ each year, and on which their breeding success may depend – remains a mystery.