More food!

Begging beaks

Reed warblers usually produce four chicks, whose gaping beaks stimulate the parents' feeding behaviour. The more wide-open beaks in the nest, the more food-gathering trips.

Cuckoos problem

A cuckoo chick is much larger than a reed warbler's and requires more food. But it has only one beak to gape with, and is therefore at risk of getting too little food. But the cuckoo has a trick!