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What first drew you to cuckoos?
I鈥檓 fascinated by the evolutionary arms race between the cuckoo and the birds that it parasitises. It鈥檚 a constant ding-dong battle. And there is always more to learn. We have recently discovered that the UK鈥檚 cuckoos winter in the Congo rainforest, thanks to . The birds spend the winter with lowland gorillas in the rainforest, then cross the Sahara during a 60-hour, non-stop flight to the UK. It鈥檚 a tough migration: several of the tracked birds didn鈥檛 make it.

How are cuckoos faring in the UK?
Sadly, our cuckoo numbers have been plummeting, dropping by more than half in just two decades. We aren鈥檛 sure why. It may be because there鈥檚 been a catastrophic decline in many moth species in the UK in the last few decades, meaning fewer hairy caterpillars for the cuckoos to feed on. Habitat destruction might also be a cause. Whatever the reason, we鈥檙e not the only ones to have noticed it.

What else has spotted the cuckoos鈥� decline?
Reed warblers. I鈥檝e studied common cuckoos [Cuculus canorus] and the warblers they parasitise in the fens of eastern England for 30 years. A cuckoo lays one egg in a host nest. In the old days, one in five cuckoo eggs were rejected by the warblers. It鈥檚 a risky defence, because sometimes the warblers mistakenly reject one of their own eggs. Today, with fewer cuckoos around, the warblers have relaxed that defence: they鈥檙e much less likely to reject eggs.

What if the warbler misses a cuckoo鈥檚 egg?
The warblers have evolved to reject eggs that look different to their own and the cuckoo responded by evolving egg mimicry 鈥� their eggs are a beautiful match. If a cuckoo鈥檚 egg isn鈥檛 detected, the cuckoo hatchling tosses all the warblers鈥� eggs out of the nest, along with any warbler chicks that happen to be there. What鈥檚 extraordinary is that the parent warblers will sit and watch, doing nothing to interfere even as they witness the destruction of their brood.

So if the egg is missed, the cuckoo has won?
Yes, because the warblers don鈥檛 reject chicks that look different to their own. And then the cuckoo chick will trick the warbler parents into feeding it by using a begging call that sounds like a whole brood of hungry warbler chicks. In Australia, where cuckoos have probably been battling with their hosts for longer, the arms race has gone to the next level. There, some host birds will eject a newly hatched chick that doesn鈥檛 look like their own. In response, those cuckoos have evolved chick mimicry as well as egg mimicry.

Why do we like these cheating, chick-killing freeloaders so much?
I think it鈥檚 partly the romance of them being a harbinger of spring, and partly because their sound is so instantly recognisable: Wordsworth referred to them as a .

Yes, they are cheats, but I think we admire them for getting away with it.

This article appeared in print under the headline 鈥淐uckoos against the clock鈥�