Each spring, reed warblers arrive here in Britain to raise their young. But so does one of the most remarkable birds: the cuckoo. One of nature's greatest tricksters, this bird fools other species to raise its chicks. This is it's incredible story.
Searching for cuckoos
Cuckoos are famous for their extraordinary parasitic behaviour — they lay their eggs in the nests of other species and trick them into raising their chicks. To understand how they do this, I went in search of reed warbler nests - since this species is one of their favourite hosts. Out of 15 reed warbler nests hidden in a reed bed, I eventually came across a cuckoo chick. Meanwhile I'd also found a reed warbler nest with four reed warbler eggs inside.
Comparing nests
I filmed both bird nests in tandem and watched as the first warbler chick hatched and, shortly afterwards, the female began to probe it with her beak - testing its strength to ensure it survives. The adults worked hard here, offering the chicks their first insect meals when they were just hours old and clearing the nest of broken egg shells to deter predators. As I filmed the reed warbler, my cameras also rolled from inside a new hide where the cuckoo chick was so big it filled the whole reed warbler nest. It was heavy too, and as it grew its weight caused the nest - built for a brood of much smaller birds - to tilt alarmingly, especially during high winds. This chick's persistent call was designed to sound like a whole brood of warbler chicks.
Reed warblers
Listening to the brood at the other nest, you couldn't tell the sound apart. At four days old, the chicks needed food and the parents were constantly arriving with beakfulls of insects. A series of black dots, thought to act like targets, guiding the parents to deliver food into each beak, appeared inside their beaks as they gaped.The cuckoo chick required so much upkeep and the warblers worked tirelessly to keep it fed - even stuffing a damselfly into its wide, red gape. They too were tidy, I watched them take away the chick's faecal sack shortly after feeding it. And, for one comical moment, I noticed the female tried to brood this enormous chick.
Raising a cuckoo
Raising a cuckoo chick took the reed warblers twice as much time and a LOT more effort than bringing up four chicks of their own. But the reed warblers were tireless workers. In the final chapter of this story, I discover how, even when heavy rain pushed them to their limits, reed warbler foster parents never give up on their cuckoo chick imposter.
