Reed Warblers Work Twice As Hard To Raise Giant Cuckoo Chick | Discover Wildlife | Robert E Fuller

By comparing two reed warbler nests, one where they were raising a brood of their own chicks and one where they raised a cuckoo chick, I discover just how hard-working this species is! 

For this film I documented two nests and saw how, even when heavy rain pushed them to their limits, reed warbler foster parents gave their all to raise a chick that was not their own.

Cuckoos v reed warblers

Cuckoo chicks hatch in a nest that is not their own, relying on a host species to raise them. I found this one after searching 15 reed warbler nests and followed the story as a pair of reed warblers dedicated their time to parenting. Meanwhile I also followed a reed warbler nest where a pair were raising four of their own kind.

Diet & feeding

In the reed warbler nest, feeding four mouths was no easy feat. At four days old the chicks seem to be constantly hungry.  Interestingly the parents delivered small insects, appropriate for their small-sized beaks. Meanwhile the cuckoo's foster parents were also hard at work, in this case bringing in larger insects like damselflies and even bumblebees - as though each menu had been carefully tailored to suit each chick.

Cuckoo calls change

The chicks in both nests called out for their parents to feed them whenever there was any movement nearby. But at about 16 days old the cuckoo chick began to make a different sort of call. The sound, a 'cheeping' was intended to help the reed warblers locate it once it had left the nest. Once its foster parents turned up, the sound switched back to its original 'begging' call. 

Brooding 

The reed warbler chicks grew quickly and the female soon struggled to brood them all. When they were genuinely too big to brood she switched to sitting at the side of the nest and also turned her efforts to hunting. Meanwhile the cuckoo chick threatened to outgrow its nest at just one week old and by 16 days you could barely see the nest beneath it.

Feather development

Once that they were no longer being brooded, the reed warblers began to quickly grow their feathers, and over the course of just two days the difference was palpable - which was just as well because when bad weather hit these tiny chicks became very vulnerable.

Rescue

Thankfully the storm didn't affect this brood of four, or the cuckoo chick, but when I went to check on them I discovered another reed warbler nest had collapsed and a tiny chick had fallen. After waiting to see if the parent birds would feed it where it was and realising the chick was too cold to respond I picked it up and warmed it in my hands before putting it back in the nest.

Fledging

At 11 days old, the reed warbler chicks fledged, making their way up through the reeds. But at this stage these young birds were so vulnerable and I watched nearby kestrels with concern. Thankfully the chicks survived and it was amazing to watch them practise their flying skills. In a few weeks time these birds would make their long migrations back to Africa.
In contrast, the cuckoo left the nest at 18 days but it was by no means able to fly. Thankfully after its first venture outside the nest it landed in an old moorhen nest and not in the water below. As the cuckoo learned to fly, the reed warblers continued to feed their giant chick- sometimes standing on its back to reach its beak.

Reed warblers can raise two broods in a normal breeding season, but when their nest is taken by a cuckoo they are unlikely to breed again.