How the cuckoo exploits nature's mothering instinct

 

Cuckoo and Reed Warbler, painting by Robert E Fuller

As Mother's Day approaches I'm reminded of how strong the nurturing instinct is in parent birds and how some birds run themselves ragged trying to keep up with the demands of their young. This really hit home one spring when my father and I watched a reed bunting that had been tricked into thinking a cuckoo chick was its own. The poor thing was so exhausted fetching more and more food for a chick that was three times its size - it was like watching a modern mum tirelessly providing for overgrown teenagers!

We had been photographing water birds at a flooded disused gravel pit. From makeshift hides we had our sights on a great crested grebe nest, a little grebe nest and a reed bunting nest. While we were hidden away, a pair of cuckoos, fresh in from Africa, attracted our attention with their unmistakable call. These birds are the true harbingers of spring and a sure sign that the warmer days are on their way. The cuckoo has a unique breeding habit. It is notoriously parasitic, finding foster nests for its eggs and tricking other birds into raising its chicks. Favourite hosts include meadow pipits, dunnocks, pied wagtails and reed warblers. And, remarkably, different female cuckoos will target different species. One will focus on tricking dunnocks each year and lay matching bright blue eggs. Another will lay green-ey blue eggs, speckled with brown - a perfect match to trick the reed warbler.





My father and I watched as the male announced his presence from a high perch with his namesake call: “cu-coo”. The female, meanwhile, was already carrying out lengthy and detailed investigations into the comings and goings of her chosen ‘host’ species. She was flying down into the reed beds to search out their hidden nest sites. We knew her speciality must be the reed warbler’s nests.

Timing is of course essential for a successful deception. To avoid detection, the cuckoo needs to remove an egg from her host’s nest and lay her own matching egg all without arousing suspicion. My father and I started chatting about how great it would be to find a nest with a cuckoo chick in it.

“Yes, but it would be like looking for a needle in a haystack, Robert”, my dad warned. That didn’t stop us trying. We carefully combed through several acres of reed bed, both of us in chest waders, in search of the suspended warbler’s nest - the size of a teacup. No joy. We refocused on our grebes and then some days later, when I’d stopped even thinking about it, my father called to me.

“I’ve found it!

“Found what?”

“The egg.”

“Which egg?”

“The cuckoo’s egg! I’ve found it.”

The egg was almost a perfect match, just fractionally larger than the others. We kept a close watch on the nest and sure enough the cuckoo’s hatched first. An unusually large chick came out and, immediately, its instincts kicked in. Though still blind, the chick set about using its specially hollowed back to flip its competitors overboard into the boggy water below. There would not be enough food or space for anyone else.

We decided to build a hide to study this unusual event – just a bit of a challenge given that we were three-feet in water. We built a scaffold tower and covered it in hessian sacking so that we could watch unnoticed from six feet away. The duped foster parents cared for the cuckoo chick, assuming it to be their own, and put in an extraordinary 16 hours a day to raise it.

The cuckoo chick has a special tactic to drive its adopted parents on. Rapid shrilling calls trick its surrogate parents into thinking their nest contains a whole brood of chicks and their instinct to feed these young is unstoppable. The chick grew at an astonishing rate. Even its parents were looking a bit surprised and, by now bedraggled, by the chick’s unquenchable hunger as they came back again and again with more food supplies.

And then I heard the same shrill call close by. I realised that this wasn’t the only young cuckoo in the reed beds. There was another in the reed bed on the other side of the water and yet another in the corner of the lake. Now that I had my ears and eyes tuned in, I quickly located a further five reed warbler’s nests with cuckoo chicks in, planted, I suspect, by the same pair. My cuckoo was now overflowing the nest. His flight feathers had come through and he was getting close to fledging, building up his wing muscles by regularly flapping. This nest was only designed for a neat clutch of reed warblers and not this heavyweight. I made some running repairs, tying up the reeds below the nest to give it extra support.

A thunderstorm the previous night made me head down to the gravel pits early one morning, but the nest was empty. Thankfully, just three metres away I spotted the cuckoo chick clinging onto a reed half-submerged in water. It was time to get the chest waders out again. I brought the chick back to the bank and put it on a branch to dry off. No sooner had I turned around to get my camera, it was calling its surrogate parents for food, making up for lost time. The reed warbler’s arrived on the scene immediately and frantically fed their soggy chick. Sometimes they had to perch on its back to supply it with food as it was now so big. But no other cuckoo chicks were calling. On closer inspection, three cuckoo chicks were floating dead in the water and there was no sign of the other four. I suspect that the surviving chick was the one whose nest I had modified.

Cuckoo chicks leave the nest after three weeks of birth and are feed by their surrogate parents for a further two weeks outside the nest. Three weeks after that the clumsy chick will follow its true parents on their 3,000 mile migration down south. Reed warblers normally have two broods of four or five chicks per year. But will only be able to raise one cuckoo chick in that time. Cuckoos are nature’s favourite cheats and also one of our shortest visiting migratory birds as they leave our shores in August. For reed warblers, at least, it is maybe just as well. After watching this one I feel it deserved a mother's day reward!