The idea for this post actually came from a guest I had the other day. I was genuinely stumped by a question they asked:
“What is the evolutionary advantage of cuckoos being brood parasites?”
I suddenly realised that, even though we hear them calling all around us now that they’ve migrated back to South Africa, I’d never stopped to think about the fundamental ‘why’. I knew what they did, but not why it was a better strategy than simply raising their own chicks. As things often unfold out here, one simple question sent me down a rabbit hole of learning once again, which is the beauty of this place. Learning never stops!
One of the first cuckoos we hear during our summer, the Klaas’s Cuckoo
For anyone unfamiliar with the term, a brood parasite is a bird that lays its eggs in the nest of another species and relies entirely on those other species to incubate and raise its young. Instead of investing time and energy into building nests, defending them, and tirelessly feeding hungry youngsters, cuckoos hand over the parental duties and disappear.
At first glance, it appears almost deceitful, a shortcut through the usual demands of parenthood. But out here, the bush doesn’t operate according to our ideas of fairness—it works according to what delivers the best chance of survival and reproduction.
After a bit of patience waiting for it to come out, we were able to get a great view of this Diederik’s cuckoo.
The Efficiency Advantage
So why would evolution favour this seemingly sneaky strategy?
The simple answer is efficiency. Raising chicks is exhausting. It requires vast resources, food, safety, and constant parental attention. By offloading all of that responsibility, cuckoos free themselves to focus on foraging, migrating and, most importantly, producing more eggs.
A female cuckoo that isn’t tied down to a single nest can lay in multiple hosts’ nests during one breeding season, dramatically increasing the probability that at least some of her offspring will survive to adulthood. This high-volume reproductive strategy is a powerful evolutionary advantage.
Masters of Adaptation
The adaptation doesn’t stop at simply ditching the nest. Cuckoos have become masters of survival mechanisms. Over thousands of years, cuckoos have become masters of mimicry. Their eggs often match the colour and size of their chosen host’s clutch so well that even the most attentive parent can’t tell the difference.
Cuckoo chicks often hatch more quickly and are able to demand food with such intensity that they outcompete the host’s own brood.
While the majority of cuckoos can perfectly match their host’s eggs, this isn’t universally true. Some species, like the Jacobin Cuckoo, which parasitises the Dark-capped Bulbul, do not mimic their eggs, yet the bulbul still raises the chick as if it were its own. The mechanisms behind this successful non-mimicry are still being studied.
Common Cuckoo (Cuculus canorus) egg between Marsh Warbler (Acrocephalus palustris) eggs. Russia, the Ryazan region (Ryazanskaya oblast), the Pronsky District, Denisovo.
Now that the cuckoos have returned and their calls echo through the reserve again, their strategy makes a lot more sense. It is a system perfectly engineered for a season of abundance. Why spend weeks raising a single brood when the environment offers countless opportunities to survive and reproduce more efficiently?
A Burchell’s Starling feeding a Great Spotted Cuckoo chick.
The Instinctual Journey
On a side note, one of the most fascinating aspects of cuckoos is their instinctual migration. Even though they are not raised by their own species, they know exactly when and where to migrate to avoid our winters. They have never had the luxury of learned behaviour; their entire journey is purely instinctual. There are always moments like this in the bush where I will be stumped and sit back in amazement.
Beautiful birds, these cuckoos. Thanks for this article on the “why”, Ross.
Yes, it’s true that they safe a lot of energy and their chances of survival are probably much better when they use other birds’ resources.
There is also another question left: Why do the other birds do this? Can’t the realise that the chicken jaust hatched is a parasite and not their own one? They could also kick this parasite out of the nest or stop feeding it. Do they really believe that the chick is their own one?
Anyway, a totally fascinating behaviour. And how clever nature has kind of in-built solutions, regarding the way, e.g. how these birds or also any other birds who migrate for the first time and maybe without parents know when to leave and where to fly to when summer in a place is over.
Hi Ross, I am always in awe of the beautiful birds in Londolozi! Yes this is curious, the instinctual journey. Also other social species, when raised alone, still have the ability to perform it. As you mentioned them, warbler are among the “obligated migrators”. It is triggered by external conditions as you know but genetically programmed too…
Hi Ross, thanks for this information on the Cuckoo and how it lays it’s egg in another birds nest. It is quite distressing for the other bird to keep on feeding the Cuckoo chick as it seems to me can never get enough to eat. We have seen this before on our previous property where our Robin Chat was feeding this Cuckoo chick almost all day long. Can’t remember what Cockoo chick it was, but to me it was stressful watching the Robin Chat feeding this huge chick all day long.
Nature is both amazing and confounding sometimes! Do other bird or even non-bird species exhibit this behavior? Wouldn’t other species ‘benefit’ from the same behavior? Does it encourage promiscuous females? All fascinating!
What seems really amazing to me is that even though the cuckoos are not raised by their parents, instinctively they know to migrate before winter sets in – that’s definitely a mystery of nature. Also, although birds have incredible eyesight, how do they not know a cuckoo chick is not their own? Thanks Ross for exploring your guest’s question and adding more details to the mystery of brood parasites.