Why do birds care for cuckoo chicks?
The strange world of brood parasites
The cuckoo is probably the most well known brood parasite – an animal that lays its eggs in the nest of a host species and relies on them to raise their young. But why do the hosts fall for its tricks (an act parson-naturalist Gilbert White called ‘a monstrous outrage on maternal affection’)? Why don’t they recognise that they’re raising a completely different species?
The hawk has landed
The first hurdle for the cuckoo is getting to the host nest to lay its egg.
This is tricky because the common cuckoo’s favoured host species – reed warblers, meadow pipits and dunnocks – are very alert to the threat they pose, and will drive them off if they see or hear them.
But here the cuckoo can rely on its first bit of mimicry. The common cuckoo bears a passing resemblance to the sparrowhawk – a killer of small birds – and the female can make a chuckling call a bit like one, too. This act scares away the host parent just long enough for the cuckoo to swoop in and lay an egg. They lay just one in a host nest, but may visit up to 50 nests during a breeding season, laying somewhere between 12 and 22 eggs in total.
Cracking egg forgery
This is the moment of truth. While cuckoo eggs are larger than those of the host bird, they mimic the latter’s colouration. But if the egg is not a good enough mimic, the host will cast it from the nest, bury it under more nesting material, or abandon the nest altogether.
Even though the common cuckoo as a species has been reported to parasitise the nests of 291 different species, individual females specialise in a single host. And the genes regulating cuckoo egg coloration are passed down exclusively along the maternal line, allowing females to lay convincing forgeries in the nest of the species they specialise in (and meaning that the male cuckoo, who might have been brought up by a different host, doesn’t give their child the “wrong” mimicry genes).
But this specialisation is a double-edged sword.
In one sense, it’s crucial, because the cuckoo is in a constant evolutionary arms race with the host species for egg colouration. Hosts evolve eggs with complex colouration and patterning, acting like a sort of unique signature to help spot fakes, and the cuckoo needs to keep up.
But individual dependence on one host species also makes the cuckoo’s existence precarious: if it can’t adapt quick enough, its line will come to an abrupt end.
This has recently been reported with two African species, cuckoo finches and their tawny-flanked prinia hosts.
Tawny-flanked prinia usually produce red, white or blue eggs, overlaid with a variety of patterns. Cuckoo finches have evolved eggs that mimic these colours and patterns, but now the prinia have started laying olive-green eggs, and the cuckoo finches can’t (at least, for now) produce the mix of pigments needed to make eggs of that colour, which puts that line of cuckoo finches at risk of extinction.
The changeling
If the cuckoo egg is accepted, the chick hatches after about 11 days, just before the hosts’ chicks do. This gives the common cuckoo chick the chance to remove the competition, rolling the other eggs or chicks out of the nest by pushing them over the edge with its back. It’s even been witnessed doing this while the parent is on the nest.
Honeyguides – yep, the same ones that help us find honey – are even more brutal. Their parasitic chicks are born with sharp, hooked bills with which they bite and shake their fellow nestlings to death.
Oddly, while the host species have evolved the ability to discriminate against cuckoo eggs (when the egg mimicry is not convincing enough), they have not evolved the ability to discriminate against cuckoo chicks. This makes for a tragi-comic sight when, at 14 days old, the common cuckoo chick is about three times the size of its “parent”.
The adult bird responds to the “gape” – the open-mouthed expression of baby birds when begging for food – of the cuckoo chick. But to compensate for the fact there’s only one of it (compared to a whole brood that the parent expects), the cuckoo chick makes loud, rapid begging calls to simulate an entire brood. That way, the parent still gathers enough food for the large cuckoo chick. It will also stay with, and be fed by, the host parent for longer than the host’s actual offspring.
Purple indigobirds, another species of brood parasite, have even evolved to mimic the unique gape markings of their host’s chicks to make this ruse even more convincing.
Goodfeathers
So why do host parents continue caring for these parasites?
Part of it seems to be down to “supernormal stimulus” – an exaggerated version of a stimulus that elicits an instinctual response. For the brood parasite, this typically means producing a larger egg, being larger than the host birds chicks, and having a louder begging call. It tricks the host bird into thinking that it has a very large, healthy chick, and it prefers that over its real offspring.
Another explanation, in some instances, is mutual protection. Chicks of great spotted cuckoos, for example, don’t remove the hosts’ chicks but grow up alongside them. One of the great spotted cuckoo’s host species is the carrion crow, but the crow doesn’t seem to bother chasing off the cuckoos. Why? For one, crow chicks are larger than cuckoos, so they are never out-competed and a few usually survive alongside the cuckoo chick. Also, the cuckoo chicks provides an added a layer of protection to the whole nest. When disturbed, the cuckoo chicks release a sticky, smelly substance to deter predators – saving both itself and its fellow crow nestlings.
There’s also the “mafia hypothesis” (or what we might call “Goodfeathers”). The great spotted cuckoo and the brown-headed cowbird both repeatedly visit the nests they’ve parasitised. If they find that their egg has been rejected, they will destroy the host’s nest and kill the remaining nestlings. By doing so, the cuckoo or cowbird encourages the host bird to build another nest that it can then attempt to parasitise. But if the host bird is aware of this threat, it might keep the parasitic chick around so it doesn’t lose the rest of its real brood.
But brood parasitism isn’t exclusive to birds.
Bonus: cuckoo bumblebees
Following last week’s post about anti-social bees, I learnt that there’s a line of bumblebees that opted out of communal life and became parasites instead.
These cuckoo bumblebees only parasitise the nests of their fellow bumblebees, and they’re so closely related that they’re now included under the Bombus genus of “true” bumblebees.
First, the cuckoo female has to wait until her social bumblebee sisters have established their nests. With no young to care for and no ability to collect pollen or build a nest of her own, she acts like a male bee, just flying around, drinking a bit of nectar and then lazing around until she’s hungry again.
Like their avian namesakes, cuckoo bumblebees rely on mimicry and subterfuge. For one, they look very similar to their host species. The red-tailed cuckoo bumblebee (Bombus rupestris), for example, is easy to mistake for the species it parasitises, the red-tailed bumblebee (Bombus lapidarius).
They also use chemical trickery. Bumblebees recognise each other by emitting a unique mix of complex hydrocarbons, but the cuckoos have evolved to copy these signature scents to introduce themselves into a host colony.
And once she’s in, the cuckoo female is geared up for a fight. She has a longer sting than a true bumblebee and a more heavily armoured abdomen – and because cuckoos don’t exude wax like ordinary bumblebees, there are no chinks in her armour. The host queen has little chance of winning.
The cuckoo then usurps the nest by subduing or killing the host queen. She tears open the host’s brood cells, waits till the host workers have discarded the larvae from these cells and then lays her own eggs, building back up the egg cells from spare wax in the nest (because she can’t produce her own). She’ll then continue to exploit the host workers to feed her and her developing young. This usually spells the end of the whole nest, because the cuckoo and her larvae consume the colony’s resources but contribute nothing to the nest.
But even then, she can’t let her guard down. It often happens that another cuckoo bumblebee will invade an already-parasitised nest and attempt to usurp the usurper. All very medieval.