When we think of parasites, we typically think of unpleasant creepy-crawlies like leeches or ticks. Today I’d like to introduce you to the brood parasite, the most famous of those being the cuckoo bird. You might recognise the cuckoo as the little bird that pops out of old clocks, but it’s really their parenting habits – or lack thereof – which make them truly noteworthy.
Parasitic relationships are those where the parasite is dependent on the host, while also causing them harm. There are six different methods of parasitism, all of them unsettling. Brood parasites don’t feed on their hosts, but these birds do lay their eggs in another’s nest, shunting off the responsibility of raising their children and dumping them onto someone else.
Cuckoos are the most famous brood parasites, but others include cowbirds, whydahs, and black-headed ducks. Unlike vultures, these birds have earnt their none-too-positive reputations. You might think that the host would recognise their own eggs, or failing that, realise the mistake once the chick had hatched. But brood parasites have developed several strategies to work around this.
A female cuckoo will wait for the host to leave its nest before laying its own eggs. The common cuckoo even resembles a sparrowhawk so that it can invade the host’s nest without interruption. They’ll even lay every egg in a different nest to improve the chick’s chances of survival.
Some brood parasites count on their victims not falling for the trap. Certain birds are smart enough to recognise an egg that doesn’t belong. But evolution is an amazing thing; over the years, some eggs have adapted to have harder shells so that the hosts can’t break them; yet others have adapted to resemble the eggs of the host to blend in.
Once the parasitic egg has hatched, it would be easy for the host to refuse to feed the changeling chick. It’s still not certain why they raise cuckoo birds, but there are some interesting theories. It’s not uncommon for birds to raise the offspring of other species; even entirely different animals! Chickens in particular are known to care for ducklings, rabbits, and even kittens! So it could be that the host bird’s parental instincts are so strong that they just see a hungry mouth, and don’t pay any mind to the fact that it’s much, much bigger than their own young.
However, this clearly doesn’t apply to every kind of bird. Some hosts are quick to spot an imposter, and will take extra measures to prevent them from hatching. They’ll weave grass and sticks over the egg, or build an entire new nest on top of it.
But the brood parasites don’t give up that easily. To ensure that their eggs are being properly taken care of, some of them employ a strategy known at the “mafia hypothesis”. The parasite will watch the host’s nest, and if their egg is rejected, the parasite will destroy the nest and injure or kill the host’s offspring. It’s theorised that threat of this scares the host into complying.
These brood parasites and their hosts are great examples of an evolutionary arms race; as the parasites adapt to make their eggs harder to reject, the hosts become more able to recognise an intruder. It’s really in the host’s best interest to avoid parasitism; the cuckoo chicks pose a real threat for the specie’s survival. The invading chicks take up valuable resources from the host chicks, seeing as they’re typically much larger in size and require more food. They’ll even fight their adoptive siblings for a meal and let them starve. On top of that, cuckoo chicks make an incredibly loud begging call when hungry, sounding like entire brood of chicks. This increases the amount of time the host spends finding food, but it also has the added drawback of attracting predators to the nest.
If we apply human morality to these birds, it’s a pretty insidious practice. Cuckoos and other brood parasites avoid the responsibility of building a nest and raising a child by making other birds do it for them. No matter how well the poor victims try to escape this forced adoption, the parasites always find a way to sneak their way back into the family. With the host family suffering major disadvantages from taking in a parasite brood’s egg, it seems like an especially malicious way to live. But to these birds, it’s just a matter of survival.