House crow (left) and Asian koel. Photos: Muhammad Mahdi Karim (GNU FDL) and Doug Janson (CC BY-SA).
In their breeding period, birds mate, lay eggs, rear their offspring and protect them. In India, birds usually breed in summer because their chicks will have enough food in the following monsoon.
While most breeding activities are common across species, protecting their eggs and offspring is more of a headache for some.
I spent a summer watching two bird species, one of which spent valuable time trying to lay an egg in the other’s nest. The former is known as a brood parasite: a species that imposes the cost of rearing its offspring onto another individual – the host – by laying its eggs in the hosts’ nests.
This evolutionary adaptation relieves the parasitic parent from investing in its young or building nests, and frees them up to spend more time foraging and producing more offspring. Humans have known of brood parasitism for centuries but don’t yet have an explanation.
The host bird species that I observed, in villages in Chittoor district, was a house crow (Corvus splendens); its brood parasite was an Asian koel (Eudynamys scolopaceus). Some of us have grown up listening to the tale of the koel laying its egg in a crow’s nest. But most people are unaware of the dynamics of this act that remain out of sight.
The crow’s breeding season coincides with the start of the monsoon and lasts until August. The koel’s breeding period, from March to September, overlaps with the crow’s.
The koel lay their eggs in crow nests to ensure there’s a higher chance of clutch formation. (The clutch is the number of eggs laid.) The koel’s eggs resemble those of the crow in pattern and colour. Researchers have observed such mimicry in other parasitic bird species as well. For example, the egg of the common hawk cuckoo (Hierococcyx varius) mimics that of its host species, the jungle babbler (Turdoides striata) in size and colour. Such mimicry is thought to have evolved to prevent the host from rejecting any eggs.
Among some other species, the parasite’s eggs are non-mimetic – like those of the pied cuckoo (Clamator jacobinus), whose host is the cape bulbul (Pycnonotus capensis). However, the bulbul doesn’t reject the egg.
Some parasitic eggs are cryptic: the host has a hard time detecting the impostor egg. For example, the koel’s eggs are slightly smaller than those of the crow. Since smaller eggs require less incubation time, the koel’s eggs hatch in good time.
A crow lays five eggs on average during its breeding period, laying one egg each day. The koel lays its egg(s) in the crow’s nest in these five days. If the parasite lays its eggs before the host, the eggs are likely to be rejected. An egg laid after the host has completed its clutch may endanger the eggs.
Before I started my observations, our team checked the state of the nest by counting the number of eggs – crow’s and koel’s – in the nest and marked them for identification. Then I picked a safe spot and waited with binoculars, from dawn to dusk.
Host birds respond to brood parasites using different defence strategies. They attack the parasite outright at times, and at others issue warning calls, hide the nest, look for and remove the parasite’s eggs and aggressively defend their territories.
The nests I observed were on Indian cork and tamarind trees, which have dense foliage. Crows also look after their nests in pairs or small groups as a defence mechanism. House crows are monogamous through successive breeding seasons. I observed one crow in a nest at all times while another crow helped by staying on the lookout and at times feeding the individual staying back.
In a cooperative group of carrion crows (Corvus corone) found throughout Europe and much of Asia, a set of helper crows allow an incubating female to spend more time in the nest, preventing parasitic cuckoos from laying their eggs. I even observed crows chasing female koels even outside the nest when they spot each other.
Both the host and the parasite exhibit secretive behaviour as an adaptation and counter adaptation. Crows entered and left their nests with great caution. The female koel in turn spent a lot of time surveilling potential host nests from concealed perches. The female koel has a hawk-like plumage and prefers trees with dense foliage, so she can hide effectively among trees.
Some studies have reported that brood parasites remove one egg and before laying theirs. I once saw a koel enter a crow’s nest while both crows were away, but she was quickly chased out. The koel flew in my direction and I saw that she had an egg in her beak. When we checked the nest later, we found one egg was missing. The koel had successfully removed the egg, but hadn’t laid one in its place. The following morning, we found a new koel egg. It could have belonged to the same female who flew away with the egg or a different one.
The eggs of the host as well as the parasite kept vanishing, and new eggs kept appearing in the observed nests. A koel reduces its risk by not putting all of its eggs in one nest, but distributing them between multiple. When the koel visits a host’s nest, it also punctures or breaks eggs irrespective of the species so that her offspring isn’t starved of food even during a shortage.
One evening, I saw three koels sneaking into a crow’s nest one after another when the adult crows were not around. There were only two koel eggs in that nest as all the crow eggs had been removed by then. The koels also removed the remaining eggs that day. We believe the crows had abandoned that nest.
If a host happens to see a parasite laying an egg in its nest or recognises an intruding egg, it will abandon the nest or reject the egg. Both the crow and the koel have a responsibility to ensure the clutch size is not beyond what the crow can take care of. Once, our team found 21 eggs in one nest; the crows abandoned it, and bonnet macaques raided consumed the eggs soon after.
It is very difficult to ensure the success of a crow’s eggs, should they manage to hatch, if there is a parasite chick in the nest as well. But this is a separate story. Meanwhile, crows and koels continue their coevolutionary race to survive.
Dincy Mariyam is a doctoral student at the Centre for Wildlife Studies. Her work focuses on understanding nature-based tourism in Indian protected areas.