Why do Asian koels sing during the monsoon?

I get to listen to their concert all day: it starts off as a gentle ‘ku-oo’, that gradually builds into a crescendo. I can hear them even as I type this from my desk by a window: insistent, with a certain yearning. It has been a week since the koels started singing in our neighbourhood. Their song signifies a lot of things. For one, it signals the arrival of the monsoon.

“I can hear them right now in our garden,” says writer, historian, and naturalist S Theodore Baskaran, who lives in Bengaluru. “I live in the periphery of the city amidst a lot of trees, and it is a great blessing to listen to the koels sing.” The song, in essence, is a male’s mating call. While the males are jet black, the “females are ashy with spots,” points out Theodore, adding: “One can hear their song from afar.”

The 81-year-old has edited Mazhaikaalamum Kuyilosaiyum (which translates to ‘The rains and the song of the koel’), a collection of essays by the late M Krishnan, who was a pioneering wildlife photographer, writer and naturalist. “In an essay in the book, Krishnan talks about how a lot of Tamil poets have written about the birds’ sweet song,” recalls Theodore, adding: “He names the [15th Century] Tamil poet Kalamega Pulavar, among them.”

Chennai wildlife photographer Munish Palaniappan has been enjoying a koel couple’s symphony from the bedroom of his Ambattur home. “They are regulars at a tree nearby,” he says, adding that the birds keep up their seasonal appointment without fail every June.

Among the most well-known facts about the Asian koel, is that it does not build its own nest: it is a brood parasite, which means females lay eggs inside the nest of another bird that also brings up the koel’s offspring. “In most parts of our country, koels use a crow’s nest for the purpose,” says Bengaluru-based Suhel Quader, who is a scientist with the Nature Conservation Foundation. “The koel’s breeding season is hence tied to the breeding season of the hosts’,” he adds.

Koels may seem to have it easy, what with no parenting responsibilities. But that does not mean the birds lead a carefree life. Females, points out Suhel, keep a keen eye on their hosts’ nests. “They have to lay their eggs when the host too has started doing so,” he says. “And if the host is done laying eggs, and the koel does so after this, the crow might become suspicious.” The trick is to find that perfect timing to fool the crow. When females lay their eggs, they remove one or two of the crow’s eggs. Says Suhel, “Koel chicks grow alongside crows.”

Suhel, who grew up in Hyderabad, says that he has long associated the koel’s song with rains. “They usually sang a month before the rains began,” he recalls. When it is time to mate, males go “all out” to advertise themselves, points out Suhel. So much so that some of them even develop a hoarse throat at the end of the season. “They perhaps take a break for a little while then,” he laughs.