In a perfect world, one would hope to always spot an animal without disturbing it, to always be able to watch an animal without worrying about the effect of your presence. But realistically, many animals notice you long before you notice them. You only see them from the corner of your eye as they’re fleeing.

But no creature creates quite as much of a ruckus about an approaching predator (or person) as a nesting bird because nesting birds aren’t really fleeing — they’re staging a diversion. Many birds chirp raucously, and even fake injury, to distract predators from the eggs or chicks inside their nest. It’s this sort of drama that first drew my attention to the nest of a red-vented bulbul in a patch of thorn-scrub forest in Kutch. Resting amid the thorny succulent branches of a cactus-like euphorb, branches filled with noxious sap, this nest was impeccably defended. The bulbuls’ defence seemed effective — the chick in this nest did not get eaten. But the geographic range of the red-vented bulbul extends into ecosystems quite different from thorn-scrub forest, into regions where there are not only fewer euphorbs in which to safely nest but also myriad other species with which they must interact, not the least of which is the red-whiskered bulbul.

A large portion of the range of the red-vented bulbul overlaps with that of red-whiskered bulbuls. And in many ways, they seem near-identical — they eat a similar mix of fruits and insects, build similar nests from twigs, roots and cobwebs, and make a fuss about predators. On realising that such similar birds occupy overlapping ranges, one is compelled to ask how they coexist.

In peninsular India, these two species coexist by not fully coexisting. They seem to prefer slightly different weather conditions — red-whiskered bulbuls live in marginally wetter, more humid environments than red-vented bulbuls. Such fine separation isn’t unusual — the natural world is rife with pairs of closely related species that divide up their shared habitat along almost imperceptible axes, giving the impression of coexistence, while carefully avoiding each other.

What is remarkable about these bulbuls, however, is that they maintain this separation even in a new home established over 12,000km away, on the island of Oahu in Hawaii. In the ’60s, red-whiskered bulbuls were introduced into some of the wetter parts of Oahu. By the ’80s, they hadn’t spread too far from where they were introduced, unable to cross drier habitats to reach new patches of moist forest. The red-vented bulbuls, however, spread much further in the same time, into drier regions. These two species therefore recreated, in miniature but also in quick time, a distribution pattern that closely resembled their geographic distribution in India. Why would species that have spent millions of years balancing avoidance with coexistence do it differently on the other side of the world? Similarly, I imagine wherever they are, red-vented bulbuls put as much effort into defending their nests from predators as did the pair of birds nesting in the euphorb — if a survival mechanism isn’t broken, why fix it?

(Ambika Kamath studies organismic and evolutionary biology at Harvard University)

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