At the beach a few weeks ago, as I scoured the sand for seashells, my colleague Rachel Moon turned her attention to a flock of terns sitting a little distance from the sea. Every now and then, a tern would take off, fly out to sea, and eventually return with a silver-grey fish in its bright orange beak. Each arrival was accompanied by a chorus of raucous calls. As the birds in the flock shuffled around to make room for the returning hunter, the flock gradually grew quiet again.

But a few calls persisted even after the commotion died down, and Rachel soon discovered their source. These piercing cries were being made by some terns, each of whom had singled out another tern from the flock and had partially prostrated itself before its chosen bird. Maintaining this flattened position, the calling terns followed their targets around insistently. Rachel began to feel annoyed on behalf of the targeted terns, but the birds themselves remained placid, resolutely ignoring the irritants.

At first, we couldn’t figure out what was going on. Was this some sort of mating ritual? A signal of submission from omega males towards the alphas? Just as we were leaving the beach, it struck me that the prostrating birds looked a little fluffier, a bit more unkempt, than the birds they were troubling, and then it suddenly all made sense — these were young terns begging their parents for food! But we hadn’t recognised the interaction as such because none of these parents had actually fed their young.

What we were watching — a pleading child and an unmoved parent — was an elegant bit of evolutionary biology in action. Over the last century or so, evolutionary biologists have developed a way of talking about how we expect natural selection to work. At the heart of this logic is the concept of relatedness. The more closely related you are to someone, the more likely you both are to have inherited the same DNA from your common ancestor. Therefore, the more closely related you are to someone, the more invested you are in their survival and reproduction, because there’s a bigger chance that they’ll pass on that shared DNA to their own babies. But at the end of the day, you are most closely related to yourself, so evolutionary theory says you’ll always put yourself first.

Now consider a parent tern with several chicks. Because parents are equally related to all of their offspring, they aren’t supposed to pick favourites. But a chick shares all its DNA with itself and only about half its DNA with its siblings, so a chick would prefer that its parents feed it instead of its siblings. This asymmetry in the preferences of the chicks and the parents led to the interaction we saw. The chicks had clearly grown up. They were big enough that their parents had decided they did not need to be fed any longer; the chicks begged to differ.

I suspect the chicks we were watching will be fine without being fed. Their perseverance in pestering their parents will serve them well if applied to catching fish. But despite parent terns’ best efforts at fairness, sometimes they simply cannot feed all their chicks equally. Some baby terns thus suffer parental neglect. Born in times of scarcity, and often the youngest of the bunch, these neglected chicks are fed far less than their siblings, and thus grow more slowly. Chicks that grow slowly are unlikely to survive to adulthood. So as soon as a neglected chick can walk, it faces a decision — to stay or to go?

The fate of the underfed babies that choose to go reveals an unexpected dimension to terns’ parental behaviour. Because terns nest in large colonies, a chick that leaves its own nest is bound to find another nest quite soon. Upon doing so, the chick tries to infiltrate the nest, to pass as one of the resident chicks. Surprisingly, this subterfuge often works, and the wanderer is accommodated into a new family.

On the face of it, tern adoptions make no sense. According to the relatedness paradigm, caring for an unrelated chick at the expense of your own seems like all cost and no benefit. To understand how this cost-benefit equation may even out, biologists have hunted for evidence of the possible advantages of adoption to the parents. So far, they’ve come up empty. But there is another possible explanation — adoption is an error that parent terns make to avoid a much costlier mistake.

Imagine you’re a tern parent that returns to the nest to find a new chick in it. One option, of course, is that the new chick is an interloper, in which case it should be shooed away immediately. The other option is that you previously miscounted the number of chicks in your nest. Evicting one of your own chicks when you’re capable of caring for it is a big mistake, if your goal is to ensure that your DNA is passed into the next generation. So biologists now think that tern parents sometimes adopt others’ chicks only because they aren’t quite sure if these strange wandering babies are their own or not. Not exactly the uplifting self-sacrificial act that some may hope for, but when accidental acts of kindness emerge in creatures otherwise focused on themselves and their own, they’re worth celebrating.

Ambika Kamathstudies organismic evolutionary biology at Harvard University

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