Alone but not lost

Adistinct feeling of discomfort accompanies the sudden realisation that you’ve found yourself somewhere unexpected. The puzzled looks, the subtle unfamiliarities, the disoriented panic that you will embarrass yourself before retreating to familiar ground. I associate this feeling with accidentally entering men’s bathrooms at the airport, where returning to a place I belong involves just taking a few steps towards the door while assiduously avoiding eye contact. But imagine then getting on a plane that takes you to the wrong city, country, or even continent, and you can begin to imagine the predicament of a vagrant bird.

Vagrants are individuals that end up far outside their normal geographic range, far from most other members of their species. The normal ranges of birds can be immense — many spend summers and winters in different parts of the world and migrate vast distances between them. Vagrants are often examples of this truly impressive feat of migration gone wrong. But over long periods of time, these errors play a crucial role in generating the diversity of life. Through a combination of chance and opportunity, single individuals can reshape nature.

How do these errors in migration occur in the first place? Sometimes they result from abnormal weather events, like cyclones that blow birds off-course. But more frequently, vagrants land up thousands of kilometres away from their destination because of errors in their migration programme. Older birds, experts at long-distance flight, use a complex variety of cues to figure out where they’re going, but first-time migrants depend entirely on internal, inherited instructions that specify only the direction and distance they need to travel. A two-part instruction, however, is easy to break, because even a small mistake — go west instead of go east — has severe consequences. Geographic patterns of where vagrants end up relative to their usual migration routes reveal that they travel the right distance, but don’t do as well with direction.

An individual vagrant’s chances of surviving its mistake are not great, but on longer timescales, some of these mistakes turn into opportunities. Take the cattle egret, a bird you see perched on the backs of grazing cows and buffaloes, picking off insects that scatter out of the cattle’s path. Cattle egrets are native to Africa and Asia, but have spread to the Americas within the last 150 years. In this age of people shuttling every manner of plant, animal, and microbe across the world, cattle egrets are unique in having found their way to South America without human assistance. The first cattle egrets in the Americas, therefore, were vagrants, which managed to hang on and establish a population that has persisted and spread across two continents. People have played a role in their expansion, though — the human-mediated spread of cattle across the world provided vagrant cattle egrets with the opportunity to continue living as they did on the distant continent they journeyed from.

Though cattle egrets in the Americas still look and behave like their counterparts in Africa or Asia, the persistence of vagrants coupled with the right ecological opportunities can, over millions of years, yield impressive diversity. Islands that lie in the path of migrating birds and cyclones present the best prospects for vagrants to diversify. Islands often lack the complex ecological communities present on larger continental landmasses, so vagrants that reach islands aren’t presented with much competition. Consider the Hawaiian honeycreepers, a group of birds that originated when vagrant Rosefinches flew over from Central Asia, likely in search of seeds to eat. On landing in Hawaii, these birds encountered not just seeds but a host of other potential food sources untapped by other creatures — nectar, fruit, insects that live under bark. Though their short, stout beaks weren’t well-suited to eating these foods, some of their descendants acquired mutations that altered their beak dimensions, getting better at sipping nectar, for instance, but worse at eating seeds. A seemingly poor strategy, but since these variants had little competition for the nectar, they managed to survive. Their descendants in turn gradually got better at capitalising on these novel resources, eventually splitting into many species, each excelling at feeding on a particular food. A complex community of over 50 species of Hawaiian honeycreepers came into existence because vagrant Rosefinches chanced upon an opportunity.

Vagrant birds have the capacity to create communities of humans of a particular kind as well — birdwatchers (birders). Birders avidly track the occurrence of vagrants, chasing into hurricanes to spot the birds they bring with them. They travel long distances for the chance to see a vagrant Ross’ Seagull for 45 seconds, spend days camping in the snow to watch a vagrant Calliope hummingbird and arrive in hoards to spot a single vagrant Pink-footed goose among a flock of Canada Geese. We owe everything we know about such vagrant birds to birders’ curious passion for tracking them down — decades of observations of solitary birds coalesce into global patterns. Given enough time, even the loneliest of birds can fit into a community.

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

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