Change in the Air 



Songbirds with divergent migratory patterns may be 



a rare example of a hotly debated way of forming new species. 



On a winter day in 1961 , a cat prowling the 

 Irish city of Dublin stalked a European 

 blackcap, a small, gray warbler with a flute- 

 like song. The cat got lucky — and so did I. Shortly 

 after the cat captured the bird, a passerby chanced 

 on the bird's remains. On closer inspection, the find- 

 er noticed a metal band around the victim's leg, in- 

 scribed with numbers and an address in Austria. So 

 began what turned into an extraordinary piece of 

 good fortune for ornithologists and evolutionary bi- 

 ologists, including me. 



Ornithologists often fit birds with uniquely iden- 

 tifiable bands, but the chances of recovering them 

 aren't much greater than finding a message in a bot- 

 tle cast into the ocean. Out of more than 450,000 pied 

 flycatchers banded in Europe during the past fifty to 

 sixty years, for instance, fewer than five were recap- 

 tured in their wintering areas. But in this case, a Good 

 Samaritan — perhaps an amateur birder — mailed the 

 band to an Austrian ornithology society, about a thou- 

 sand miles away. There it became the first break in 

 solving a baffling migratory mystery. 



In the 1950s and 1960s, substantial numbers of 

 blackcaps began wintering in Britain and Ireland. 

 No one could figure out where the birds were com- 

 ing from. Ever since then, populations of blackcaps 

 overwintering in Britain and Ireland have contin- 

 ued to grow. A recent survey by the British Trust 

 for Ornithology reported blackcaps in more than 

 2,000 gardens throughout Britain and Ireland. 



The Dublin cat's successful hunt provided the key 

 clue for determining where the birds had started 



Pair of European blackcaps (Sylvia atricapilla) pause together 

 on a branch, the female facing left, the male facing right. Dis- 

 tinct subpopulations of blackcaps may be undergoing sym- 

 patic speciation — diverging into two species despite living (at 

 least part of the time) in the same place. The members of each 

 subpopulation tend to choose mates only within their own 

 group, in accord with their divergent migration patterns. 



By Stuart Bearhop 



their journeys. Even more significant, the cat and 

 serendipity had combined to give important new 

 insight into what Darwin called "that mystery of 

 mysteries," the origin of species. 



When it comes to blackcaps, the mystery of the 

 origin of species is wrapped in the enigma 

 of migration. Aristotle was among the first to at- 

 tempt an explanation for the seasonal appearance and 

 disappearance of bird species. European robins that 

 appear in winter, he noted, seem to be replaced by 

 European redstarts in the summer. One species, he 

 suggested, is "transmuted" into the other. Aristotle 

 also proposed that birds hibernate, an idea that re- 

 tained much popular support until the end of the 

 nineteenth century. 



Such notions may seem fanciful today, but the re- 

 ality is even more astonishing. In some cases a bird 

 weighing less than a third of an ounce flies 8,000 

 miles across large tracts of inhospitable habitat. 

 Around the world, billions of birds take off in the 

 fall for winter homes, unerringly navigating through 

 night skies, hailstorms, and heavy fog. With equal 

 skill and punctuality, they return to their birthplaces 

 in the spring, to nest and breed through the end of 

 summer. Yet despite long-standing fascination with 

 this behavior, knowledge of many aspects of avian 

 migration is still surprisingly limited. 



Thus when European blackcaps began wintering 

 in Britain and Ireland, ornithologists could reach no 

 quick consensus about their origins. Some argued that 

 the birds most likely came from Scandinavia, to the 

 north; blackcaps breeding there, they noted, passed 

 through Britain and Ireland in late autumn. Other or- 

 nithologists suggested that the visitors might have 

 originated from any number of directions: blackcaps 

 are among the most widespread songbirds in Europe. 



The puzzle was harder than it might seem at first. 

 One major obstacle was — and still is — that many bird 



September 2006 NATUIVAI HISTORY 



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