Allopatric speciation, the classic way new species originate, is shown in the schematic drawings. In 

 spite of a small stream in the midst of a population of lizards, individual lizards on both sides of the 

 stream readily interbreed with each other. Then, over time, the stream swells to become a river the 

 lizards cannot cross, separating them into two subpopulations that can no longer reach each other 

 to mate. As random genetic mutations develop within each group, the two groups diverge until two 

 distinct species form, no longer capable of interbreeding. The geographic barrier that leads to 

 allopatric speciation need not be a river: deserts and mountains, for instance, can play the same role. 



newly formed river. Genetically, the two isolated 

 subpopulations then diverge, until they form two 

 distinct species no longer capable of interbreeding. 



In blackcaps, however, the subpopulations would 

 be divided by time instead of space. That is, the birds 

 all breed in Central Europe but get started at differ- 

 ent times. Blackcaps would thus be evidence of how 

 subpopulations can become isolated in the same 

 place, the first stage in a hypothetical process known 

 as sympatric (literally, "same place of origin") speci- 

 ation. The idea that speciation could take place sym- 

 patrically has been hotly debated for years. 



So the stakes were high as Berthold set about try- 

 ing to prove that blackcaps were diverging sympatri- 

 cally. To succeed, he needed a way to identify the two 

 kinds of avian migrants reaching the German breed- 

 ing grounds. During several winters in the early 

 1990s, he visited the British Isles to mark blackcaps 

 with bands. But back at the breeding grounds, he en- 

 countered the same problem I described earlier. Find- 

 ing small numbers of marked birds lost in a multitude 

 of unmarked birds breeding across vast tracts of south- 

 ern Europe calls to mind the words "needle" and 

 "haystack." The scientific developments that enabled 

 his hypotheses to be tested were still several years away. 



Almost a decade passed before I came into the 

 picture. I had been exploring a technique 

 known as stable-isotope analysis. Isotopes are forms 

 of the atoms of a given chemical element that differ 



slightly in mass. An example is deuterium, or "heavy 

 hydrogen," a stable, nonradioactive isotope that 

 occurs in nature. Deuterium makes "heavy water" 

 when it combines with oxygen to form H 2 0. 

 Throughout much of the Northern Hemisphere, the 

 proportion of deuterium in rainwater tends to be 

 correlated with latitude. Plants and insects drink the 

 rainfall and birds eat berries and insects, so a "signa- 

 ture" of the ratio of deuterium to ordinary hydro- 

 gen is transferred to the birds' tissues. 



During my discussions with a colleague, the or- 

 nithologist Robert W. Furness of the University of 

 Glasgow, we realized that the deuterium-hydrogen 

 signatures might be just the right information to test 

 Berthold's ideas. Our hope was that all we would 

 need were a few clippings from the tips of the claws 

 of migrants recently arrived on the German breed- 

 ing grounds. Blackcaps' claws grow slowly through- 

 out the year. If we could analyze the deuterium in 

 them, we might learn where the birds had wintered. 



The first step was to test whether the deuterium 

 signatures in the claws of birds in the two winter- 

 ing areas were sufficiently distinct. So for two con- 

 secutive winters, starting in 2001, I traveled around 

 Britain, Ireland, Portugal, and Spain, collecting 

 claw clippings from as many blackcaps as I could 

 lay my hands on. 



To narrow my search, I posted a call on a British 

 bird-watching Web site. Happily, a slew of bird- 

 watchers with gardens responded. In each garden, I 



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NATURAL HISTORY September 2006 



