put up fine polyester "mist nets," so as to catch the 

 blackcaps flying past. One trick was to attract them 

 with a recording of their flutelike songs. My field- 

 work often amounted to sitting in the kitchen of a 

 keen garden bird-watcher, drinking tea, and listen- 

 ing to the recording until a blackcap arrived — not at 

 all unpleasant work. (I owe a great debt of gratitude 

 to all of the householders who supported the pro- 

 ject.) Eventually, a bird 

 would get tangled in one 

 of the nets, and then fall, 

 unharmed, into a pocket at 

 the bottom of the net. 

 Then I would hold the bird 

 in my hand while I gently 

 clipped less than a milli- 

 meter off a toenail. 



Meanwhile, in Ireland, a 

 team led by Brendan Ka- 

 vanagh of the Royal Col- 

 lege ot Surgeons in Ireland 

 collected samples from 

 birds in and near Dublin. 

 When I got to Iberia, 

 things were a bit more ar- 

 duous — I got up before 

 dawn every day and spent 

 two New Year's on my own 

 (almost too hard for a 

 Scotsman to bear). 



Male blackcap begins singing to attract a 

 mate as soon as he arrives at the blackcap 

 breeding grounds in the spring. Female 

 blackcaps, like most other female birds, 

 do not sing. 



B 



ut when the results of 

 the isotope analysis 

 came back from Jason Newton at the Scottish Uni- 

 versities Environmental Research Centre, all the 

 work seemed worthwhile. We found a large and con- 

 sistent difference between the deuterium values in 

 the claws of the two wintering populations. The deu- 

 terium signatures would indeed mark the breeding 

 birds according to their various winter habitats. 



We now had to collect information from black- 

 caps in Germany. That part of the fieldwork was co- 

 ordinated by Wolfgang Fiedler, one of Berthold's 

 colleagues. Our first aim was to find evidence of as- 

 sortative mating — that would confirm the existence 

 of a hitherto undocumented path toward speciation. 

 To do so, we needed to capture breeding pairs of 

 birds. Ifblackcaps were mating assortatively, the deu- 

 terium signature in the claws of the male would cor- 

 relate with that of the female he was paired with. 



We were extremely excited when two years' 

 worth of data told us exactly this story. We calcu- 

 lated that blackcaps were about two and a half times 

 more likely to pair assortatively than mate at ran- 

 dom. We had discovered the first concrete evidence 



that changes in the migratory behavior ofbirds could 

 lead to some degree of reproductive isolation, prov- 

 ing what Berthold had surmised so long ago. 



So what might be driving the assortative mating? 

 We estimated the day of arrival on the breeding 

 grounds for a number of males, simply by noting the 

 day each bird started singing (males sing as soon as 

 they reach their breeding grounds, whereas females 

 do not sing at all.) The males' arrival dates correlat- 

 ed with the deuterium signatures in the birds' claws: 

 the birds that wintered in the north were the earlier 

 arrivals on the breeding grounds. Their head start is 

 an important reason why assortative mating ensues. 



To fit the final piece of the jigsaw, we had to see 

 if any reproductive benefits grew out of the new mi- 

 gratory route. Those benefits would help explain 

 why the northern population had increased so rapid- 

 ly, the last of Berthold's longstanding ideas we were 

 able to test. And, sure enough, females that paired 

 with males from northerly winter homes laid more 

 eggs than the female partners of southern males. 



The extra fecundity could arise in several ways. 

 Northern males, as early arrivals, would tend to set- 

 tle on the best territories. For their part, the females 

 might possess what could be described as "intrinsic 

 female quality." Thus the "best" females would set- 

 tle on the best territories, either by excluding oth- 

 er females or by becoming selected as the mates of 

 the males first to arrive at the breeding site. 



We also discovered that females wintering in 

 northerly latitudes were more likely to produce suc- 

 cessful clutches than females wintering farther 

 south. We speculate that, indeed, the shortened mi- 

 gration route to the north saps less energy from the 

 northern females, leaving them with greater re- 

 sources to devote to reproduction. 



Much more work will be needed before we can 

 begin to answer even a few of the new ques- 

 tions our research has raised. Why, for instance, do 

 blackcaps spend the winter in the British Isles today, 

 when they never did before? Perhaps an increasingly 

 more hospitable climate played a role. If so, behavioral 

 shifts such as the one observed in blackcaps might be 

 one way natural selection could buffer the effects of 

 environmental change, at least among migratory birds. 



Finally, since migration is such a plastic trait, is the 

 new blackcap behavior just a short-term change, 

 which will eventually disappear? Or is it the first step 

 in the formation of a new species? If it is the latter, 

 we bird lovers are privileged to have witnessed a nov- 

 el kind of branching point in the history of life on 

 this planet: a mode of speciation dramatic for its 

 abruptness, compared to the glacial changes that usu- 

 ally characterize the time scale of evolution. □ 



NATURAL HISTORY September 2006 



