Two winter vacation spots attract blackcaps from their summer 

 homes in Germany and Austria: the British Isles (Britain and Ire- 

 land) and the region encompassing Spain, Portugal, and north- 

 ern Africa. When spring comes, the birds return to central Eu- 

 rope to breed. The ones that winter in the British Isles (the 

 "northern" blackcaps) return first because they set off about 



could claim the best territory for mating and caring 

 for their young, with good cover for nest protection 

 and an abundance of insects for feeding chicks. In 

 many bird species, territorial quality is known to be 

 an important factor in breeding success. 



Second, the northern birds would be ready to 

 breed earlier, too. Because of the timing and travel 

 differences, then, it seemed reasonable that the two 

 blackcap populations would be apt to mate "assor- 

 tatively" with respect to their wintering trait. The 

 northern-wintering males would pair with north- 

 ern-wintering females, and the southern-wintering 

 males would pair with southern-wintering females. 



ten days earlier than their "southern " counterparts, and their 

 migration paths are shorter. The shorter paths save them ener- 

 gy for breeding and protecting their nests, and their early ar- 

 rival enables them to seize the best territories with the best for- 

 aging opportunities. All those factors may help explain why the 

 northern blackcaps lay more eggs than their southern brethren. 



Assortative mating may seem like a minor and 

 esoteric academic concept. Yet if blackcaps pair 

 off assortatively, their behavior has major implications 

 for biologists' understanding of how breeding pop- 

 ulations can become reproductively isolated. Such 

 isolation is the first stage in speciation, or the forma- 

 tion of a new species. 



Evolutionary biologists usually think ot speciation 

 in its "classic" — or, more formally, allopatric — form, 

 and they have amassed plenty of evidence in favor 

 of the process. In allopatric (literally, "other land of 

 origin") speciation, a once-unified population be- 

 comes divided by a geographic barrier such as a 



September 2006 NAT U K A 1 HIS ion. Y 



39 



