species cannot be tracked throughout their annual 

 cycles. Recent technological advances, such as trans- 

 mitters that send signals via satellite, remain too large 

 to attach to the smaller bird species. And the chances 

 of recovering a band remain slim. 



Making matters trickier still, migration is a highly 

 plastic trait; that is, the genetic determinants of mi- 

 gratory path and direction vary a great deal from bird 

 to bird. Hence migration patterns can respond quick- 

 ly to natural selection. 



Today's migration patterns in the Northern Hemi- 

 sphere, for instance, must have evolved in the past 

 10,000 years or less, because much of the area from 



Four blackcap chicks beg for food from their mother. Because 

 migration routes are genetically programmed, these chicks, 

 once grown, will winter where their parents did. 



which or to which birds now migrate was under an 

 ice sheet until then. On the evolutionary time scale, 

 10,000 years is a mere instant. All in all, it is extremely 

 difficult to gauge whether present-day changes in mi- 

 gratory behavior, such as the blackcap's sudden win- 

 ter preference for the British Isles, are a result of nat- 

 ural or human-generated pressures, no matter where 

 the birds started from. 



Faced with such subtleties, the eminent or- 

 nithologist Peter Berthold and his colleagues 

 from the Max Planck Institute for Ornithology in 

 Radolfzell, Germany, had ample reason to be grate- 

 ful to the Dublin cat. In the early 1990s, they were 

 still exploring the hypothesis that the blackcaps win- 

 tering in Britain and Ireland had migrated from Ger- 

 many and Austria (where the cat's prey had come 

 from), even though German and Austrian blackcaps 



natural history September 2006 



traditionally go south for the winter, to Iberia and 

 North Africa. 



To test his suspicion, Berthold captured blackcaps 

 that were wintering in England and transferred them 

 to Germany. He kept the birds there until the fol- 

 lowing autumn, when they would feel the innate urge 

 to migrate. He confined the birds to a large enclo- 

 sure with a transparent roof and an ink pad at its base — 

 a common research technique of the time. The birds' 

 feet, coated with the ink, left a ready record of the 

 direction they hopped. And he found that, rather than 

 hopping toward the southwest, as German blackcaps 

 do when heading for Iberia, the captured blackcaps 

 hopped slightly north of west, in the direction of the 

 British Isles. 



Even more important, Berthold and his colleagues 

 showed that the new migration pattern was herita- 

 ble. The team bred blackcaps that wintered in Eng- 

 land or Ireland with blackcaps that wintered in Spain 

 or Portugal. The hybrid young of those pairs hopped 

 in an intermediate direction, toward a perhaps fatal- 

 ly inappropriate destination somewhere off the At- 

 lantic coast of France. The results strongly suggest- 

 ed that the new flight path from central Europe to 

 the British Isles evolved because of a genetic shift. 



Berthold's team argued that such a genetic shift 

 could have provoked the explosive growth in the 

 blackcap population wintering in the British Isles 

 only if there were substantial survival advantages to 

 the shift. But what were the advantages? One idea 

 was that since the Second World War, birds had been 

 attracted to Britain and Ireland by increased provi- 

 sioning of bird feeders, additional planting of berry 

 bushes, and a series of mild winters. Berthold also 

 pointed out that migration routes between Ger- 

 many and the British Isles are much shorter than the 

 ones between Germany and Iberia or North Africa. 

 That could leave the northwestward migrators with 

 more energy to devote to reproduction. 



A more subtle, but possibly more important, ad- 

 vantage was the effect of the winter home on the 

 timing of seasonal events. In the spring, blackcaps 

 begin their migration and reproductive development 

 only when the length of daylight reaches a certain 

 threshold. Berthold realized that the daylight in the 

 British Isles reaches that threshold about ten days 

 earlier in the season than the daylight in Iberia or 

 North Africa, because of the Earth's tilt. The tim- 

 ing difference seemed crucial in two ways. 



First, the northern blackcap populations would 

 probably arrive at the German breeding grounds 

 first. After all, not only do they have a shorter dis- 

 tance to travel, but their departure would also be 

 triggered ten days earlier than the departure of the 

 southward-migrating populations. The early birds 



