Migratory birds rely on genetic information to prompt them to begin moving at the appropriate time and in the correct direction. When migration begins the vast majority of birds will go in the 'right' direction, but occasionally a few go a different way. Normally these few get into trouble and do not survive. Evolutionary change needs these occasional random genetic differences between individuals so that natural selection can eliminate the harmful and reward the useful.
Studies of a bird population that traditionally breeds in Germany and winters in Spain show an interesting development in recent years (Current Biology).
Blackcap Warblers in Germany
Blackcaps (Sylvia atricapilla) nesting in German forests leave at the end of summer for warmer climes. The traditional direction of flight takes them 1,000 miles south to Spain where they overwinter.
It seems reasonable to assume that there have always been a small number of birds that set off in the wrong direction towards England, and that these usually did not survive to breed the following year. But things have changed!
The climate in England has got warmer, and many people regularly feed birds. 30% of German Blackcaps now winter in England where a good reliable food source is available in winter – Blackcaps can eat from bird tables. They do well and are fit for the spring migration.
The distance from England to Germany is much shorter than from Spain, so the English-wintering birds (both males and females) arrive before their Spanish-wintering counterparts. This means that the English-wintering birds are kept genetically separate from the Spanish-wintering population, although they still breed in the same forests.
With early arrival in spring the English-wintering birds also get the best territories, giving them a clear advantage.
New Species Evolving
The Current Biology paper points out that in less than 30 generations these new English-wintering birds have not only become numerous, but have started showing differences when compared with the Spanish-wintering population. They have:
- more rounded wings, which sacrifice long-distance flying power for increased manoeuvrability
- bills are becoming narrower and better-suited to summer insect diets (a heavier bill is needed to feed in Spain in winter)
- slightly darker plumage (maybe to make the differences apparent to other Blackcaps).
Martin Schaefer (one of the authors of the Current Biology paper) said, "this is reproductive isolation, the first step of speciation", and "it’s prompted by a very innocent human activity".
Bird Feeders and Breeding Ranges
When bird food is made reliably available for wintering populations (or early spring arrivals: Hummingbirds feeders in Canada) successful breeding can be influenced by man.
With so many examples of the harm that human activities are doing to wildlife it is good to find a couple of examples of man being useful!
Sources:
- 'Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide', Rolshausen, Segelbacher et al., Current Biology, 2009.
- 'Feeding Birds Could Create New Species', Brandon Keim, Wired Science, 2009.
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