Feeding the birds is the most innocuous of pastimes, but it could have a unintended effect: changing the course of evolution.
Researchers have discovered that the enthusiasm of many Britons for feeding birds in winter and the gradual warming of the British Isles due to climate change have helped change the appearance of the blackcap, a warbler as commonly recognized in European gardens as robins are in North America.
The differences have been modest, involving a slightly altered beak size and wing shape, according a paper on the finding published online in the journal Current Biology, but they've occurred in just a few decades, a pace that has stunned scientists.
"What's really catching people's attention with this paper is the speed at which evolution can manifest itself in short periods of time," said Keith Hobson, a research scientist in Saskatoon with Environment Canada who helped with the study. Mr. Hobson said it was like seeing evolution "happening before our eyes."
Scientists have been able to detect natural selection at work in the blackcaps because one population group of the birds began changing its annual migration route in the 1960s, appearing in Britain in significant numbers during the winter.
Until then, all blackcaps breeding in southern Germany and Austria each summer migrated south to the warmer Mediterranean areas of Spain to escape the central European winter.
The first blackcaps wintering in Britain were reported in 1959, and then with increasing frequency in later years, indicating the change in migratory route is a relatively new development.
The practice of putting suet and other rich foods in feeders and relatively warm winters in recent years allowed a British migratory population to evolve separately from its Spanish kin. Researchers believe the birds ended up in Britain, about 1,200 kilometres north of their regular winter grounds, because they flew off course from their usual migration routes.
If winters had been colder and bird feeders didn't exist, these wayward birds would have soon died due to starvation and inhospitable weather, but they thrived and now amount to about 10 per cent of the blackcap population.
"The British are very avid bird watchers and so I think we can be fairly certain … [the northward migration]only started in the 1960s," said Martin Schaefer, a professor in the department of evolutionary biology at the University of Freiburg in Germany and lead researcher of the study.
The British blackcaps don't have as far to migrate to get back to southern Germany each summer, only about two-thirds of the distance of those that travel from Spain, so they arrive in the breeding grounds about 10 days earlier. This has created a condition known as reproductive isolation that is helping to maintain the different populations.
The birds have begun to look different because the British birds have evolved to have rounder wings better adapted for manoeuvrability and not as good for long-distance flying. They also have developed longer, thinner beaks because they no longer need an adaptation for eating large fruits, like olives, as they do in Spain.
The Canadian connection to the research is an Environment Canada laboratory in Saskatoon that analyzes feathers for their isotope content. Isotopes are atoms of the same kind of element, such as hydrogen, that have slightly different weights. Comparing isotope levels helps determine the latitude where birds developed their feathers, allowing researchers to figure out which came from Spain and which from Britain.
Some signs that animals and plants in Canada may be adapting to warmer temperatures:
Canada Geese: The geese used to migrate south for the winter, and many still do, but some populations stay in Canada year round, feeding on lawns and becoming a general nuisance because of their droppings.
Mallards: Some of these ducks are also staying around all winter instead of going to sunnier climes, provided temperatures remain warm enough to maintain open water.
Robins: With global warming, robins, the harbinger of spring for many Canadians, have been spotted for the first time in Arctic areas, where the Inuit had no name for the birds.
Tree swallows: They're also being found in the high Arctic, a surprising migration considering that they're hundreds of kilometres north of the tree line.
Sockeye salmon: With global warming, the salmon are moving northward and are being caught in the Arctic.
Mountain pine beetles: Warmer winter temperatures are allowing the pest to spread to through forests in Western Canada, laying waste to great expanses of woodlands.
Aspens: A common tree in northern boreal forests has put on a growth spurt due to rising levels of carbon dioxide in the air, growing up to 50 per cent faster.
Staff and Canadian Press