Wednesday, 13 October 2010

Transatlantic origins

The changing of the seasons marks a turning point for much of our wildlife, often prompting large-scale movements to other habitats or countries. October is the month that I, as a birdwatcher, feel reinforces the sense of connection with far away places, underpinning the complex movements taking place. Our position on the western fringe of Continental Europe means that we are well placed to see migration in action; as summer visitors depart so winter ones arrive and others pass through on passage.

Over recent days I have watched the weather forecasts, charting the movement of systems from both east and west and anticipating what they might deliver to our shores. For example, a storm system hitting the eastern seaboard of North America might pick up birds migrating south and carry them across the Atlantic. It seems such a great distance but such is the speed of these grand weather systems that they have the potential to deliver even the smallest and seemingly least robust birds to our shores. The birds that make landfall here, thousands of miles from where they should be, are known as ‘vagrants’. While certain vagrants reach us fairly regularly, with a dozen or so turning up annually, others are rare, perhaps with just a single individual recorded.

A fortnight ago I was on Blakeney Point watching one such visitor, a bird whose arrival the day before had attracted a crowd of birdwatchers even though the chances of securing an identification were slim. The bird in question was a small American flycatcher, one of a group of similar species that are notoriously difficult to separate in the field. Although originally thought to be a Yellow-bellied Flycatcher, it soon became clear that the bird was either an Alder Flycatcher or a Willow Flycatcher. These two species were, until very recently, thought to be a single species and it is only because of subtle differences in their songs and advances in genetic analyses that each has been elevated to species status. Only one of the two, Alder Flycatcher, has occurred in Britain before and its identification was only secured because it happened to be caught by a licensed bird ringer.

Alder Flycatcher breeds from Alaska to Newfoundland, occupying a more northerly breeding range than its relative, and it winters from Columbia to Peru. Examination of weather maps for the period leading up to its discovery, suggests that the bird had been caught in that east coast storm, sweeping it across the Atlantic, perhaps via Iceland. Its arrival in Norfolk may seem miraculous, especially with vagrants from the east present nearby at the same time, but it does underline the role that weather systems can play in bird movements. 

No comments:

Post a Comment