Words: Kate Risely, of the British Trust for Ornithology (BTO)
If you haven’t been following the Migrant Birds in Africa blog (http://migrantbirdsinafrica.blogspot.com), I urge you to do so. As I have mentioned before, severe and ongoing declines of many of our African migrants, such as Cuckoo and Turtle Dove, have led BTO and RSPB researchers to West Africa, to better understand the challenges that migrant birds face during the winter.
During the past few months, fieldworkers have been monitoring birds in five different areas, ranging from the semi-desert Sahelian region in Burkina Faso to the tropical rainforest in southern Ghana.
In the February instalment, BTO research biologist Mark Hulme wrote about listening to Nightingales while sitting on a tropical beach, and reflected that it had been worth battling through various challenges in order to collect such valuable data on our migrant birds.
To most people who are not active birdwatchers, Nightingales are near-mythical birds, celebrated in literature and poetry for their beautiful song (though I have to say that, to me, the most remarkable thing about Nightingale song is its sheer volume).
Their reputation certainly goes before them, for although everyone knows the name, few people in this country will have ever knowingly heard a Nightingale, let alone seen one. They are especially hard to find these days – previous BTO Atlases exposed a really shocking range contraction between 1970 and 1990, and figures from the Breeding Bird Survey show that Nightingales have declined by over 40% just since the start of the survey in 1994.
Provisional maps from the 2007-11 Bird Atlas show that Nightingales are now very sparse indeed – the core population has retreated to coastal areas of Essex, Kent and Sussex, with a few patchy and isolated records over their former range (south-east of a line drawn from the Wash to Exeter).
Thanks to research by BTO ornithologists, the habitat requirements for breeding Nightingales are well known – this species needs closed-canopy scrub, with areas of bare ground under the canopy, and also areas of dense low vegetation.
This combination offers safe foraging opportunities (on the bare ground), suitable nest sites, and cover from predators. However, changes in woodland management, increased grazing pressure by deer and loss of scrub habitats may all have caused a loss of this kind of habitat structure.
These factors are the subject of ongoing studies, including a project in which BTO researcher Chas Holt radio-tracked breeding Nightingales to see exactly how deer numbers affected the birds’ habitat use.
Although loss of suitable breeding habitat in England may go some way to explaining the decline in the numbers of Nightingales, habitat loss alone is not enough to account for such a dramatic change.
Many species that spend the winter in sub-Sahelian Africa, such as Willow Warbler, Cuckoo and Nightingale, are all showing severe declines, and this has thrown the spotlight on their African wintering quarters as a likely source of the problems.
BTO researchers are trying to find out exactly what is happening, and have started work on two exciting new projects – firstly, the ‘on-the-ground’ research in West Africa mentioned above, and secondly, a scheme designed to track individual Nightingales on their wintering grounds.
Twenty birds were caught in their East Anglian breeding grounds in 2009 and were fitted with geolocaters – tiny devices, weighing less than 1g – which will record exactly where the bird goes.
The aim is to re-catch the same individuals on their breeding grounds in 2010, remove the tags, and download the detailed information about where these birds spent the winter. Together with data from birds tagged in Switzerland, Italy and Bulgaria, this study should provide unique information on the wintering distribution of Nightingales, and will provide a focus for our work in West Africa.