Monday 23 January 2012

Why are our African migrants declining?

Contributed by Dick

A recent paper in Bird Study (see reference at the end) has looked at correlations between population trends of migrants in Britain and a number of 'bioclimatic' factors in Africa. Of course, Swift is included in the 26 species they looked at.

Click for larger image
Some African migrants are increasing, including those which might have similar pressures to Swift, e.g. Swallow, House Martin and Sand Martin, but many are decreasing, especially Turtle Dove, Cuckoo and Spotted Flycatcher.

Two of the factors analysed are 'bioclimatic zone' with categories 'southern', 'humid', 'arid' and 'north', and 'winter habitat' with categories 'general', 'wetland', 'open' and 'wood'.

Swift is categorised as bioclimatic zone 'southern'  and winter habitat 'general'.

There are 6 other species classified as winter habitat 'general': Hobby, Sand Martin, Swallow, House Martin, Reed Warbler and Chiffchaff. According to BBS, these birds have all increased since 1994, Swift is the exception.

Three species are classified as bioclimatic zone 'southern': Hobby, Swallow and Swift. Hobby and Swallow are increasing and, again, Swift is the exception.

Therefore if something bad is happening to Swifts in Africa, these general categories do not explain what it is.

While loss of nest-sites is assumed to be a significant factor in the decline in Swifts in the UK, it seems that their rate of decline may be too large for this to be the only factor, so this paper raises at least this question in the case of Swift: what is happening in Africa to reduce the survival rate of Swifts? Quite possibly it is something happening in the stopover sites in Mozambique and Liberia. Hopefully more work with geolocators will help answer this question.

Thanks are due to Chris Hewson for sanity checking an earlier draft.

Nancy Ockendon, Chris M. Hewson, Alison Johnston & Philip W. Atkinson (2012): Declines in British- breeding populations of Afro-Palaearctic migrant birds are linked to bioclimatic wintering zone in Africa, possibly via constraints on arrival time advancement, Bird Study, DOI:10.1080/00063657.2011.645798
To link to this article (behind a paywall):

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