Abstract from BTO Research Report No 323:

Marchant, J.H. & Wernham, C.V. (June 2003)

Phenology of autumn passage of Willow Warblers at Dungeness: 1960-2000 ISBN 1-902576-65-9

Executive summary

1. The Willow Warbler Phylloscopus trochilus is perhaps the commonest bird that is a summer visitor to Britain & Ireland. It nests in all regions except Shetland and in almost every 10 km square. In autumn, it is one of the earliest species to depart. Autumn passage birds are especially abundant on the southeast coasts of England, in East Sussex and Kent, and have been ringed there in large numbers at sites including Dungeness and Sandwich Bay.

2 The Common Birds Census has revealed a substantial decline in the UK breeding population of Willow Warblers, estimated at 40% for the 31-year period 1968–99. This decline has been strong in southern Britain and relatively minor in northern England and Scotland, and has been linked to a decrease in adult survival rates among birds breeding in southern Britain.

3 This study investigates daily counts of passage Willow Warblers at Dungeness Bird Observatory for autumns from 1960 to 2000, and draws upon data from the British & Irish Bird Ringing Scheme, held by the BTO. The national ring-recovery data set for Willow Warbler contained 2,856 records from birds ringed in Britain & Ireland at the time of this analysis, and 98 recoveries from birds ringed abroad. Only 284 recoveries, however, were from birds passing through coastal southeast England in autumn.

4 The phenology of the autumn passage of Willow Warblers at Dungeness shows a double or treble peak during August. The origin of this multi-modality is not clear but may relate to birds of different age-classes or breeding areas. Between the 1960s and 1990s there was a tendency for the passage peaks to fall later in the year.

5 Ring-recoveries of birds passing through southeast England in autumn show that birds on passage in July, when passage is beginning, are from breeding areas close to Dungeness. In early August, birds arrive from localities as distant as the north of Ireland and southern Scotland, whereas later in that month birds also arrive from central Ireland and from northern and central Scotland. Birds on passage in September are linked by recoveries to locations mainly in eastern Britain, suggesting that many such birds are of Continental origin.

6 The apparent shift towards later passage may stem from the decreases shown by CBC among southern breeding birds, since recoveries show that these are the birds that pass through Dungeness relatively early in the autumn. An alternative hypothesis, supported by the later timing of all three passage peaks in the 1990s compared with the 1960s, is that all elements of the population are passing through later in the autumn. The latter hypothesis may link to global warming, if warmer weather tends to delay departure – but evidence from observatory data from the Baltic region suggests that a warmer climate in the breeding areas might promote earlier rather than later autumn departure.

7 Some further work to help resolve these issues is suggested.

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