Summary of results from ESF Migration Network, 1994-96

Results continue to be published from the European Science Foundation Network on western Palaearctic-African Songbird Migration in which MRG ringers at Woolston participated from 1994 to 1996. This project co-ordinated ringing across 50 sites in 18 countries in Europe and Africa, with standardised measurements taken on a variety of migrant species, mostly warblers. Participants attended courses held at Falsterbo Bird Observatory, Sweden, to learn new techniques and calibrate their measuring techniques against other ringers, with David Norman and Chris Benson representing Woolston, so that measurements can be reliably compared across the whole of Europe. Professor Dr Franz Bairlein, the project coordinator and Patron of MRG, has kindly provided the following figures to illustrate some of the analyses carried out.

Morphological characters often follow a geographical gradient which can be used to track birds of different breeding origin along their migration. For instance, as shown in Figure 1, the wing lengths of Sedge Warbler (ACRSCH = Acrocephalus schoenobaenus) and Whitethroat (SYLCOM = Sylvia communis) in northern Europe increase significantly from west to east, with Woolston, at 2.5°W, recording smaller Sedge Warblers than anywhere else.

As shown in the following map (Figure 2), western European Garden Warblers (Sylvia borin) are shorter winged than birds trapped during autumn passage at more easterly sites and in particular in the eastern central Mediterranean. Again, the mean wing length of Garden Warblers measured at Woolston was the shortest of any site in the Network, and similar to those in Iberia and Morocco. These results give strong evidence for parallel migration of birds of different origin.

It is no surprise to find that the timing of autumn passage varies with latitude, many birds leaving northern areas earlier than the more southerly sites, as Figure 3 shows for Reed Warbler (ACRSCI = Acrocephalus scirpaceus) but there is no significant difference for Sedge Warbler (ACRSCH). This is perhaps because they undertake long-distance flights to fattening areas whereas Reed Warblers move more slowly and tend to feed on the way.

However, there are also longitudinal differences in the timing of autumn passage from northern Europe. The plots in Figure 4, using only the sites north of 50°N and with at least 30 birds caught, show that Garden Warblers (SYLBOR = Sylvia borin) and Willow Warblers (PHYTRO = Phylloscopus trochilus) migrate later the farther east they are, while Blackcaps (SYLATR = Sylvia atricapilla) show the opposite trend, but there is no significant difference for Reed Warbler (ACRSCI). However, the analysis cannot look just at the coordinates and has to consider some local factors. Woolston’s birds are mostly relatively local breeders, with few long-distance migrants, whereas Fair Isle (1.6°W) has none of these warblers breeding, and all of the migrants are from northern Scandinavia and much later than would be expected for its longitude. Excluding the data from Fair Isle gives a much stronger west-east correlation with the R²-values much higher: SYLBOR = 0.4204; PHYTRO = 0.6304.

David Norman