Wednesday, May 5, 2010

Whimbrel migration.

This is the first in what might best be described as a "mini series" of occasional summaries relating to bird migration, or similar subjects, here on Islay.

Each spring we witness Whimbrel migrating through northwards and both their presence and ringing cries herald the emerging new season. Numbers differ, based on the influence of weather, observers , the sheer good luck of being in the right place when a group sweeps through or alongside a field where they taking time out to feed and rest.

Actual migration past Frenchman's Rocks, SW Islay is minimal contrasted against the numbers that are recorded around Loch Indaal and in the NW of the island. These birds are, I believe, exclusively associated with the Iceland and Faeroe's population, although it's feasible that some of the birds could cut across the Great Glen , at height, heading for Scandinavia. Remember too that we do have a small breeding population in the UK mainly centred on the Shetland Islands. The majority of birds in flight over Islay tend to be moving north or NW, either over our land mass or along the coast. The first birds come through in mid April and passage continues to mid May, but with stragglers outside of those dates. Groups often number around 20 , but flocks in excess of 60 are not uncommon.

Some years ago, when associated with the Wintersett Ringing Station in West Yorkshire, spring birds were invariably singles or in low numbers and most often passed to the N or NE. Later knowledge emerged of a roost at a reservoir in west Bowland, North Lancashire, of birds on Morecambe Bay and of a favoured feeding area on farmland near to the Bleasdale Estate. Invariably any of these latter birds seen leaving flew off to the north east suggesting they were part of the Scandinavian or Russian populations. Birds are recorded on the Solway Firth which suggests passage up the Irish Sea northwards towards Islay. At this stage they've probably completed two thirds of their long migration from their wintering quarters predominantly in West Africa.

Ferns et al (1979) counted spring migrating Whimbrel and, at that time , there were notable concentrations associated with sites around the Severn Estuary. A peak spring passage count showed ca. 2500 birds to be involved, with a notable single site where birds concentrated being the Somerset Levels. This phenomenon now appears to have ceased. Interestingly birds caught on the Severn Estuary in spring had fat reserves sufficient for them to make a direct flight to Iceland, which suggests many birds might simply pass over Islay. Work in Ireland has seen huge roosting concentrations on occasion, i.e. 1000+, and substantial numbers passing through northwards, again presumed from the Iceland population. In other years only small numbers are seen which suggests the route taken is quite narrow rather than being across a broad front. In the last two years, when we had more easterly winds in spring, fewer birds passed through suggesting they were being displaced westwards over to Ireland. Strangely enough, in all these western areas passage in autumn is much lower and flock sizes smaller.

Whilst we're in the middle of the 2010 passage period at present I've already records of almost 400 birds (399!! ) passing through and doubtless the actual total already is much higher. An interesting local phenomenon providing an absorbing subject each spring!!

No comments: