BTO migration blog

Spring and autumn are exciting times for anyone who watches birds. Here on this blog we will make predictions about when to expect migrant arrivals and departures, so that you know when and where to see these well-travelled birds.

Friday, 27 April 2012

North and west, east and south divide.

It has very much been a week in which migration has been a game of two halves, with south and east coast watchpoints having had a fairly quiet time as far as visible migration and grounded migrants are concerned.  In contrast, during the early part of the week, Fair Isle, Shetland, and Bardsey, Gwynedd, had their busiest days of the spring so far.

Some birds have been getting through the heavy rainstorms south of the UK. In the south, more Nightingales arrived back on territory, birds could be heard throughout southern Britain. Cuckoos continued to trickle in, along with a small number of Whitethroats and Grasshopper Warblers. Hirundines continued to arrive too but the numbers for the time of the year are low. Seawatchers in the south did experience a good passage of Great and Arctic Skuas, along with a small number of Pomarine Skuas. Arctic Terns were also still much in evidence and were joined at many sites by Little Gulls and the odd Black Tern.

Pomarine Skua by Joe Pender

Migration in the north and the west has been much more in evidence. At the beginning of the week a large number of Robins, Dunnocks, Song Thrushes and Ring Ouzels arrived on Fair Isle, along with three Tree Pipits, a Blue Headed Wagtail, ninety-one Wheatears, four Swallows, Two Wrynecks and singles of Hoopoe and Common Crane. At the same time Bardsey was teeming with migrants, which included over two-hundred Willow Warblers, over one-hundred Swallows, around eighty Blackcaps and eleven Grasshopper Warblers.

Wryneck by Jill Pakenham

Wales also held the lion’s share of southern European migrants. A Little Bittern was found in Pembrokeshire, Ceredigion hosted a Kentish Plover and six White Storks were seen over Colwyn Bay.
It is interesting to look at the distribution of migrants this week and relate them to the weather we have been experiencing. The birds that arrived on Fair Isle did so when the wind turned east/south-easterly, and those on Bardsey when the wind was from the north.

 It could be that these migrants drifted east in the westerly airflow that southern Europe have been experiencing as weather fronts arrived from the Atlantic. Having drifted east it is likely that they then made their way north on the wrong side of the North Sea, the weather here has been more settled at times. 
As they made their way north the anti-cyclonic weather fronts would find them in a more easterly airflow and by utilising this they would make their way back across the North Sea to the UK, arriving much further north than they might have been aiming for. This might help to explain a busy north and a quiet south migration-wise this week.

Looking at the weather forecast for this weekend, Saturday morning looks like the time to be out and about in search of migrants, as the winds south of the UK will be lighter than they have been for a week or so. By Saturday afternoon a low pressure system is due to cross central France, bringing heavy rain and fairly strong winds with it. It could be that Sunday will see a repeat performance of earlier in the week with the north and west again being the place to be.

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