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.

Wednesday, 13 April 2011

Strong north-westerlies slow things down

After the rush of the weekend, the moderate to strong North-Westerly winds that set in on Monday made it difficult for Northward-bound migrants to make any headway. Prior to migration, birds feed up frantically, storing fat all over their bodies - in the pit of the throat, in loops around their bellies, in their wing-pits and even on their thighs.

Once fully loaded, a small songbird can, on average, fly around five-hundred miles if the weather conditions are suitable - light winds from any direction are good but light Southerlies are ideal. Flying into a strong headwind reduces the distance a fat-loaded bird can fly, so it is better to land, rest and feed up in readiness for the next leg of the journey.

That it seems is what many birds have done during the early part of this week. On Monday, Willow Warblers did just that but on this side of the Channel. Visible migration watchers at Hegistbury Head, Dorset, awoke to a fall of Willow Warblers with at least 400 grounded there overnight on Sunday.  Take a look at the BirdTrack reporting rate (and animated map) for Willow Warbler which shows how dramatically numbers of this species have increased.

Above: Willow Warbler by Jill Pakenham

For the rest of the week the winds are turning Southerly and will be fairly light. In these conditions we can expect the arrival of more birds, and a small low pressure system over the Channel on Thursday could see many of them grounded on the south coast.

Birds from the Med continue to be found, giving further evidence of the scale of the arrival that happened last weekend. More Hoopoes and Woodchat Shrikes have been found, along with a single Iberian ChiffchaffOn the common migrant front, there have been more Grasshopper Warblers and Nightingales. The first Turtle Doves are here and the first migrant Golden Oriole has been seen in Devon - our breeding birds won't be back for at least another two weeks.

So for anybody out and about this week, Thursday looks like the best bet to witness visible migration and the spectacle of seeing large numbers of grounded migrants. It's certainly the day I plan to be out looking for grounded migrants at Dungeness.

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