… migration forecast #2
This migration overview is in two parts. Today, we concentrate on Africa where interesting developments are taking place as I write this:-
The Inter-Tropical Convergence Zone (ITCZ) boundary has finally shifted. Under its influence, circulation patterns in the region are beginning to change and – perhaps most importantly for our pre-migration birds – the Azores high-pressure area has been displaced by a major south-tracking Atlantic low.
|Temp & Sea-level isobars for 12:00z 4/3/13 (Meteoradar.co.uk)|
From tomorrow (Monday) we are predicting a step-change in daytime temperatures. For the past few months, things have been fairly cool (for west Africa) but this week will see midday shade temperatures > 36 DegC with peaks around 40 DegC in inland areas. Coastal winds, which have been fixed at north and north-easterly during the winter, will swing westerly and south-westerly, force 4-6, even > 7 over Morocco and the Straits.
These factors will aid north-bound migrants for several days, enabling them to get (literally) a flying start to the race.
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On reflection, perhaps “race” is the wrong word. Spring migration is more like a car rally than a track race: The object is to arrive on schedule at each Time Control, and there are stringent penalties for a crew that clocks in too late – or too early.
For migrating birds, the analogy holds good: Get to your breeding grounds late, and the nest sites and territories might all be taken. Even worse, your mate from last season might have got fed up waiting and decided to start a family with someone else! But there could also be a penalty if you get there too early: the weather might be – probably would be – insupportably bad, and your summer food supply might not even have arrived yet. (This is certainly true for coastal ospreys. Don't forget that fish migrate, too...)
So it's all a matter of timing - with or without stopwatches.
Next week, we'll look at weather patterns over Europe and see how these might influence our birds as they make their way north.