Thursday 7 February 2013

The winter movements of a pair of Swifts (part 2)

This is an update to our story about where the Swifts were in December and January, here.

Contributed by Lyndon Kearsley

It is February 2013 now and some common swifts have been seen in Malta and Israel over the last two weeks and a number of House Martins are in Lisbon (and Malta) along with lots of Swallows. That is normal timing for Southern and Mediterranean Europe, but what are "our" swifts doing. Maybe, they are in habitat like this:

Orange River region in Northern Cape Province / S Namibia in 2011. Photo F. Ambrogio.

Not possessing a crystal ball we can't of course answer that, but here is what the Belgian pair discussed in my last blog entry were doing in February 2012 last year.

February movements (click to enlarge)
The female moved around an area between Lake Malawi and the headwaters of the River Congo in south east Congo Republic (RDC) in February.  This range is about 1000 km in a SE to NW direction. It is very consistent with a second Belgian female who covered a similar range size that month, but a little to the NW in Central Congo.

The male partner which as you remember spent January in a rather small part of South Africa near Johannesburg moved slowly north into southern Zimbabwe and then headed WSW swinging across the south of Botswana in to western Namibia by mid month. It remained thereabouts for 12 days before heading quickly due north into the middle of the Congo Basin. As the crow flies about 4000 km, and always keeping between 1000 and 1500 km south and west of his mate which it neatly bypassed by month end.

Kalahari rain clouds. Photo Oompie
Male track superimposed on SABAP density map
(Click to enlarge)
You'll probably be thinking what on earth made it head over to Namibia? The east coast southern rains form a separate weather system and move due west across the Kalahari through January and February, greening it and triggering insect eruptions, particularly of various termite species. When the track is plotted on the South African Bird Atlas (SABAP) smoothed map of Common Swift observations one sees some correlation, although I have not had access to the dates of the field observations.

Please bear in mind that the geolocator data is only precise to within about 150 km and that Common Swifts are thought to spend the whole of their winter period flying. The dawn and dusk fixes give then only approximate midday and midnight averages. Those plotted here are smoothed averages taking into consideration the movements the day before and day after. The swifts themselves fly a huge multiple of these distances as they dart around going about their business. Since all geolocators are light level archival data-loggers, a constantly flying aerial platform (a swift) is quite ideal, even if not stationary, and the data surprisingly good. The calculated plots when considered on an African scale certainly reveal broad distribution, strategies and timing adequately. 

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