Sunday 10 March 2013

The winter movements of a pair of Swifts (part 3)

Contributed by Lyndon Kearsley

Getting into position. A further look at the winter timing of a pair of Belgian swifts in 2012. This is the third instalment of our story about where the Swifts were in December and February.

March is a difficult month for tracking birds using light level dataloggers. From about 15 days either side of the spring equinox on the 20th, the day length is about the same across all latitudes and so no north / south fix is possible. However the timing of local midday is not affected, so longitude (how far east or west) can be calculated. 

In the following map I've plotted the locations during March (shown in closed or open black dots) by using the longitude combined with a fabricated latitude derived by simply dividing the first and last fix before and after the equinox by the number of days. The February and April positions are also included to show how this ties in before and after the equinox

You can see that the female leaves East Africa crossing rapidly to a position which coincides with her December location on the lower Congo River. She remained there at about that longitude for the last two weeks of the month and then departed towards West Africa at the beginning of April.

The male moved very little in this period probably remaining above the lower Congo River wetlands in the Democratic Republic of Congo.

                   Red dots are female positions in February and in April
                   Black dots are female positions in March
                   Blue dots are male positions in February and in April
                   Open dots are male positions in March
                  [Click map to enlarge]
The April tracks show how this story continues and marries up with this theory. The male indeed remains in the lower Congo until about the 20th of April when he too makes his move out to West Africa (Sierra Leone).

The female had already been in West Africa for quite a while by that time having crossed the Gulf of Guinea in early April, and her positions show her to have been first in Ivory coast, then Liberia, Sierra Leone and Guinea. Bear in mind that geolocator fixes are at best approximate and the tracks shown are smoothed 3 day rolling averages. Never the less at this scale one gets a clear idea of the birds timing and strategy.

On 29th and 30th April both birds are shown to be somewhere in Sierra Leone and closer to each other than they have been all winter, nicely placed for the dash back to Belgium.

Recently a  paper discussing a statistical track estimation solution for the latitude problem during both equinoxes was published by Wahlstrom et al (2012). 

Wahlstrom, N., Gustafsson, F., & Akesson, S. (2012, July). A voyage to Africa by Mr Swift. In Information Fusion (FUSION), 2012 15th International Conference on (pp. 808-815). IEEE.

1 comment:

  1. Interesting article. Just saw first swift of the year. Gloucestershire, England.