Wednesday, January 21, 2009

Migration and stopover of Manx shearwater

An interesting paper recently appearing in Proc Royal Soc B. Using small data-loggers placed on 6 breeding pairs of birds captured and recaptured on Skomer Island workers have been able to work out the route and stop-over sites of these shearwaters as they enjoy their pelagic 'winter' away from their breeding grounds.
The data loggers used not only recorded their approximate positions but, at 10 minute intervals during the recording period (mid Aug 2006 to June 2007), they also logged whether they (the logger attached to the birds' leg) was submerged or not. This then permits estimation of just how much time each bird spent sat on the water.

The first image shows the route taken by each of the 12 birds, and the inset the occupancy contours during their over-wintering stay off Brasil.
The second image shows the stopping off locations - big circles 10 days, small 5 days, outward from the colony with a crossed circle and open circles movement back to the colony. The 'on-land' stopovers serve to indicate possible position location errors rather than feeding at inland sites.
Apart from a longer return migration in males there does not appear to be any significant differences over migration between the sexes, except for something termed 'pre-laying exodus' when females leave the breeding colony to, it is assumed, visit rich feeding areas in order to amass the nutrients sufficient for it /them to develop its one egg - which comes in as a massive 15% of its body weight.
The full article is "Proc R. Soc. B doi:10.1098/rspb.2008.1577" (on-line publication)


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