We believe that the problem relates to the tags’ batteries. The tags are solar-powered and need to be recharged by the sun. It is possible that during the bird’s moult, due to a change in the woodcocks’ behaviour, the tags were not receiving sufficient sunlight and have run completely flat. To fully recharge a battery from flat will take about 4 hours of bright sunlight. It may be that the tags are not receiving this. Day length is short and sunlight intensity low in the northern locations where the birds have spent the autumn, not to mention the fact that the birds are often hidden during the day.
Those who followed Woodcock Watch last year will remember the same issues in our first autumn. However, this year’s results have been noticeably better and the tags have returned far more data. Crugith and Lanyon for instance have transmitted useful results through the majority of their autumn migration and we hope that more such data is on its way.
It is more than likely that some of the birds have died. Crousa, who we have not heard from since May, and Elissa, who has not sent data since June, are probably no longer alive. Annual survival rates are around 60% for woodcock - that is to say that around 40% of the adult population do not survive from one year to the next. Lifespan does not typically exceed 4 years. It is perfectly natural that we lose a few of our woodcock over the course of a year.
But for birds like St Patrick, Skittle, Amy and Rebecca, who last transmitted in August or September, there is still hope. It is possible that their tags will receive the sunlight they require in the coming weeks and will then begin transmitting data again. We observed this pattern in 2012 with several individuals sending winter locations after an autumn quiet period. It is possible that some of these birds, when we next here from them, will be back here in the UK.