The catching this morning of a Sedge Warbler which weighed in at 17.5 grams set me looking back through our records. Sedge Warblers usually weigh between 10 to 11 grams so this bird had put on weight mainly in the form of fat amounting to an increase of approximately 75% of its body weight. Fat is the fuel that the bird will use on its nocturnal migration towards its wintering area in Africa.
Over the past ten years we have only caught eight Sedge Warblers in October of these 50% show signs of accumulating fat. But today's bird was the heaviest. It is not however the heaviest we have recorded, this is one trapped on 24/9/04 which weighed 18.1 grams. How long does it take a bird to accumulate this amount of fat? We have a good pointer in a bird ringed on 24/9 weighing 12.9 grams and 7 days later it was re-trapped and weighed 16.5 grams.
However Ian Newton's superb book on Bird Migration gives some figures for Sedge Warblers migrating through Kenya in spring. The maximum rate recorded was a bird which went from 12.2 grams to 19.6 grams in just three days another bird more than doubled its weight in 17 days.
We also caught two Reed Warblers today one was a normal weight of 10.6 grams the other weighed 13.1 grams. Looking back through our records the heaviest Reed Warbler I could find was 14.8 grams.