The last few days have seen very little activity on the gritting trays at Leighton Moss RSPB Reserve. From September 23rd to November 27th we have recorded a total of 307 sightings of colour ringed birds involving 64 different individuals. Of these 36 were adult birds and 28 this years young.
We don't record every day but we have some interesting statistics on the number of visits birds make to the trays.Sixteen have been recorded on only one day. At the other end of the scale the most recorded bird was a juvenile female which visited on 15 days visiting first on September 30 and last on November 27th.
At each visit the birds sort through the grit which is mainly builders sharp sand. They can stop on the trays for up to 10-15 minutes and they appear to swallow regularly. This is in line with some research in Germany where they found an average of 609 small stones and a maximum of 850 stones in the gizzard in early winter when they are feeding mainly on reed seeds, but only 38 in spring when they turn to invertebrates.
One question I am often asked- Does this consumption of grit increase the birds weight. I looked at the weights of birds we have caught and weighed this year. In July the average weight of 22 birds was 14.1 gms. in October the average weigh of 40 birds was 15.2 and of 22 birds in November it was 15.7. So they have certainly increased their weight by around a gram and a half.Would need a much larger sample and a statistical test to prove the increase. But its interesting and does at first sight suggest that taking grit may well play a part in the increase.We have checked birds for any fat and there is very little.
John
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