Avon BTO

Friday, July 04, 2008

Independence Day

I have both breeding season results for 82 tetrads, which gives an opportunity to compare distribution with 1992. The comparison is not exact because that survey asked observers to spend one hour in a tetrad twice; so that this time, with double the observation time, less common species are more likely to be observed. 62% of 98 species show a distribution within 10 points of that in 1992, implying no real change in distribution (though this may not be the case for a few species whose distribution was less that 10% previously). Of those that show a greater change, all but five have a positive change; of course additional information may alter that. The five that show a significant distribution decline, of between 11 points (GW) and 33 points(WW) are Y. RO and CK; There are no surprises there.
Rooks are interesting, because we know how many there were in 2005; we counted 6000 nests, hence the breeding population was 12,000 birds- and there would be an additional say 2000 non breeding juveniles. This was a 20% decline on the 2000 rookery count figure of 7400 nests. This year the 82 tetrads have counted (taking the maximum of the two surveys), 1700 rooks, which, if true for all tetrads, would give an estimated population of 8500 birds, or about 4000 nesting pairs, a 30% drop on the 2005 figure. I have not yet added up the number of rooks nests recorded by observers.
The winter rook maximum total was 7623 from 248 tetrads which would imply a winter total of 12,400 birds, very much closer to the results of the 2005 Rookery count. The question is are we undercounting in the breeding season, which is probable for early counts, or do we have birds here in winter that dont nest here. BBS results have shown a clear decline in rook numbers in recent years, and it will be interesting to see this year's results- because as they compare like with like they are a much more precise measure of change than is possible from the Atlas survey.
Watch this space; And, as song is falling off by the day the more second surveys that can be completed soon the better.


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