Avon BTO

Tuesday, July 08, 2008

A century of tetrads

I have the breedingseason results from 102 tetrads, a quarter of the region, and the comparison between the distribution of species in 1992 and today is becoming clear. 108 species have ben recorded in tetrad surveys, and of those 65% are within 10 percentage points either way of their distribution in 1992- ie no significant change in distribtion. Only five species show a significant (ie more than 10 points) change, and they are CK, GW, WW, RO and Y. By contrast 33 show a positive change. In part this is because the time allocated for observation is double that in 1992, which has allowed elusive species to be seen more often. Thus Sparrowhawk has increased from 16% to 26%, Goldcrest from 20% to 47%. But The increase of MS from 11% to 24%, CG from 2% to 17%, MA from 39% to 72%, BZ from 18 to 71%, LB from 21 to 57% and HG from 10 to 47%, CD from 53 to 83%, GS from 19 to 49%, BC from 59 to 87%, Lt from 27 to 58%, Jay from 24 to 55% and GR from 66 to 91% all represent real change.
Because there were no counts in 1992 it is not possible from this data to assess changes in density, but because BBS began in 1994 we have useful figures for density measured as a rate/hour which will give a measure of density change; and it will be interesting to compare winter and breeding season densities directly as well.


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