Avon BTO

Monday, July 28, 2008

Almost there

The end of the first season approaches, and I have had the results from 160 tetrads to date- and there are plenty more to come, boith paper and on the web.
lessons learnt?- A lot of people only do things at the last minute- and it is probably better to do things early- certainly in the breeding season.
Identifying juveniles, and providing breeding status generally, has been a novelty for some and is trickier than might at first appear. Luckily someone doing a 10k square usually comes up with proof of confirmed breeding- though there are some slightly embarrasing blanks. But no odds- targetting species next year will fill the gaps.
Overall change in the breeding population since 1992 is also pretty clear; 27 have a significantly (ie more than 10 points increase) wider distribution now than they did; 17, mostly uncommon, species have decline further or gone. 11 new species have appeared, mostly in very small numbers. Quite how much of the wider distribution is caused by the change in methods has yet to be calculated.
There are no surprises, but it is good to have precision about the processes of change, which are, of course, continuous, complex, and driven by a mass of interacting causes. (For those who believe climate might be relevant, the four years 1988-91 of the previous survey had an average annual temperature of 13.9C .The last twelve months have an average of 14.2C).

Once the final figures are in i can look at the breeding season density compared with the winter, and the changes in density since 1994,

Monday, July 21, 2008

Statistical snippets

Results from 130 tetrads (33% sample) in the breeding season show some pretty clear changes.

The bad news;- Lapwing down from 17% to 8%, Cuckoo from 26% to 3%, Willow Warbler from 67% to 35% , Marsh Tit from 10% to 3%, and Yellowhammer from 47% to 37% (though much of its key habitat has yet to be explored).

The good news; MS up from 11%-24%, CG from 2% -11%, SH from 16% to 24% , BZ 18% to 72%, PE 0%-5%, even K 30%-38%, MH 29%-52%, LB 21%-55%, HG 105-45%, CD 53%-81%, GS 19%-50%, BC 59%-85% ,GC 20%-51%, RN 0%-15% ,GR 66%-92% ,and GO 55%-87%.

These figures will change a bit with further results, especially as both cotswold and mendip areas are underrepresented, and urban and suburban areas over-represented- 60 of the tetrads are from ST57, ST58, ST67, ST68. But there are few surprises. However more difficult is to puzzle out the causes.

Sunday, July 20, 2008

Ten days to go

Just back from a weeks holiday in kent- few birds, lits of houses and gardens.
Back to find a lot of activity has produced a total of 174 tetrads with one second visit and 118 with two. So 52 second visits to go, and I hope some more with both visits, as we clocked up 203 winter computer visits.
One thought- a lot of visits have not recorded confirmned breeding for House Martin- but they are all busily feeding young in the nest now. If your 10km square has not recorded confirmed breeding for House Martin a quick check on your nearest nest would give a Roving record.
Good luckj for the last few days- then we can all relax until November!

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.

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.