Tuesday, November 15, 2011

Those Chaffinch

We ringed lots of Chaffinches at Out Rawcliffe this autumn, but most of the time Will and I were too busy on a daily basis to look in detail at the age/sex ratio of the birds we caught, other than mentally note how many were both females and first calendar year/juveniles.

The ringing site on Rawcliffe Moss is approximately 6 miles from Garstang, a town on the very western edge of the Pennines. In North West Lancashire during autumn mornings there is a marked but mainly inland movement south of Chaffinches, a phenomenon which is often very pronounced in the first few or occasionally several hours after dawn. The visible passage begins in August, peaks during September, declines somewhat during October to then virtually fizzle out in November. This dispersal of Chaffinches has been described by several observers recording the visible migration of many bird species in the Pennines area of Lancashire.

I looked at our captures made between the months of August to October in the two years 2010 and 2011. During 2010 we caught 332 individuals, and then 375 in 2011, a total of 707 captures in the two autumn periods.

Chaffinch

The BTO Migration Atlas states that Chaffinches involved in autumnal movements are thought to be 90% first year birds, predominantly females. Our data from Out Rawcliffe supports the idea that the Chaffinches passing through there are largely juvenile/1st Calendar Year (1CY) females. The wing lengths of the birds involved also show that the same Chaffinches are probably entirely from the UK population.

Chaffinch wing lengths occur in the range of 81mm to 98mm for males and 73mm to 89mm for females, so although there are overlaps in biometrics, plumage differences make separating the sexes easy. Birds from northern and eastern populations can be slightly larger, often with a longer wing length, whereby males with a wing more than 93mm or females with a wing length greater than 86mm may originate from more migratory Scandinavian or Eastern European populations.

Chaffinch

Of the 707 Chaffinches captured during the autumns of 2010 or 2011 none could be positively identified as of Scandinavian or continental origin. In the Autumn of 2010, there were 2 Chaffinches caught on 1st November each with a wing length of 93mm which might be considered none-UK birds, but not definitely so as both were large adult males. In the autumn of 2011 there were 2 Chaffinches both on 28th October 2011, with wing lengths of 92 and 93mm respectively which might be considered non-UK birds. The timing of these 4 captures fits well with the suggestion of continental birds reaching the north west of the UK in the later part of autumn via south east England when more migratory continental Chaffinches spread north and west.

Table: Age/Sex ratios of Chaffinches

Month

Total Captures

Adults (% of Total Captures)

1CYs (% of Total Captures)

1CY male (% of 1CYs)

1CY female (% of 1CYs)

Aug 2010

27

9 (33%)

18 (67%)

5 (28%)

13 (72%)

Aug 2011

44

5 (11%)

39 (89%)

17 (43%)

22 (57%)

Sep 2010

263

48 (18%)

215 (82%)

71 (33%)

144 (67%)

Sep 2011

228

35 (15%)

193 (85%)

88 (45%)

105 (55%)

Oct 2010

42

16 (38%)

26 (62%)

8 (31%)

18 (69%)

Oct 2011

103

17 (17%)

86 (83%)

37 (43%)

49 (57%)


From the 700+ Chaffinches the number of recaptures was very low, supporting the idea of onward dispersal. If anything the low number of meaningful records of birds first ringed in 2010 and then recaptured in 2011 without recapture in-between, suggest that some experienced Chaffinches use exactly the same dispersal route and stop overs each year.

Chaffinch

5 comments:

Mary Howell Cromer said...

What very interesting facts on these beautiful Chaffinches! I find this all so wonderful what you do. I do not quite understand it all, yet I do understand that it is an important element in knowing the numbers, what is going on with them, their travels and how many recatures have survived for longer periods, etc. I wonder how many birds total have come through your hands...would be a stunning number! You do good things Phil~

lp.are.the.best said...

How did you manage to ring more than 700 Chaffinches ? Only mist nets ? And again the photos are unique :)

David Norman said...

As the author of the Migration Atlas account for Chaffinch, I am very pleased to see this analysis. I ended that text by commenting 'some of the main gaps in knowledge are the true differences in behaviour between local birds and immigrants and between males and females ...'

It is still the case that most Chaffinches are ringed in south and east England - and this has led to incorrect generalisations - so any studies elsewhere are especially valuable.

Well done Phil (and Will)!

Russell said...

Very interesting. Great information with superb illustrations. Man, I am so unobservant (I'd be happy to ID it by its common name and possible sex). To separate them so carefully with such fine details needs skill and knowledge Nice work.

Paco Sales said...

Que buen trabajo tan perfectamente documentado Phil, es un placer visitarte y aprender sobre la gran diversidad de aves con las que estás trabajando. Un abrazo para ti Phil

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