Changes in the size of Blackbirds wintering in Co. Tipperary between 1982 and 2006.

Christopher J. Wilson. ‘Uisce Bhan’, Ballinamorragh, Curracloe, Enniscorthy, Co. Wexford.

The Blackbird Turdus merula breeds throughout Europe in a variety of habitats. Some populations are completely migratory, some are partial migrants and some are completely sedentary (Cramp 1985). In Ireland, the resident population is sedentary, with an influx of birds in winter from Northern Britain and Scandinavia (Hutchinson 1989). Bergmann’s rule states that “in warm-blooded animals, races from warm regions are smaller than races from cold regions” (Mayr 1970). Wing length has been found to be the most reliable and reproducible measure of overall body size for a range of species (Redfern & Clark 2001). Measurements of museum specimens from throughout Europe show that wing length increases with latitude (see table 1 below), consistent with Bergmann’s Rule.

Table 1. Wing Length (mm) of Blackbirds in Europe (mean and range) from The Birds of the Western Palearctic. (Cramp 1985).

In the past 100 years, Europe’s climate has warmed by more than the global average, with an increase of 0.95^oC since 1900. Temperatures in winter have increased more than in summer with the warming being greatest in northwest Russia and the Iberian Peninsula (Voigt & van Minnen, 2004). Climatic factors are known to affect the distribution of organisms (James et al. 2005).
The migration route to Ireland for Blackbirds from northern Britain has largely been abandoned in recent years, and fewer migrants from northern Europe are spending the winter in Britain and Ireland. This is probably due to milder winters accompanied by a move into more hospitable suburban habitats in northern Europe (Chamberlain & Main 2002). In this paper I use Blackbird biometric data collected in Co. Tipperary over 25 years to ascertain if there has been any temporal variation in Blackbird body size, which will allow me to investigate the hypothesis that global warming has resulted in fewer large Blackbirds from northern Europe wintering in Co. Tipperary.

Tipperary is an inland county in the southern half of Ireland (latitude 52^oN to53^oN).

Blackbirds were trapped, using mist-nets, during routine ringing activities in Co. Tipperary from 1982 to 2006. Birds were trapped in a variety of habitats such as gardens, broadleaved woodland, orchards and farmland with hedgerows. Each bird was aged and sexed using criteria in Svensson (1975 & 1984). Wing length was measured to the nearest 1.0mm with a stopped rule, using the maximum chord method (Spencer 1972 & 1984). Almost all measurements were taken by Christopher J. Wilson (CJW) and by the author (KPC), who was trained by CJW in bird ringing techniques. A few measurements were taken by other trainees under the supervision of CJW or KPC. Both CJW and KPC left Tipperary for work reasons, so there are no data for the years 1992 to 1998.

Because the distribution of wing lengths was approximately normal, (see figures 1 and 2) I have used the t-test to compare the mean values for wing lengths in the different age/sex categories.

Figure 1. Distribution of wing lengths of adult and juvenile male Blackbirds in Tipperary in winter from 1982 to 2006

Figure 2. Distribution of wing lengths of adult and juvenile female Blackbirds in Tipperary in winter from 1982 to 2006

Details of the wing lengths of 166 Blackbirds ringed in Co. Tipperary in winter (the months of November to February incl.) between 1982 and 2006 are set out in tables 2 and 3 below. Adult and juvenile data are presented in two separate tables because juveniles are usually significantly smaller than adults with wing lengths 2mm to 3mm shorter (Leverton 1989). Five wing lengths of adult females were clearly errors and were not included in the analysis. The data is presented in 5 year periods from 1982 to 1986 and from 1987 to 1991. No ringing data were collected between 1992 and 1998. The third period is 1999 to 2006.

Table 2. Wing lengths (mm) of adult Blackbirds wintering in Co. Tipperary from 1982 to 2006. (mean, standard error, sample size and range)

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Adult male Adult female

1999-06 132.3 ± 1.43 (n=9) 126-139 128.2 ± 1.82 (n=6) 123-133

Mean wing lengths decrease in the order adult male > juvenile male > adult female > juvenile female.

Table 3. Wing lengths (mm) of juvenile Blackbirds wintering in Co. Tipperary from 1982 to 2006. (mean, standard error, sample size and range)

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Juvenile male Juvenile female

There is clearly a progressive reduction in mean wing length of adult male and juvenile male Blackbirds over the period 1982 to 2006. The situation with females is different. There is a decrease in mean wing length in adult females from 82/86 to 87/91 but there is a slight increase in 99/06. The sample size for juvenile females is too small to be of any value in this analysis.
Note; The wing lengths given in table 1 are not to be compared with the data in tables 2 and 3 because table 1 is based on museum specimens which are known to shrink by up to 4% (Winker 1992).

This study clearly shows a progressive reduction in the wing length of male Blackbirds wintering in Co. Tipperary from 1982 to 2006. The difference in the mean wing length of adult male Blackbirds was not statistically significant between 82/86 and 87/91. However, the difference in the mean wing length of adult males between 82/86 and 99/06 was significant (t = 2.19, P > 0.05, df 32)

The difference in the mean wing length of juvenile males between 82/86 and 99/06 was also significant (t = 3.05, P > 0.01, df 23).
There are two possible reasons for the absence of a significant decrease in the mean wing lengths of females from 82/86 to 99/06. The first is that the sample size is too small and if more females were trapped, we might get a better picture of the real situation. This may account for some of the lack of data for juvenile females. The second possible reason is that female Blackbirds from northern Europe are still coming to Ireland in winter.

Chamberlain & Main (2002) show from ringing recoveries that, there are more ontinental immigrants amongst British and Irish female Blackbirds (both adults and immatures) than amongst males. It has been shown for Meadow Pipits that the population wintering in the north of the range consists mainly of males ( Hotker 1989).
This entire study hinges on the consistency of wing measurements. To avoid errors in the data due to different measuring techniques, we confined the study to birds ringed in Co. Tipperary by or supervised by CJW and KPC.

Because the mean wing length of male Blackbirds wintering in Co. Tipperary has decreased significantly between 1982 and 2006, we can reject the null hypothesis – for males only. The results of this study are inconclusive in relation to females and further trapping is required to increase the sample of female wing lengths in the data.

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