26/12/2011
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Freezing winter followed by summer of love for resident birds

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The cold winter weather may have reduced the numbers of thrushes and finches nesting in our countryside during the summer of 2011, but those that survived the freezing temperatures produced a plentiful supply of youngsters. In contrast, the migrants returning to our shores in abundance during the spring experienced a very average breeding season.

Constant Effort Ringing (CES)

Following yet another very harsh winter, it is not surprising that the abundance of nine resident species was significantly lower than the average over the previous five years, with adult numbers for Dunnock, Song Thrush and Reed Bunting falling to their lowest levels since the CES scheme started in 1983. However, a total of 10 species did exhibit significant increases including four residents (predominantly seed-eaters) and six migrants (all warblers). The abundance of Blackcap, Chiffchaff and Blue Tit was the highest recorded since the beginning of the scheme.

Ten species, nine residents and one migrant (Blackcap) had a particularly productive breeding season, displaying significantly higher juvenile:adult ratios than the average reported over the previous five years. This increase in breeding success may help to redress the reduction in numbers experienced by many resident species over the last two winters. The productivity of Dunnock, Chaffinch and Bullfinch was the highest recorded since 1983. Breeding success was significantly lower than the five-year average for Garden Warbler, Lesser Whitethroat, Willow Warbler and Blue Tit.

Table 1: Changes in captures on Constant Effort Sites in 2011 (from 98 sites). Significant increases are in green; significant decreases in red. Lower and upper confidence intervals are given in brackets. '*' denotes a small sample size.

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Nest Record Scheme

In contrast to the 2010 season, during which Barn Owl brood sizes fell to one of the lowest levels on record, 2011 appeared to be a very productive year for all three of the small-mammal specialists included in these analyses: Barn Owl, Tawny Owl and Kestrel. Anecdotal reports of well-stocked prey larders suggest that rodents were plentiful during the breeding season, and the warm, dry spring weather provided perfect hunting conditions. Kestrel appeared to fare particularly well, exhibiting a 15% increase in the number of fledglings produced per nest, welcome news for a species whose recent population decline, particularly in England, has raised concerns amongst conservationists.

The NRS results from 2010 indicated that it had been a very good year for woodland box nesters, with Blue Tit, Great Tit and Pied Flycatcher all producing large numbers of fledglings. This positive trend continued into 2011 thanks to the warmest, and one of the driest, spring since the mid-1960s. Great Tit fared particularly well, producing the highest number of fledglings per nest since 2002. The same could not be said of the two other cavity nesters, House Sparrow and Tree Sparrow; while brood sizes of the latter were significantly higher, a greater proportion of nests failed during incubation, and the number of fledglings produced by both species was close to the average.

Many of the additional species for which preliminary trends were produced in 2011 were open-nesting passerines, including a good mix of residents (Song Thrush, Robin, Dunnock), short-distance migrants (Blackcap, Chiffchaff) and long-distance migrants (Reed Warbler, Whitethroat, Willow Warbler). There was some suggestion that migrant breeding success was lower than in recent years, particularly for Sylvia warblers, with Whitethroat clutch and brood sizes both significantly reduced. However, the number of offspring fledged did not differ significantly from that recorded during the previous five years for any migrant species. Residents appeared to fare better overall, with Song Thrush and Dunnock productivity particularly high, although again these differences were not statistically significant.

Table 2: Percentage change in breeding success in 2011 relative to the average for the previous five years. Significant increases are in green; significant decreases in red.

Written by: BTO