The CES Scheme uses standardised bird ringing to monitor the abundance, breeding success and survival of 24 common breeding songbirds. Over 120 sites across the UK are monitored by qualified volunteer bird ringers through the breeding season, with twelve standard visits between May and August. Changes in the total number of adults caught provides a measure of changing population size, whilst the ratio of juvenile to adult captures provides an index of breeding success. Recaptures of adult birds ringed in previous years are also used to estimate annual survival rates.
Preliminary CES results reveal poor productivity across most species
Cold, wet and windy conditions across most of Britain and Ireland posed a challenge to both birds and volunteers (Graphs 1a & b). Ringers had to dodge gales and rain, many even wading through floods to fit in their CES visits - without this valiant effort, we would have much less information about how birds cope with these extreme events. Thanks to the rapid submission of data, we have already been able to produce these preliminary trends.
Graph 1: Showing a) temperature and b) rainfall in spring and summer 2012 (red line)
relative to the 2007-2011 average (blue line)
A poor breeding season for migrants
Despite early fears that migrant populations would be decimated by huge storms in North Africa and Southern Europe most arrived in good numbers (Table 1), if a little later than average (as evidenced by the Nest Record Scheme preliminary laying date trends). Short-distance migrants, Blackcap and Chiffchaff, which winter in the Mediterranean were particularly numerous, with both species recorded in the highest numbers since CES began in 1983 (Graphs 2a & b). The long-term decline in the abundance of Sedge Warbler, a long distance migrant, continued however, possibly due to deteriorating conditions on the African wintering grounds, with numbers reaching an all-time low (Graph 2c).
While trends in abundance varied, breeding success was uniformly poor. Seven of the eight migrant warblers monitored by CES experienced very significant reductions in productivity compared to the five-year average (Table 1) and all eight exhibited the lowest productivity since CES began in 1983. Many of these species feed on caterpillars and flying insects, which may have been hard to find during periods of heavy rain. The majority also build open-cup nests, so eggs and chicks may have become chilled during wet spells. However, the declines in productivity recorded by CES were generallygreater than the reduction in fledgling production recorded by Nest Record Scheme participants, suggesting that the survival of chicks once they had left the nest was also affected by the weather. This is perhaps unsurprising, as young birds are likely to be less effective at finding food and their plumage is also relatively poor in quality, increasing their chance of becoming waterlogged.
Graph 2: Adult abundance for a) Blackcap b) Chiffchaff c) Sedge Warbler
since 1983, 2012 figure in red.
Tits also fare badly
Numbers of adult tits were very similar to the five-year average though the number of Willow Tits is now so low that any changes need to be very large to record as significant. A combination of mis-match of food supplies and fledging and the cold and wet conditions meant juvenile productivity on CES continued the trend demonstrated by the Nest Record Scheme. Great and Blue Tit thus demonstrated signifiant reductions in productivity (Table 1) with the second and third, respectively, worst seasons since CES began. (Graphs 3a & b).
Graph 3: Productivity for a) Great Tit b) Blue Tit and c) Long-tailed Tit since 1983. 2012 figure in red.
Mixed results for other resident species
Numbers of Blackbird and Robin were below average at the start of the season (Table 1), possibly due to low survival rates during the recent run of cold winters. However, the weather had relatively little impact on their breeding success, with both producing average numbers of young. The same was true of Song Thrush, and it is notable that all three species feed primarily on soil invertebrates and molluscs, which are unlikely to be adversely affected by heavy rain and may even increase in availability under damp conditions, helping to counter-act the physical effects of cold, wet weather.
Most other species showed poor productivity (Table 1). Species more heavily dependent on insects gleaned from shrubs may have found food items less accessible. Problems would have then been compounded by wetting of the chicks in open-cup nests (Dunnock, Chaffinch, Bullfinch) and the young as soon as they had left more enclosed nests (Wren, Treecreeper). Reed Bunting and Cetti’s Warbler, which both breed in marginal vegetation around ponds and lakes, are also likely to have been badly affected by flooding.
Graph 4: Productivity for a) Reed Bunting b) Cetti's Warbler since 1983. 2012 figure in red.
Does poor productivity in 2012 matter?
Songbirds have the potential to produce many offspring per nest and numbers can therefore bounce back after a single poor breeding season. However, the adverse conditions in 2012 were the result of a southward shift in the jet stream and climatologists do not know what impact future temperature increases will have on its trajectory. This summer has demonstrated what happens to our songbirds when the jet stream moves – if this becomes the norm then many of our songbird populations could be adversely affected.
We’re extremely grateful to all Constant Effort Site ringers and for the support of the JNCC/BTO partnership that the JNCC undertakes on behalf of the Country Agencies and the BTO Ringing Scheme which is funded by the BTO/JNCC Partnership, The National Parks and Wildlife Service (Ireland) and the ringers themselves.