Trends in the abundance of farmland birds: a quantitative comparison of smoothed Common Birds Census indices

1. Widespread declines in the populations of many British farmland birds have occurred since the early 1970s, We must understand the causes of these declines to make recommendations about conservation and agricultural management, and this can be approached by investigating the relationships, across species, between abundance and agricultural change. We describe novel, quantitative approaches to the interpretation of abundance indices from which reliable inferences about conservation status can be made.2. We calculated farmland Common Birds Census indices for 42 species, smoothed the series to reveal underlying trends and estimated confidence intervals for the changes in abundance.3. Between 1968 and 1995, the abundance of 12 species declined significantly and that of 14 species increased.4. Specialization was the only significant determinant of changes in abundance (of 10 tests against species characteristics): 13 farmland specialists declined, on average, by 30%, whilst 29 more generalist species underwent an average increase of 23%, confirming that farmland birds should engender conservation concern.5. Smoothed abundance curves, transformed to emphasize trend direction and timing, were then compared quantitatively to identify whether groups of species had shared common trends.6. Species tended not to be strongly grouped, but small groups of species with common trends were identified. Similarities in ecology among grouped species clarify the possible environmental causes of their population trends, indicating future research priorities.7. The groups identified included: one group consisting of three thrush species Turdus and the skylark Alauda arvensis L. which all declined from the mid-1970s after being stable previously; one group comprising three trans-Saharan migrant warblers (Sylvidae), whose abundance fell in the early 1970s and later increased, and a diverse group of six smoothly increasing species.8. Turning points were identified as where each species' population trend turned significantly, revealing critical periods during which populations are likely to have been affected by environmental change.9. Three collections of downward turning points were found, including one in the mid-1970s when many farmland bird declines began. Four other periods each included many upturns. The groups of turning points should facilitate the identification of environmental changes which have had widespread effects. Management prescriptions can then be designed to reverse or to mirror such key changes and thereby focus conservation effort effectively.