TRENDS IN THE ABUNDANCE OF FARMLAND BIRDS - A QUANTITATIVE COMPARISONOF SMOOTHED COMMON BIRDS CENSUS INDEXES

1. Widespread declines in the populations of many British farmland bir ds have occurred since the early 1970s, We must understand the causes of these declines to make recommendations about conservation and agric ultural management, and this can be approached by investigating the re lationships, across species, between abundance and agricultural change . We describe novel, quantitative approaches to the interpretation of abundance indices from which reliable inferences about conservation st atus can be made. 2. We calculated farmland Common Birds Census indice s for 42 species, smoothed the series to reveal underlying trends and estimated confidence intervals for the changes in abundance. 3. Betwee n 1968 and 1995, the abundance of 12 species declined significantly an d that of 14 species increased. 4. Specialization was the only signifi cant 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 2 3%, confirming that farmland birds should engender conservation concer n. 5. Smoothed abundance curves, transformed to emphasize trend direct ion and timing, were then compared quantitatively to identify whether groups of species had shared common trends. 6. Species tended not to b e strongly grouped, but small groups of species with common trends wer e identified. Similarities in ecology among grouped species clarify th e possible environmental causes of their population trends, indicating future research priorities. 7. The groups identified included: one gr oup consisting of three thrush species Turdus and the skylark Alauda a rvensis L. which all declined from the mid-1970s after being stable pr eviously; one group comprising three trans-Saharan migrant warblers (S ylvidae), whose abundance fell in the early 1970s and later increased, and a diverse group of six smoothly increasing species. 8. Turning po ints were identified as where each species' population trend turned si gnificantly, revealing critical periods during which populations are l ikely to have been affected by environmental change. 9. Three collecti ons of downward turning points were found, including one in the mid-19 70s when many farmland bird declines began. Four other periods each in cluded many upturns. The groups of turning points should facilitate th e identification of environmental changes which have had widespread ef fects. Management prescriptions can then be designed to reverse or to mirror such key changes and thereby focus conservation effort effectiv ely.