The effect of dairy farming on barn swallow Hirundo rustica abundance, distribution and reproduction

1. Recent changes in agricultural practice have been hypothesized to affect the abundance and reproductive success of farmland birds. The influence of dairy farming on barn swallows Hirundo rustica was investigated by compari son of their abundance, phenotype and reproduction on the same farms before and immediately after dairy farming ceased, while a control sample of farm s without change in farming practice in the same years was used to check fo r temporal changes unrelated to farming practice. 2. The abundance of barn swallows decreased significantly when dairy farmin g ceased, with an average reduction of 48%, while there was no significant difference in the sample of control farms. This was mainly due to a decreas e in the abundance of yearling immigrants. The abundance of insect food mea sured with sweep nets decreased significantly in the absence of cattle, whi le there was no significant change in the sample of control farms. 3. The mean phenotypes of adult barn swallows breeding on the same farms wi th and without dairy farming did not differ significantly for any of the 16 variables measured (11 morphological variables, body mass, parasite load, haematocrit, leucocyte counts, and arrival date), nor were there any signif icant differences recorded in the sample of control farms. 4. The mean phenotypes of barn swallow nestlings on the same farms with and without dairy fanning differed for tarsus length, body mass, haematocrit, leucocyte concentration and T-cell mediated immune response, with nestlings being of poorer quality in the absence of cattle. There was no significant differences recorded in the sample of control farms. 5. Start of reproduction was not delayed in the absence of cattle, but size of first clutches was reduced by the absence of cattle. The frequency of s econd clutches decreased in the absence of cattle, and laying of second clu tches was also delayed. Hatching, fledging and breeding success did not dif fer between the two types of farming practice. This led to an overall reduc tion in annual reproductive success in the absence of cattle. None of these significant differences was recorded in the sample of control farms. 6. These observations suggest that termination of dairy farming reduces loc al population size, reproductive success and the quality of offspring produ ced. There is little evidence of the distribution of phenotypes of adult ba rn swallows being affected by the presence of cattle.