Summer diet and body condition of Song Thrushes Turdus philomelos in stable and declining farmland populations

Summer diet and body condition were compared in two farmland Song Thrush Turdus philomelos populations in south-east England. One population on mixed farmland was stable and the other on arable farmland was rapidly declining. Summer diet was dominated by earthworms, snails, beetles and insect larvae (mainly Coleoptera and Lepidoptera). In both populations there was a pronounced seasonal decline in the quality of the diet, with preferred earthworms dominating prey items during March-April and snails dominating during June and July. Dry weather during late summer was associated with reduced proportions of earthworms and snails, and increased proportions of spiders in the diet, and dry soil conditions had a weak negative influence on the body weights of chicks (in the arable population) and adults. Despite a tendency for earthworms to constitute a higher proportion of the diet in the stable population, and for snails to constitute a higher proportion of the diet in the declining population, the body condition of chicks and adults appeared to be unrelated to diet composition and did not differ between study populations. Changes in agricultural practices have probably caused a major reduction in the availability of key summer food resources for Song Thrushes on lowland farmland, and we speculate that breeding thrushes mitigate the impacts of food shortage on chicks by confining their nesting attempts to localities and periods where invertebrate food resources are adequate to raise a brood of young.