Insect availability for breeding yellowhammers on lowland farmland

Granivorous farmland passerines have declined in Europe as a result of agricultural intensification. Many of these species' require insects during the breeding season, the availability of which has declined in recent decades, potentially impacting on demographic rates via reduced adult foraging efficiency. Sympathetic management of foraging habitats may be crucial in promoting population recovery of farmland birds. This requires data on .the underlying food availability and the way in which birds' exploit these resources. The foraging behaviour of a key granivorous passerine, the yellowhammer Emberiza citrinella, was investigated during the breeding season in relation to insect abundance and accessibility in key foraging habitats (cereal crops and field margins). Insect abundance differed temporally between crop types under varying management, due in part to the timing of crop development. Insect distributions exhibited considerable spatial heterogeneity within cereal crops, with evidence of localised aggregations. Field margins were used heavily by foraging yellowhammers in early summer. However, margin use declined in late summer, with a marked shift to cereal crops. This shift occurred despite margins supporting higher insect abundance than cereals, and was most likely due to seasonal vegetation growth within margins, restricting accessibility to insects. At a finer patch-scale, yellowhammers selected foraging sites with short, sparse vegetation and high insect abundance.' These results suggest that accessibility to food, mediated by vegetation structure, is an important determinant of habitat selection at both the habitat and patch scale. Field margins were experimentally cut to create open patches,within the sward, and the response of foraging yellowhammers tested. The use of such patches increased between early and late summer, and patches were used more frequently as vegetation height increased in the adjacent margin. These results suggest that open patches within tall, dense swards may increase accessibility to food for foraging birds. Nestling provisioning rates and body condition were significantly higher in the presence of experimental margin patches, possibly indicating that such patches may enable greater adult foraging efficiency. The posi~ive response of yellowhammers to margin manipulations suggests that creating heterogeneity in vegetation structure within a habitat may be a means for enhancing food availability for farmland birds. Field margins managed within agri-environment schemes may require more frequent, targeted cutting to ensure accessibility to insects for foraging birds in late summer.