Crop sward structure explains seasonal variation in nest site selection and informs agri-environment scheme design for a species of high conservation concern: the Corn Bunting Emberiza calandra

Abstract

Capsule Corn Buntings nested in tall dense grasses and cereals, selecting for sward density over height, and cereal fields with high weed scores.Aims To measure the vegetation attributes of fields selected as nest sites by Corn Buntings, to inform the design of safe nesting habitat measures in agri-environment schemes for this rapidly declining ground-nesting passerine.Methods Nesting activity was monitored across 32 farms in eastern Scotland during 2004–09. Land use and singing males were mapped to measure habitat availability for nesting females within their mate's territory, and crop swards measured. Effects of sward characteristics on field selection for nesting, and seasonal variation in crop use, were modelled.Results Nests (95% of 580) were mainly in cereals and grasses. Taller denser swards were selected, with sward density a stronger predictor of field use than sward height, and cereal field use strongly associated with high weed scores. Variation in sward structure between crop types, and changes due to crop maturation or harvesting, largely explained seasonal patterns in crop use.Conclusion Conservation measures targeting nesting Corn Buntings should provide uncut or late-cut grasses or cereals 30–100 cm tall with a dense ground layer of weeds or crop vegetation.