The influence of crop tiller density on the breeding performance of a cereal-nesting specialist.
In the pursuit of maximizing agricultural yields and profits, crop management under intensive agriculture has reduced the heterogeneity of crop sward structure within fields. Several farmland bird specialists that nest within arable crops have undergone marked population declines during recent periods of agricultural intensification. Understanding the mechanisms that link crop sward structure to demographic fitness and population change may be important for developing effective conservation solutions. We studied whole-season breeding performance of a declining cereal crop specialist, the corn bunting Emberiza calandra, to test the relationships between crop sward structure and both nest site selection and reproductive output within intensively farmed landscapes. Experimental double-drilling of cereal seed was used to manipulate crop sward structure and explain the observed within-field nest distribution. The density of crop tillers (reproductive cereal stems) was a strong predictor of nest site selection and a disproportionate number of nests occurred close to (15-30 m) crop edges where regular seed sowing overlaps (operational double-drilling) occurred during crop establishment. Nest survival was subject to a strong crop edge effect and re-nesting rate increased with mean tiller density in the surrounding crop. Experimental double-drilling that simulated seed sowing overlaps increased crop tiller density (by 25%) and corn bunting nest density (by 406%) compared to adjacent areas of operational double-drilling. Corn buntings breeding in intensively managed cereal crops therefore place their nests in areas of high tiller density which under normal sowing practice tend to occur close to crop edges where nest survival is low. A simple population model indicated that productivity under conventional, intensive cereal management may be too low to sustain local populations in the absence of immigration. Modern cereal husbandry therefore creates an ecological trap for nesting corn buntings. Synthesis and applications. This study highlights a novel mechanism by which modern cereal crop management lowers the potential reproductive success of a conservation priority species nesting in intensively managed cereal crops. Unavoidable overlapping of cereal seed along crop headlands, close to crop edges, systematically produces patches of dense sward that attract nesting corn buntings. This creates an ecological trap because nests close to crop edges are subject to high predation rates, and this may be linked to the loss of arable weeds elsewhere within crops. When double-drilled strips are intentionally provided adjacent to these areas, nesting preference switches to these strips, suggesting a potential conservation solution. Provision of mid-field double-drilled strips at least 100 m from crop edges, if similarly selected by nesting females, should raise nest survival rates and improve population trends. Such interventions are simple and cheap to implement, unlike most conservation measures in high-value cropping systems.