Does sward density affect prey availability for grassland birds?

Measurements of the effects of sward height, density and heterogeneity (usually % bare soil) are often confounded in field studies of bird habitat preferences and their effects are difficult to disentangle. This study experimentally investigated how changes in sward density alone affected the foraging behaviour of starlings Sturnus vulgaris L., small passerines that feed on soil invertebrates in grassland. High- (24,000 tillers m−2) and low-density (12,000 tillers m−2) ryegrass Lolium perenne L. swards, created using a combination of cutting and fertilization treatments, were grown under greenhouse conditions. These density treatments were representative of sward densities in extensively and intensively managed fields in the wider countryside. Trios of starlings were placed in individual enclosures on top of these turfs, which covered trays containing a sand base topped with randomly located invertebrate prey (mealworms). Starlings foraged successfully on both dense and sparse sward treatments. No differences in foraging behaviour, intake rates or intake efficiency were attributable to sward density.