Experimental cutting of the shrub layer did not improve capercaillie Tetrao urogallus breeding success during wet summers in Scots pine forests, Strathspey, UK

Understanding factors causing the low breeding success of capercaillies Tetrao urogallus is important for the conservation of this species. Here we investigate possible causes of spatial variation in breeding success in two neighbouring Scots pine Pinus sylvestris woods in Scotland, Abernethy Forest and Craigmore Wood. Breeding success declined with increasing June rainfall at both sites, but there was a stronger effect at Abernethy. Average productivity (chicks/female) during 2000-2011 was 1.61 (95% C.I. 1.08-2.41) times greater at Craigmore than Abernethy. It was possible that the difference was due to increased wetting of chicks by vegetation during and after rain at Abernethy, where the vertical density of the shrub and grass layer was greater than at Craigmore. Wet chicks may then die. To test this hypothesis, 2 m wide routes were cut through tall heather Calluna vulgaris at Abernethy, so that broods could move between bilberry Vaccinium myrtillus feeding areas without having to brush against tall dense vegetation. However, there was no improvement in breeding success in the treated area compared to a control area. Possible explanations are that the capercaillies did not use the cut routes, that cutting did not provide sufficiently short vegetation, that rain affects capercaillie chicks in other ways (e.g. through insect availability), or that broods shelter from rain using pine thickets.