LAUSR

Black grouse Lyrurus tetrix declines continue throughout much of temperate Europe, following habitat loss and population isolation, associated with reduced productivity. In northern England, only 1437 lekking males remained in 2014, genetically isolated from other UK populations, and typified by low productivity but high adult survival. Their distribution overlaps with that of red grouse Lagopus scotica on moorland managed for red grouse shooting. Here, quasi‐cyclical fluctuations in red grouse numbers are driven by the parasite Trichostrongylus tenuis. Whilst management for red grouse may benefit black grouse survival through reducing predators, we hypothesised that parasite transmission from red to black grouse may contribute to low black grouse productivity. We measured T. tenuis prevalence and mean intensity from 186 black grouse carcasses (40 radio‐tagged birds, 30 shot birds, the remainder found by chance) and compared them with equivalent measures in red grouse. Trichostrongylus tenuis occurred in 63% of black grouse at a mean intensity of 390 worms per bird, and in 95% of red grouse at 1535 worms per bird. Seventeen black grouse contained Heterakis spp. and two contained the tapeworm Parionella urogalli. Trichostrongylus tenuis intensity in black grouse was 2–4 times higher in predated birds than in ones that died from other causes. Body condition and productivity were both lower when T. tenuis abundance in black grouse was higher, supporting the hypothesis that parasites contributed to low productivity. Despite T. tenuis abundances varying temporally by approximately 15‐fold in both grouse species, patterns of worm abundance between the species were not correlated, nor did we detect any relationships between black grouse breeding success and red grouse worm abundance. Despite the lack of direct evidence for parasite cross‐transmission, we suggest ongoing precautionary surveillance of parasitic worms in areas where red and black grouse overlap.