Helminth gut parasites of black grouse (Lyrurus tetrix) in northern England, their impact on productivity and possible sources of infection

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 1,437
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. T. tenuis occurred in 63% of black grouse at a mean intensity
of 390 worms per bird, and in 95% of red grouse at 1,535 worms per bird.
Seventeen black grouse contained Heterakis spp., and two contained the
tapeworm Parionella urogalli. T. 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.