Effects of the caecal nematode Trichostrongylus tenuis on egg-laying by captive red grouse.

Captive hen red grouse infected with Trichostrongylus tenuis larvae started to lay later in spring and laid fewer eggs at a slower rate than uninfected hens. March-infected hen grouse showed greater effects than December-infected ones, probably because developing larvae were more pathogenic than adult worms. The mechanism by which parasites interfered with egg production involved reduction in food intake. Infected hens also gained less weight than controls before laying.