Effects of strongyle worms

Once the infective larvae have developed into adult worms in the caeca they then burrow into the caecal wall where they can cause considerable damage. They reduce the digestive efficiency of the grouse and ultimately their condition. Weak and dead grouse are often seen from late February onwards, with the classic ‘owl-like’ flight from the red grouse the first indication of a parasite outbreak. When the birds are flushed they have a weak gliding flight usually in a downhill direction. Grouse can be found sitting in the heather, too weak to get up when disturbed. Dead birds can be found anywhere on the hill, but mainly in gutters and streams, too weak to climb back out.

Grouse with high infections of strongyle worms appear to give off more scent during incubation than hens with reduced worm burdens. This increased scent emission can increase predation rates on adult grouse and eggs during incubation.

Worm infections are likely to increase dramatically when high grouse densities coincide with mild, damp conditions. Worm burdens in excess of 3,000 worms per bird start to have an impact on a bird’s condition and its subsequent reproductive capacity. In some cases numbers of worms can build up to very high levels with extreme examples of worm burdens in a bird of over 30,000 worms recorded.

Recent studies have found up to one free-living infective larva per gramme of heather. (The average daily intake of heather by red grouse is between 60 and 80 grammes per day.) Worm survival in the grouse is good with an average life expectancy of two years.