Effects of dietary supplements of newly growing heather on the breeding of captive red grouse.

I. The effects of dietary supplements of heather at different stages of growth on the breeding of captive red grouse were studied. 2. A supplement of heather which was beginning to grow in spring stimulated captive red grouse to lay more eggs and to lay at a faster rate than birds given a supplement of dormant winter heather. 3. Although laying was stimulated, no significant effect on the date of laying, the hatchability of the eggs or the survival of the young was demonstrated; however, the possibility of such effects occurring in the wild is not ruled out. 4. The experimental effect occurred although the heather formed only a small proportion of a largely artificial diet, which provided a high plane of nutrition in terms of known nutrients. 5. The effect is attributed to an unidentified factor contained in the new growth.