Effects of Food Enrichment on Numbers and Spacing Behaviour of Red Grouse

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Abstract
Heather, the main food of red grouse, was enriched by spreading nitrate fertilizer on an area where grouse numbers were fluctuating over the years. Certain aspects of the heather''s response to fertilizer differed in the 3 experiments, but its N content increased by a similar amount in all 3. Numbers in the 1st experiment increased, while grouse on the rest of the moor were breeding poorly and declining in density from the moderate peak. All grouse were removed from the experimental and control area at the start of this experiment, but not from later ones. Immigrants reared bigger broods on the fertilized area; many of the young from these broods were recruited into the breeding population, which increased and subsequently remained at a higher density than on the control for 4 yr. The 2nd experiment was started while grouse on the rest of the moor were breeding well and increasing to a very high density. Fertilizing did not improve breeding, relative to the control, where grouse were breeding well anyway. It was followed by more immigration and increase recruitment to the breeding population, which again exceeded that on the control for 4 yr. Experiment 3 took place while densities were declining rapidly from the high peak during experiment 2. Numbers on the fertilized area decreased as much as on the control, until both areas held no birds. When food enrichment increased numbers, this followed changes in territory size. The ratio of hens to cocks in spring also increased. Natural fluctuations in numbers on control areas had a greater amplitude than increases due to fertilizer on enriched areas. Variations in our measurements of food quantity and quality could not explain changes in grouse numbers on control areas.