A PRELIMINARY INVESTIGATION OF THE FACTORS REGULATING POPULATION SIZE IN THE WOODPIGEON COLUMBA PALUMBUS

Summary A Woodpigeon Columba palumbus population in East Anglia was studied for five years and the factors regulating numbers examined particularly the autumn and winter food supply and the shooting mortality in February and March. Woodpigeon numbers increased on average 2–4 times through breeding then dropped steeply between September and December. A small proportion of the loss was caused by emigration. Local movements occurred in late December and early January if snow forced the birds out of the study area to feed on nearby Brassica (none was available in the area). Numbers increased again when snow disappeared and then fell to a minimum in late February and March. In some years a slight rise occurred through immigration (Continental immigration is discussed), thereafter numbers decreased very slightly until the breeding season. Grain stocks were measured on the autumn stubbles by making random sample counts. Stocks were good in 1960 and 1962 owing to wet seasons and delayed ploughing, but were poor in 1959 and 1961. The number of Woodpigeons at the beginning of winter was positively correlated with the average amount of grain in October and November. A low early winter population was associated with a low proportion of juveniles. Adult weights in November and early December were unaffected by the autumn grain supply, but juveniles were much lighter at this time in the two years of low autumn grain stocks. It is presumed that they were unable to grow properly on a clover diet. Clover stocks dropped to a minimum in late February and early March and the pigeon population appeared to be limited by this food supply. During the winter the pigeons regularly ate up to 46% of the available clover, indicating that intraspecific competition was real. The amount of available clover increased rapidly in late March with the spring flush of vegetation. The survival or mortality rate of pigeons from December to early April was not correlated with clover density, nor was the mortality in autumn obviously correlated with grain stocks, and this is discussed. Battue shooting in February and March was not intensive enough to exceed natural winter mortality, and was more efficient at low population densities, but could not provide an effective regulatory mechanism. It is concluded that pigeons were shot between one and six weeks before they would have died in any case. No other important shooting mortality occurred during the winter. On average over the five years, population size increased from 63 birds per 100 acres in July to 154 (91 juveniles) after the breeding season. Of the total post-breeding population, 36% died between September and December, 18% between December and February and 5% between February and July. Of the adults, 22% died between September and February compared with 77% of the juveniles, thereafter their mortality rates were equal. The investigation showed that adult annual mortality averaged 30% compared with 36% obtained from the national ringing recoveries (Murton 1961). It is concluded that the Woodpigeon population is primarily determined by the level of the food supply. Fecundity variations and any possible self-regulatory mechanisms are discussed and considered to be unimportant. Under the present conditions it is unlikely that enough pigeons could be killed by shooting to bring about a sustained population reduction and certain economic aspects are briefly discussed in relation to damage prevention and the above findings.