In the early 1960s the mean Woodpigeon population fluctuated between 60 and 112 birds per 50 ha during the winter months. Subsequently there was a steady decline until the population levelled out in the early 1970s at around 30 birds per 50 ha. From a very low point in 1977, the numbers of birds increased reaching in 1986 a mean of 55 birds per 50 ha. Marked changes occurred in the types of cereals grown in the area. In the 1960s spring-sown barley was the major crop with winter-sown wheat occupying most of the remaining area. Throughout the 1970s winter wheat was increasingly grown and winter-sown barley replaced the spring-sown variety. Although the area of permanent pasture did not change significantly, the area of clover ley was reduced markedly in the late 1960s. In the mid 1970s silage and oilseed rape were introduced and the cultivation of peas was expanded. On the basis of the crop types grown, the study period was divided into two 11-year blocks 1961–1971 and 1975–1985. Crop preferences of Woodpigeons were calculated for each month within each of these 11-year periods. A series of stepwise regressions was then conducted on both of the 11-year periods in order to see how well the areas of preferred crops in a particular month could account for the mean population size of that month. In the winters 1961/62 to 1971/72 the winter Woodpigeon population size was influenced by two crops. The area of cereal sowings largely determined how many birds remained in the area in December. The availability of clover ley, which depended upon both the area of clover ley and the amount of snow cover, controlled the population size in January and February. In the winters 1975/76 to 1985/86 the population size was again affected by two crops. In November the amount of grain on both stubbles and sowings influenced how many young birds stayed in the area. The area of oilseed rape in December then set the population size which changed very little throughout the rest of the winter (Inglis et al. 1990).
The gregarious feeding behaviour of Wood-pigeons (Columba palumbus) was studied in a January-March period in a Cambridgeshire study area when the birds were feeding on clover fields; many individuals were marked with wing tags enabling them to be identified under field conditions. Normal and underweight (arbitrarily under 450 g) adults survived equally well provided they remained in the flocks living within the study area, where they held dominant positions in the social hierarchy. Juveniles weighing over 450 g had a poorer survival and underweight juveniles the lowest survival of all within the area. Juveniles were more likely than adults to move to places outside the study area (5 to 53 miles) where they had the same survival rate as any adults of similar weight which moved more than five miles. Displaced adults presumably lost the advantages of their position in the established hierarchy and of local experience and now competed on more equal terms with the juveniles. The mean weight of pigeons was highest in those flocks feeding where clover leaf density was highest. Smaller flocks, containing a higher proportion of under-weight birds and fewer juveniles, were found on fields where the leaf density was low (less than 100 leaves/ft2 Dominant birds co-ordinated their searching (paces/min.) and pecking actions to obtain the best components of the feed. Subordinate birds had lower weights and reduced survival prospects compared with dominant birds. Subordinate birds unable to feed successfully in one flock attempted to establish themselves in other feeding groups but if low food densities prevented this they were eventually forced to feed alone. It was found that birds feeding in isolation obtained even less food per unit time than subordinate individuals living in the flock. Nevertheless, solitary birds can feed successfully if the food supply is adequate. Solitary birds devoted much time to looking around as if afraid of being surprised by a predator. But it is more probable that when feeding on certain items pigeaons benefit from the combined from the combined feeding experience of the group and this possibility is discussed. Because a solitarily feeding Wood-pigeon cannot exploit clover efficiently, social feeding behaviour improves the survival chances of the individual and in turn enables population size to be increased; social behaviour does not limit numbers before environmental resources become limiting and any apparent self-regulatory processes are shown to be artifacts (Murton et al. 1971).
Reference List:
- Inglis.R.I., Issacson.J.A., Thearle.P.J.R., & Westwood.J.N., (1990) The effects of changing agricultural practice upon Woodpigeon Columba palumbus numbers. IBIS, 132, 262-272.
- Murton.K.R., Issacson.J.A., & Westwood.J.N., (1971) The significance of gregarious feeding behaviour and adrenal stress in a population of Wood-pigeons Columba palumbus. Journal of Zoology, 165, 53-84.
