The significance of gregarious feeding behaviour and adrenal stress in a population of Wood‐pigeons Columba palumbus

Abstract

The gregarious feeding behaviour of Wood‐pigeons Columba palumbus was studied in the 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 were probably inexperienced individuals which preferred to frequent the front of the flock in order to determine what experienced birds were eating and some other experimental evidence for this view is discussed. It is shown that the number of pecks/min. taken by subordinate birds depended almost entirely on the pecking rate of dominant birds, although subordinates took fewer peckslmin. In addition, subordinates had to avoid supplanting attacks and, because they progressed by stops and starts, were prevented from co‐ordinating their feeding and searching movements and in consequence were relatively unselective in what they ate. These differences became greater if numbers were high relative to clover density so that a mechanism is demonstrated which adjusted flock size to the amount of food available; adjustment of numbers occurred when the total quantity of food available appeared to be non‐limiting judged by superficial impressions. Subordinate birds had lower weights and reduced survival prospects compared with dominant birds. Moreover, the adrenal cortex of subordinates became hypertrophied with the cortical cells exhibiting an increase in nuclear volume and RNA activity. 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.