Abstract

Jackdaws are highly social corvids. They feed in flocks, roost communally and often breed in colonies, using natural holes as nest-sites. This study describes and investigates the social organisation of jackdaws. Particular attention is payed to the functional aspects of their social organisation. The study was carried out from 1071 to 1976 in and around the village of Haren in the north of the Netherlands. Most of the jackdaws nest singly in chimneys which are scattered over the area. A small colony was created by hanging nesthoxes (15-25) on the walls of the Zoological Laboratory in Haren. It was here that most of the data were obtained. Over 600 birds were colour-banded for individual recognition. Within the population two social categories can be distinguished : resident and non-resident birds. The former defend one or more nest-sites throughout the year (except in summer). The latter defend one nest-site, only during the breeding season, if they are successful in obtaining one at all. Only adult and mated birds belong to the resident category whereas non-residents are of all ages, mated and non-mated. These categories are not static. Non-residents become residents and vice versa. Furthermore, the distinction between residents and non-residents appears to be more conspicuous in colonially nesting birds than in solitary nesting ones. In the study area 80% of the adult colour-banded birds, present in any one year, was resighted the follow-ing year. Only 48% of the first year birds was resighted again in the study area. Reproduction was approximately I young per pair per year. These figures show that the population is able to sustain itself. Estimations from other methods all suggest a much higher mortality rate. Some of these require an unrealistically high reproduction to prevent the population from declining to extinction. Some factors which may bias mortality estimations are discussed. Jackdaws are monogamous and mates remain together, both throughout the year and from year to year. Adult jackdaws are very unlikely to remate unless their mate dies. In contrast, newly mated, juvenile birds, readily change mates. However, the probability of divorce is not increased by an unsuccessful breeding attempt. Males and females react differently after losing their niate. Males try to retain their nest-site. As a result of the aggression by mated pairs, single males are seldom successful in maintaining nest-site ownership for long, but while they do, they appear to be very attractive to non-mated females. Females desert their nest-site and start to court mated (or non-mated) males possessing a nest-site. Non-mated females can gain much by seducing mated males but they seldom succeed. Jackdaws produce only one clutch per year. Resident pairs are more successful in breeding than non-resident pairs. This difference is affected by various factors, including nest-site quality, age, time of egg-laying and intraspecific interference in breeding. Pairs which failed to raise young often start to inspect other nest-sites thus harassing more successful pairs. Among the resident members of the colony a well developed, site independent rank-order was found. Males dominate females. The rank of the female depends on the presence and rank of her mate. Different dominance criteria produce different rank-orders. By allowing the birds to choose who to compete with over food, the connections between rank relationships and social preferences have been analysed. The jackdaws prefer to feed where a subordinate bird is present, avoid feeding with a dominant and seek interactions with individuals with whom the outcome of the interaction is rather unpredictable. Shifts in rank are most common during the breeding season. Unsuccessful breeders often rise in rank at the expense of successful ones. Feeding flocks are open groups with individuals and parties joining or leaving. However, resident members of a colony often feed together and form a rather stable core of many flocks, particularly in the vicinity of their colony. Flock sizes and feeding ranges vary seasonally. During the breeding season both are much smaller than during summer and winter. High ranking colony member have somewhat smaller feeding ranges than low ranking ones throughout the year, suggesting that the latter are forced to feed at larger distances. This is, however, still poorly understood. By feeding in flocks the jackdaws are able to withstand the attacks by territorial carrion crows. There are indications that the presence of crows and the vigour of their territorial defence, contribute to the flocking tendency in jackdaws. Flock feeding also increases the feeding efficiency of individual flock members by intraspecific social learning of new food sources. Birds which are familiar to one another are more likely to copy each other than are unfamiliar birds. The difference appears to be largely due to the fact that the jackdaws readily supplant a familiar bird engaged in exploiting a new food source, provided it is inferior in rank. By supplanting the birds apparently bring themselves into the right situation to copy the behaviour of another individual. The daily activities of resident pairs are centered around their nest-sites during most of the year. They defend one or more nest-sites and visit several others daily from early fall on. Both by defending more nest-sites and by visiting others, the jackdaws remain Informed of alternatives should their own nest-site become unavailable or unsuitable for breeding. High rank, the number of defended nest-sites and priority of access to nest-sites in short supply, are correlated. Hole nesting not only protects the nest's contents against most predation, it also renders it possible for jackdaws to nest inside the territories of the interspecifically aggressive carrion crow. Colonial nesting appears to give no additional protection against nest predation under most conditions. Moreover, communal defence against nest predators is practically absent and so far there are no indications that jackdaws prefer to nest colonially. Field studies of social systems have produced a growing awareness of the fact that characters of behaviour, morphology, demography and social structure are not independent but form an adaptive complex. Some of the relationships between these characters in the jackdaw are discussed. Finally, environmental factor, such as predation, the availahility of food and interspecific competition, tnould each of these characters and their relationships. In jackdaws the chief benefit of flock feeding and hole nesting seems to be that thus the jackdaws are able to withstand the interspecific aggression of the territorial carrion crow.