The behavioural ecology of the whooper swan (Cygnus cygnus cygnus)

Abstract

The behaviour and ecology of Whooper Swans (Cygnus cygnus) were studied on the wintering grounds in Scotland and the summering grounds in Iceland, with a view to extending our general knowledge of the biology of this little studied species. Comparisons are drawn between feeding behaviour shown in terrestrial, freshwater and marine habitats. In Central Scotland Whooper Swans were found to feed mainly on agricultural land and to actively select stubble fields, where they fed on waste grain, from their arrival in autumn until mid-winter. They then changed to feeding on grass from mid-winter until their departure in the spring. They were found to have adopted an activity pattern similar to that of geese, i. e. they were diurnal and flew each morning and evening between a roost-site and a feeding site. The daily activity cycle of feeding varies between habitats, but the differences are not fully explained by functional requirements. The factors affecting the timing of morning and evening flights are discussed. The length of the feeding day increased with daylength and the level of feeding per hour increased so that more time was spent feeding in the spring than at any time during the winter. Although Whooper Swans were found to compensate to some extent for the shortness of winter days by departing to the roost later relative to sunset, it is suggested that it is in the spring when their energetic requirements are highest; they need to store enough energy for migration and breeding and/or moult. The percentage of birds head-up was found to decline curvi-linearly with increasing flock size while the percentage feeding increased. Since there was no apparent relationship between peck rate and flock size, birds in larger flocks gain from increased food intake. A seasonal change in flock size was noted in Central Scotland with larger flocks occurring more frequently between autumn arrival and mid-winter than from mid-winter to spring departure. Although other factors may be involved as well, it is suggested that the advantages of flocking to Whooper Swans may vary depending on whether the food is patchily distributed (waste grain), or relatively uniformly distributed (grass). Differences in the amount of time allocated to feeding and vigilance were analyzed according to age and breeding status, and seasonal changes are discussed. The breeding success of the Whooper Swan, measured using the percentage of cygnets and the mean brood size, was found to vary greatly from year to year. Measurements of mean brood size in Iceland during summer were found to correspond well with those in Scotland the following winter. A particularly poor breeding season in 1979, a year with a very late spring, was noted both in Iceland and on the wintering grounds in Scotland. Whooper Swans are monogamous and territorial. The female does most of the nest building and all of the incubation, while the male remains on the territory, usually either vigilant or feeding. The range of behaviours exhibited by males and females during the incubation and fledging are described and the time allocated to them is analyzed. Displays between adjacent territory holders are described for the first time. These displays were found to be commoner during the fledging period than during incubation. Females were also observed to take part in defence against intruders and it is suggested that an important role of the Whooper Swan's territory is to provide a safe feeding area for the family after hatching. Behavioural co-operation between mates helped to maintain a high degree of protection for the nest and cygnets. Cygnets maintained closer proximity to each other than to their parents and tended to associate with a single parent; usually the female. As cygnets aged, distances between them and from them to their parents increased and their parents spent more time feeding and less time vigilant. The behaviour of non-breeding birds is also described and it is suggested that non-breeders tend to moult in a separate flock from failed breeders. Moult and migration are also discussed in order to provide as full a picture as possible of the Whooper Swan's