Measurement of the demographic parameters of the Mute Swan Cygnus Olor population in the Lothians
The Mute Swan population in the Lothians has been studied since 1978. Only 15 pairs bred at that time in the study area (2,000 sq km); however, this figure increased to 55 pairs by 1993 which represented a mean annual increase of 9% per annum. The breeding population comprised 39.8% of the total population in April during the period 1978 to 1985; a decrease, to 33.9%, occurred in this proportion during the period 1986 to 1993. The aim of the study was to determine the extent to which the increase in the number of breeding pairs was due to high productivity in the Lothians, or if it was due to immigration. Adult swans and cygnets which hatched in the study area were marked with individually numbered Darvic leg rings and the sex of cygnets was determined at the time of ringing. A mean of 76.5% of cygnets which fledged each year were colour ringed between 1982 and 1993. Up to 70% of small cygnets fledged with a mean of 2.7 cygnets fledged per breeding pair but only 1.4 cygnets fledged per breeding pair which included a philopatric female. Productivity was lower on rivers than on canals or still waters. Low productivity was a feature also of low altitude still waters where 2.1 cygnets fledged per breeding pair due to a high cygnet mortality rate of 43%. Biometric data indicated that the proportions of males and females at fledging were equal. The survival rate in the first year after fledging was 70% which was lower than in the subsequent nine years. There was no significant difference in the survival rates of males and females during the first six post-fledging years. The age at which swans bred for the first time ranged from 2 years to 7 years for both sexes and no significant difference was detected in the mean age at first breeding between males (3.7 years) and females (4.3 years). In addition, the age-specific number of swans which survived to breeding age was estimated. Mean annual mortality in the breeding population was 14.3% with no significant difference between males and females. There was a net loss to the breeding population of 6.4% males and 5.2% females due to established breeding birds permanently leaving the breeding population. The percentage of immigrants in the breeding population and the percentage of swans hatched in the Lothians which emigrated to other breeding populations or never bred were estimated also. A model of the breeding population was formulated using demographic parameters estimated during this study. From this model it was established that the intrinsic segment of the breeding population in the Lothians was able to sustain its numbers through its own productivity. However, the breeding population was dependent upon immigration in order to achieve a mean annual increase in excess of 2%. productivity from some breeding territories was sufficient to maintain stability in the intrinsic breeding population while productivity from others was not. Breeding territories from which poor productivity was recorded were dependent on excess productivity from the over productive territories and on immigration for persis'~ent occupation by breeding pairs. This may have inhibited the rate of increase in the number of breeding swans in the Lothians. It is shown that determination of a critical value for each demographic parameter is of little value. The need for long term continuous monitoring of a Mute Swan population is stressed, as is the measurement of all demographic parameters, in order to make a realistic assessment of a population's ability to maintain the size of its intrinsic breeding population.