Measurement of the demographic parameters of the Mute Swan Cygnus Olor population in the Lothians.

The Mute Swan population in the Lothians has been studiedsince 1978. Only 15 pairs bred at that time in the studyarea (2,000 sq km); however, this figure increased to 55pairs by 1993 which represented a mean annual increase of9% per annum. The breeding population comprised 39.8% ofthe total population in April during the period 1978 to1985; a decrease, to 33.9%, occurred in this proportionduring the period 1986 to 1993. The aim of the study was todetermine the extent to which the increase in the number ofbreeding pairs was due to high productivity in theLothians, or if it was due to immigration.Adult swans and cygnets which hatched in the study areawere marked with individually numbered Darvic leg rings andthe sex of cygnets was determined at the time of ringing. Amean of 76.5% of cygnets which fledged each year werecolour ringed between 1982 and 1993.Up to 70% of small cygnets fledged with a mean of 2.7cygnets fledged per breeding pair but only 1.4 cygnetsfledged per breeding pair which included a philopatricfemale. Productivity was lower on rivers than on canals orstill waters. Low productivity was a feature also of lowaltitude still waters where 2.1 cygnets fledged perbreeding pair due to a high cygnet mortality rate of 43%.Biometric data indicated that the proportions of males andfemales at fledging were equal. The survival rate in the first year after fledging was 70% which was lower than in the subsequent nine years. Therewas no significant difference in the survival rates ofmales and females during the first six post-fledging years.The age at which swans bred for the first time ranged from2 years to 7 years for both sexes and no significantdifference was detected in the mean age at first breedingbetween males (3.7 years) and females (4.3 years). Inaddition, the age-specific number of swans which survivedto 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 percentageof immigrants in the breeding population and the percentageof swans hatched in the Lothians which emigrated to otherbreeding populations or never bred were estimated also. A model of the breeding population was formulated usingdemographic parameters estimated during this study. Fromthis model it was established that the intrinsic segment ofthe breeding population in the Lothians was able to sustainits numbers through its own productivity. However, thebreeding population was dependent upon immigration in orderto achieve a mean annual increase in excess of 2%.productivity from some breeding territories was sufficientto maintain stability in the intrinsic breeding populationwhile productivity from others was not. Breedingterritories from which poor productivity was recorded weredependent on excess productivity from the over productive territories and on immigration for persis'~entoccupation by breeding pairs. This may have inhibited therate of increase in the number of breeding swans in theLothians.It is shown that determination of a critical value for eachdemographic parameter is of little value. The need for longterm continuous monitoring of a Mute Swan population isstressed, as is the measurement of all demographicparameters, in order to make a realistic assessment of apopulation's ability to maintain the size of its intrinsicbreeding population