Edinburgh Napier University

  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.