Population changes and mortality of the Mute Swan in Britain

Abstract

An index based on winter counts shows that the population of the Mute Swan in Britain underwent a 25% decline between the years 1960-61 and 1964-65, following a peak in 1959-60. In the last three years the numbers have remained constant in the country as a whole, though an increase has taken place in the north and a decrease in the midlands and west. There is no migration and little movement of swans other than following watercourses. The average annual mortality for swans ringed when under one year old, excluding recoveries in year of ringing, is 40.5%, and for birds ringed when over one year 38.5%. The cold weather in the winters of 1962 and 1963 increased both mortality and recovery rates, but mortality was lower than average in 1963-64. There is possibly greater survival in the third and fourth years of life than in the first two. Overhead wires are responsible for over 44% of Mute Swan recoveries where the cause of death is known: young birds are no more vulnerable to this hazard than older ones. Other causes of death include oiling, disease, cold weather, shooting and fighting.