Survival rates of young Mute Swans Cygnus olor

Abstract

The ringing and recovery data from the British Trust for Ornithology have been computerized and the following analysis made, with special emphasis on the survival rates of birds of known age (those ringed in their first 15 months of life). On average, slightly over one in four of the swans ringed in Britain are eventually reported dead. Analyses based on this reporting rate are presented showing the total number of deaths in any time period and in any area and, resulting from this, the ‘pool’ of birds still alive from which the recoveries have come. The pattern of mortality within the first year of life is examined and compared between areas within Britain. The pattern seems to be largely similar over the whole country. Mortality rises sharply in October when birds begin to leave their natal territories and remains high during the winter months but drops again in April as the birds settle down to spend the summer in an area where they eventually moult. Survival rates of the one year old cohort are followed through their first seven years of life, by which time they should be of breeding age. Comparisons are made between areas within Britain and over different time periods.