Factors affecting the number of pairs and breeding success of Mute Swans Cygnus olor in an area of south Staffordshire, England, between 1961 and 1985

Abstract

The population of Mute Swans in a 1,440 square kilometre area of south Staffordshire has been studied since 1961. Considerable fluctuation in the number of swan pairs and their breeding success has been noted. The decline within the Birmingham/Wolverhampton conurbation has been a cause for concern, human influence being a major factor. Most pairs retained the same mate from the previous season, 87% of this segment of the paired population laid eggs. Overall clutch failure was 45%, humans being the chief cause; failure in the industrial conurbation was 22% more than in the rural area. The number of cygnets fledged per breeding pair was significantly less in the industrial conurbation. Rearing success was uniform and tended to decrease with brood size. Over 50% of the broods were reared without loss. Highest mortality after fledging occurred during the first year with peak periods in October and March. Overhead wires were the commonest cause of known death. An average maximum mortality rate of 21% was calculated for breeding birds and 31% for non-breeding paired birds with deaths due to vandalism highest in the breeding birds. A low immigration of breeding birds was recorded.