A breeding population of the Mallard

Abstract

The Mallard breeding population of four reservoirs in north Somerset consisted of 150 to 160 pairs in each of the three years 1957, 1958 and 1959. The recently constructed Chew Valley Lake, which first held breeding ducks in 1954 and which held 106-110 pairs in 1957-59, has been responsible for a great increase in the population, which averaged 60 pairs (range 30-80) from 1948 to 1953, and 120 pairs from 1954 to 1956. Early nesting Mallard are almost wholly unsuccessful, apparently because of heavy predation due to lack of nesting cover in March. Newly hatched broods are unusually small, averaging 6-7 ducklings. The apparent size of broods changes little before fledging, as small broods seem unusually liable to coalesce. It is estimated that over a twelve-year period about one-third of the females attempting to breed were wholly unsuccessful, though in several years (notably 1959) there were few failures. 1950 seems to have been a particularly bad year for the production of young, and this was reflected in a small breeding population in 1951. There is a clear linear relationship between the numbers of Mallard present in late August and the numbers attempting to breed in the following spring, suggesting that the population may be nearly closed, despite substantial immigration into the area in autumn and winter. Provisional estimates indicate annual adult losses of 57% and of birds in their first year after fledging of 76%. This study will be continued. It is suggested that parallel investigations elsewhere in Britain would be valuable and could be made by teams of local observers.