Population regulation in the Mallard (Anas platyrhynchos).

Abstract

1. Key factor analysis was carried out on the number of mallards breeding on the Sevenoaks gravel quarry reserve in Kent using published counts of breeding pairs, numbers of newly hatched broods and numbers of fledged young. Individual mortalities used were clutch predation (κ₁), duckling mortality (κ₂) and overwinter loss (κ₃). Overwinter loss was investigated in mallard at thirty-five inland waters in the Midlands and southern England to determine the amount of density dependence.
2. Duckling mortality was the key factor at Sevenoaks explaining 58% of total mortality between years. The density of newly hatched broods explained 17% of duckling mortality, i.e. duckling mortality was weakly density-dependent.
3. The proportion of nests destroyed by predators at Sevenoaks increased as nest density increased; relatively higher nest success occurred between 1966-71, during which time nearly twice as many carrion crows, the major egg predators of mallard, were shot.
4. Overwinter loss was higher following years when a large number of young were produced and was the main regulatory factor. The degree of density dependence in over-winter loss determines the extent to which individuals within the population compensate for additional sources of mortality such as those caused by shooting and poisoning through the ingestion of lead shot. The large amount of overwinter density dependence existing in mallard on the Wildfowl Count sites suggests that nationally, present shooting losses and losses through the ingestion of lead shot should be compensated for.