The breeding biology and population dynamics of shelduck (Tadorna tadorna L.) at Aberlady Bay.

Abstract

The breeding biology of shelduck at Aberlady Bay in 1976 is described in detail and possible factors affecting breeding success are discussed. Factors affecting nest site selection were determined. Both parliament site and nest site tended to be as close as possible to a pair's feeding area within the bay, which was chosen in early spring. Artificial nest boxes were used to study individual birds during the breeding season. The rhythm of egg production was irregular for all birds studied. The mean rate of egg production was O.63 eggs day(^-1), and clutch size 9.3+2.4 eggs. Laying occurred between 15th April and 2nd July, the mean incubation period being 32.2+1.2 days. Hatching success varied between 80-100%. There was little evidence of predation although there was some evidence of intraspecific disturbance and mechanisms by which it may occur are described. Ducklings appeared on the bay from the end of May until the first week in July. Thirty-six broods were brought onto the bay and most ducklings were taken to a main nursery area. Mechanisms leading to creching of ducklings are described. Creching v/as extensive, although mainly between ducklings of similar ages (and usually younger ducklings were involved). The largest creche observed contained 38 ducklings. Ducklings could be separated in the field into different age classes (I-IV). Mortality of Class I ducklings was found to be 8l%. The first fledged duckling was seen on 16th July. The distribution within the bay of invertebrate food items taken by shelduck was determined. The abundance of Hydrobia ulvae within the feeding ranges of breeding birds varied between 1-100+ dm(^-3). Evidence suggested that sediment type may be important in affecting food availability in different parts of the bay. The feeding areas of individual ducks during the breeding season moved each year, either into, out of, or within the bay. Movement of feeding areas into the bay, particularly onto muddy or wet areas, usually resulted in successful breeding (duck seen with ducklings). Pairs feeding further into the bay or on muddy areas bred earlier than other pairs. Early breeding was thought to be beneficial for the maintenance of the parent/duckling bond, and survival of ducklings. Forty-two pairs were known to have bred between Musselburgh and North Berwick. The number of breeding pairs at Aberlady was thought to be restricted by competition for good feeding sites in the muddy or wetter areas of the bay, coupled with increased individual distance during the breeding season. Production of offspring at Aberlady Bay was found to be inadequate for maintenance of its population of shelduck (mainly due to high duckling mortality) and immigration from other areas was thought to occur, especially from areas with low densities of shelduck where production per pair appeared to be higher.