The aim of the work was to find out how the Redshank which over-winter on the Ythan estuary, Aberdeenshire, adapt their feeding activities to the short daylength in winter. To achieve this, the feeding behaviour and daily routine in winter were compared with those in autumn and spring.
Corophium volutator was the main prey on the estuary during the day. However, the temperature of the mud greatly affected the diet in the two areas where the feeding behaviour was studied in detail. At temperatures above 6°C, most of the biomass ingested consisted of Corophium. However, in one study area in one winter, Macoma balthica was taken more frequently at lower than at high temperatures. In the other area, Nereis diversicolor was taken more frequently at low than at high temperatures. These changes in diet appeared related to changes in the behaviour of the prey affecting their availability and not due to the birds changing their preference at low temperatures. The ingestion rate was not affected by the change in diet in the first area but decreased at low temperatures in the second. The sizes of Macoma and Nereis taken by the birds did not vary seasonally. The sizes of Corophium taken decreased in winter as a result of a reduction in the size of those present in the substrate. It was concluded that there was no evidence of Redshank increasing their ingestion rate in winter to compensate for the short daylength.
The proportion of the time spent feeding on the estuary in daylight was greater in winter than in spring. Redshank continued feeding at high water in the surrounding fields and on the estuary at night during the winter but not in either autumn or spring. During the winter they obtained less than 50% of their daily food requirements from the estuary in daylight, mainly because of the short daylength. Consequently, they had to collect the balance at night and at high water.
Seasonal changes in the numbers of Redshank were recorded. After an autumn decrease in numbers, no decrease in numbers could be detected during the winter. These findings are discussed in relation to the possible difficulties for Redshank in collecting their daily food requirements during the winter. The impact of the birds on their main estuarine prey Corophium is also discussed.