A study of the winter feeding ecology and behaviour of the bar-tailed godwit (limosa lapponica)

Abstract

Godwits, large wading birds, were studied at Holy island, Northumberland during three winters. Most fed on the polychaete worms Scoloplos armiger and Arenicola marina (the lugworm). Although 60% of items taken were Scoloplos, the larger lugworms provided over 90% of the energy content of the diet. In October and April, godwits obtained almost all their daily energy requirements during daylight hours. In December they had to feed also by night, particularly during cold weather. At sand temperatures above 3 C, godwits obtained the same amount of food each minute in mid winter as in October; they fed for longer when their energy requirements were greater. At sand temperatures below 0 C, godwits were not able to feed fast enough to meet their requirements and had to utilize some of their fat reserves, which were sufficient to balance three days' total starvation. By comparison with captive godwits, it was concluded that wild birds expend almost as much energy in capturing worms as they require just to keep alive. Godwits normally feed in flocks containing both males and females. The short-billed smaller males tend to feed at the tide-line, and the longer-billed larger females in deeper water. If males are forced into deeper water, they are less successful in feeding than females. Birds feeding alone are less successful than individuals within flocks, and are more likely to be caught by birds-of-prey. Godwits feed where the availability of their prey is highest. They rely on cast-formation (defaecation) to detect lugworms, which normally live too deep to be reached by the bird's beak. Lugworms form casts most frequently at the tide-edge and less often as the sand dries out during low tide. Hence birds move with the tide-line to feed. At temperatures below 3 C, lugworms defaecate most frequently in shallow water at the tide-edge, and godwits feed further into the water. Flocks change shape to become linear. Other birds are forced to feed at the tide-edge in cold weather, so godwits have to pack more closely and interactions increase. Potential feeding time is lost in fighting. Consequently, less food is obtained each minute than expected solely from the reduction in prey availability. At very low temperatures, godwits are forced to feed on Scoloplos, by touch. Females then gain an advantage from their longer legs and bills, since they can feed in deeper water and avoid the fights which are chiefly at the tide-edge.