Seasonal movements of shorebirds in relation to spacing behaviour and prey availability
Data on counts of shorebirds and information from ringing and colour-marking studios are examined and integrated to provide detailed information on seasonal changes in distribution, and movements of' Knot, Bartailed Godwit and Grey Plover in North- West Europe, particularly Britain. Several factors which may lead to the movements observed, particularly those factors affecting energy balance, are considered. Published information on seasonal changes in weather conditions in Britain and Europe, and on prey abundance in different estuaries, is reviewed and supplemented by studies from Morecambe Bay and the Tees and Humber estuaries. It is suggested that late autumn movements of Knot and Bar-tailed Godwit from the eastern North Sea to Britain permit birds to avoid severe winter weather conditions. Movements of many Knot northward from the Wash along the east coast of Britain also occur in November, perhaps to reach estuaries less influenced by cold sir masses from Europe. The reasons for most other movements are unclear. No evidence was found of major differences between estuaries in the timing of breeding or growth rates of invertebrate prey, which might have led to the differences in timing of peak prey biomass for shorebirds to exploit. Differences have been found in the seasonal patterns of depth distribution of Macoma balthica between Morecambe Bay, the Wash and Humber. However, the timing of movements of' Knot between these estuaries could not be related to these differences in availability of prey. Availability of Macoma on Morecambe Bay in spring is high and may partly explain why high numbers of Knot collect there to put on pre-migratory fat reserves. Movements of Knot through the Tees estuary appear to be unrelated to changes in prey abundance or availability. Two alternative survival strategiesare suggested for this species: (1) itinerancy, visiting two or more British estuaries after moult, and (2) sedentary behaviour, visiting only one estuary after moult, or staying on the moulting area. Seasonal use of the Tees estuary by, and movementsto and from there of, Grey Plover were studied in relation to spacing behaviour and prey availability. Two strategies of use of space by Grey Plover were found. Territoriality was shown to be particularly advantageous during periods of severe particularly windy, weather. Flock feeding was adopted by some birds which could not, and by some individuals which chose not to, obtain a territory. Some movements of Grey Plover from the estuary were probably determined by social status. A few of those which were unable to obtain a territory in autumn left the estuary. No evidence of movement in relation to prey availability was obtained.