The use of intertidal habitats by shorebird populations, with special reference to grey plover (Pluvialis squatarola) and curlew (Numenius arquata)
The use of intertidal habitats by Grey Plovers, Pluvialis squatarola, and Curlews, Numenius arquata, was studied on Seal Sands in the Tees estuary, north-east England during the three winters 1975-78. Birds were uniquely marked with combinations of colour rings to obtain information on the behaviour of individuals throughout and in successive winters. Grey Plovers arrived on the estuary from July to February, giving rise to peak numbers in February, but left mainly in March and April. Thus periods of stay by individuals varied markedly, M contrast most Curlews arrived in late summer and left in spring. Consequently most individuals were present for similar periods, and the number on the estuary remained constant through the winter. However, the number of Curlews feeding on Seal Sands fell to a minimum in mid winter when some moved to feed on the fields. Both species showed a high percentage return rate to the estuary in successive winters. During each tidal cycle individuals changed their feeding sites in predictable ways. For any one bird the pattern was constant over periods of several weeks or months, and sometimes repeated in successive winters. In both species, four basic patterns in use of space could be identified. These were distinguished by: i) whether one or more than one feeding site was used during a tidal cycle, and ii) whether or not a feeding site was defended. The strategy employed on a particular site could be predicted from two characteristics of that site: i) its period of exposure, and ii) the rate of drainage of the substrate. It is argued that these characteristics determine the availability of Nereis diversicolor, the main prey of both bird species on Seal Sands. Simple models based on the supposed activity and depth distribution of Nereis on different sites explained much of the variety in use of space shown by individual Grey Plovers and Curlews both during a tidal cycle and during a winter. However the models were inadequate to explain the detailed components of foraging behaviour observed. Also, variability in the foraging responses to changes in Nereis availability was considerable, both between individuals and between days for the same individual; this raises doubts over the validity of studies based on "average" birds. Curlews show marked sexual dimorphism in bill length. This influenced the pattern in use of space employed by individuals. Most long-billed birds remained on the mudflats all winter, but many short-billed individuals (i, e. males) fed upon the adjacent pastures, particularly in mid winter when Nereis was most likely to be buried out of reach of their bills.