Resource partitioning and competition in shorebirds at Teesmouth, with particular reference to grey plover Pluvialis squatarola, curlew Numenius arquata and bar-tailed godwit Limosa lapponica
Two shorebird assemblages were identified as providing high potential for interspecific competition - sanderling, knot, oystercatcher and turnstone on a rocky shore, and curlew, bar-tailed godwit and grey plover on soft substrates. Observations were concentrated on the latter group which all fed chiefly on ragworm Nereis diversicolor. Sizes of prey taken by the three species were estimated by two independent methods. Sexual and age differences in diet within species were examined. Dietary overlap between species was high only between certain age/sex classes. Depletion of prey by each species was estimated. Between 44 and 77 percent of the larger size class of Nereis was consumed in a favoured feeding area over one winter. Implications for competition are discussed. Interspecific aggression rates were very low compared to rates within species. All three species were present on Seal Sands, Teesmouth, in high densities in mid-winter, but temporal segregation between grey plover and curlew occurred on a favoured feeding site within a low water period. Segregation resulted from different micro-habitat choice by the two species, rather than avoidance, since grey plovers exhibited identical behaviour at times of year when densities of curlews were low. Within a period of exposure, grey plovers moved feeding site when their energy intake rate decreased due to drying of the sediments. Energy intake rate of grey plovers was not reduced in proportion to the density of curlew surrounding them, except at very high curlew densities. Competition between the three large species during the study was not important. Partitioning of prey size, temporal partitioning of feeding areas, and use of different sediment types enabled their coexistence. Competition may occur during years of high shorebird populations and low densities of available prey. Evidence from the rocky shore assemblage showed significant avoidance of knot by sanderling when feeding area was limited.