Roosting and associated feeding behaviour of turnstones Arenaria interpres and purple sandpipers Calidris maritima in north-east England
Both individual Turnstones and Purple Sandpipers were largely site-faithful during winter; after the breeding season, however, fewer surviving Purple Sandpipers than Turnstones returned to the study area. Both species were sedentary on small low water home ranges and were faithful to a small number of roost and high water sites. Numbers of Turnstones and Purple Sandpipers flying to the main high water roost, Hartlepool West Harbour, were greatest on spring high tides, but were reduced m high winds. Numbers at other high water sites at nearby Hartlepool Headland were greatest on neap tides. Maximum Purple Sandpiper numbers at West Harbour showed a steady decline over the period from 1986/87 to 1993/94; maximum numbers of Turnstones dropped sharply in 1993/94 after redevelopment of the harbour m 1991/92. Overall disturbance rates to roosting shorebirds at West Harbour, and those from boats and man, increased over the study period and were probably responsible for the decline in Turnstone numbers and those of two other species. An artificial island, built m 1991/92, is now the main roost site for all species. Intraspecific aggression on the feeding grounds was primarily a result of encounters over food; infringements of individual distance were relatively infrequent Dominance hierarchies on low water feeding grounds were stable for both species; adult Turnstones dominated first-winter bkds; Purple Sandpiper dominance was positively correlated with size. Survival was not related to dominance for either species. Both species were segregated at the West Harbour roost into groups from different low water feeding areas; Purple Sandpipers also into groups of different (bill-length) size class. Aggression at the roost resulted from encounters over roosting sites or infringements of individual distance. As wind speed increased, encounters over roosting sites became more frequent and intensity of aggression increased. Dominance hierarchies at the roost were stable for both species; individual roosting dominance was positively correlated, though not always significantly so, to low water feeding dominance. Adult Turnstones dominated first-winter birds; Purple Sandpiper dominance was positively correlated with size. Roosting flocks were structured in accordance with dominance relationships: adults and dominants occupying denser, middle positions and first-winters and subordinates less dense, peripheral sites. Individuals (of both species) that were subordinate at the West Harbour roost frequented it less often than dominants (at least in some seasons), probably because they could not obtain good roosting positions there. Other sites allowed these birds to feed over high water. Turnstones and Purple Sandpipers associated together frequently at the West Harbour roost in comparison to other species. There was no evidence that either species roosted with the other in order to exchange information concerning food resources or to save energy through the roost's microclimate. Both species shared vigilance with each other and probably roosted together m order to decrease the risk of being taken by a predator.