Patterns of use of intertidal estuarine areas by feeding bird assemblages: a study in the Tagus estuary (Portugal)

Full article

Published:

Volume 40(1), June 1993. Pages 39-53.

Language:

English

Keywords:

feeding bird assemblages, habitat use, waders, winter.

Abstract:

A winter survey of all bird species feeding on several intertidal areas of the Tagus estuary was made to study spatial patterns of species associations, covariation and assemblage structure. Therefore, 29 sectors were defined and the number of feeding birds of all species present counted several times during winter. At the same time, several habitat variables were measured, mainly related to substrate type and human presence. An overall positive association between species occurred. This showed that several species occurred together more often than expected by chance. Analysing patterns of covariation, it was found that two main groups of species had a similar utilisation of sectors; these were (1) Black-headed Gull (Larus ridibundus), Lesser Black-backed Gull (Larus fuscus), Sanderling (Calidris alba), Turnstone (Arenaria interpres), Oystercatcher (Haematopus ostralegus) and Ringed Plover (Charadrius hiaticula), and (2) all other species, although some subgroups could be considered to exist. All significant correlations between species abundances were positive. This suggested that strong competition for space did not occur or the spatial scale of analysis was not fine enough to detect competition processes. Five groups of sectors with similar bird assemblage structure were identified by cluster analysis. Sectors within each one of these groups had in common the most abundant species and the five dominant species were Dunlin, Avocet, Redshank, Lesser Black-backed Gull and Black-headed Gull. Nevertheless, a great structural variability inside each group was found and their location in an ordination space showed that they were not individual entities but parts of a gradient. Sediment type explained the distribution of the several assemblage types throughout the estuary. Sectors dominated by Lesser Black-backed Gull occurred near to the mouth of the estuary or in the main river. Assemblages dominated by Dunlin, Avocet and Black-headed Gull seemed to represent to various degrees a gradient in sediment type. The Redshank-dominated assemblage type was a special case of the typical Avocet-dominated one.