Summer-season abundance and foraging of waterbirds across a habitat mosaic on intertidal flats
Description
Abstract: Tidal flats exposed at low tide frequently consist of a mosaic of habitat types. In Washington State, USA, tidal flats typically contain eelgrass (Zostera marina) patches intermixed with areas devoid of vegetation. Seagrasses represent a functionally important coastal habitat, serving as protection and nursery grounds for aquatic organisms, but less is known regarding avian use at low tide. Waterbirds in different functional feeding groups may interact differently with vegetation on tidal flats, and birds may make different choices when foraging or resting. In our study, community scientists collected observational data bimonthly during daytime low tides in summer (April to August). They conducted scan surveys and focal follows of waterbirds in paired patches of two lower intertidal habitats (eelgrass and unvegetated) across five sites. Ninety-nine percent of birds seen were gulls, crows, herons, and geese, and half of all birds observed were actively foraging. Total bird abundance within a patch did not differ between habitat types; among the four functional groups (corvids, gulls, herons, waterfowl), herons were more abundant in eelgrass than unvegetated patches. Waterbirds overall were more likely to be observed foraging in eelgrass than in unvegetated patches, and this difference also was seen in the activity budgets of herons. These tidal flats were occasionally visited by humans during low tide surveys. Human presence nearby did not appear to alter bird abundance overall or for any functional group. Overall, while low-tide eelgrass patches were valuable for foraging for some functional groups, a mosaic of habitats was used by waterbird communities. Global declines in eelgrass could lead to population decreases of herbivorous birds that directly consume eelgrass as well as wading birds, such as herons, that take advantage of prey living within seagrass beds.