| Summary: | Food availability is well-known to have a strong influence on where and how shorebirds forage. A recent discovery is that microphytobenthic (MPB) biofilms, previously unknown to be used as food by any avian species, are an important component of the diet of small calidridine sandpipers. This dissertation investigates the as yet uninvestigated relations between the behavior and distribution of foraging calidridine sandpipers and MPB biofilm availability. I studied three calidridine species - western sandpiper (Calidris mauri), dunlin (Calidris alpina), least sandpiper (Calidris minutilla) - at three estuarine sites: Roberts Bank, an intertidal stopover site in Canada; and at Río Máximo and Tunas de Zaza, coastal stop-over sites in Cuba. Of these species, western sandpipers show the greatest modification of tongue and bill morphology for biofilm feeding. Infrared photography from ground and air and sediment sampling were used to assess the abundance of biofilm and the small benthic invertebrates fed on by sandpipers. Counts of shorebirds and measures of dropping density were conducted to assess bird distribution and habitat use. At Roberts Bank, the highest MPB biomass was found in the upper intertidal (0-750 m from shore, tide height range: 3.0-3.5 m), consisting of a diatomaceous biofilm, one the two major classes of MPB biofilms. Foraging dunlins closely followed the ebbing tide, exploiting the upper intertidal only briefly. In contrast, western sandpipers exploited the entire exposed tidalflat surface, spending much more time in the upper intertidal, matching MPB biomass more closely than the distribution of benthic invertebrates. Both Cuban sites showed a seasonal change in the type of MPB biofilm, shifting from cyanobacterial mats during southbound stopovers to diatomaceous biofilms during winter. In contrast to the foraging activity of western sandpipers at Roberts Bank, least sandpipers used foraging habitats in accordance with benthic invertebrate density rather than MPB biomass. I review the available evidence and conclude that the reason for this difference is that cyanobacterial mats provide poorer nutrition than diatomaceous biofilms.
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