Spatial ecology of long-tailed ducks and white-winged scoters wintering on Nantucket Shoals.

We examine the long-term co-occurrence of long-tailed ducks (Clangula hyemalis) and whitewinged scoters (Melanitta fusca) wintering at the Nantucket Shoals off Massachusetts, USA, and ask: (1) What oceanographic features attract these aggregations? (2) How are distributions of prey and sea ducks related to one another? and (3) What is the explanation for the spatial association between these two species? A winter concentration of long-tailed ducks on the order of 3 x 10⁵ birds have been present near Nantucket Island and the Nantucket Shoals since the 1970s, and there is evidence of some concentration there since the late 1800s. Despite the difference in diet, the two duck species overlap in spatial distribution in relation to a prominent tidal front on the west side of Nantucket Shoals where nested swarms of pelagic amphipods and massive clam beds occur in persistent broadscale concentrations. Gammarid amphipods were collected seasonally by bongo tows over 39 yr (1977–2016), and Atlantic surf clams and ocean quahogs were collected by hydraulic dredge over a 34-yr timescale (1982–2016). Long-tailed ducks and whitewinged scoters were counted during systematic low-level aerial surveys. Both long-tailed ducks and white-winged scoters associated with amphipod and clam aggregations at similar scales, and tracked amphipod and clam concentrations at larger spatial scales, up to ~30 km. These multi-scalar nested associations suggest that the sea ducks foraged within a hierarchical patch system structured by the tidal regime, and concentrated in areas of spatially anchored recurring prey, with highest densities in southwest areas. Enhanced concentrations of prey within a relatively narrow zone on Nantucket Shoals provide a predictable food resource to wintering sea ducks whose populations are in decline. As supported by our generalized additive models, we propose that the positive spatial associations among these species are at least partly explained by local enhancement or facilitation, whereby each species of duck derives foraging benefit from noting the location of aggregations of the other. This interaction will be important to the designation of marine protected areas. [ABSTRACT FROM AUTHOR]