Torrent Ducks (Merganetta armata) inhabit fast-flowing rivers in the Andes. Rainfall causes rivers to swell, increasing water velocity, turbulence, and turbidity. These changes may affect the ducks' foraging ability because of the harsher conditions or alteration of the aquatic macroinvertebrate resource base. We examined the relationship of Torrent Duck time budgets with food availability and rainfall in the central Andes of Colombia. Time spent foraging was not correlated with total macroinvertebrate density but was negatively correlated with density of 4 preferred families. Monthly precipitation was negatively correlated with total macroinvertebrate density but not correlated with the 4 preferred families. Time spent foraging correlated with rainfall the day before. Torrent Duck time budgets result from interactions among several factors: the ducks' foraging tactics, prey type, and river conditions that affect ducks directly, and indirectly through prey availability. Three of the 4 preferred invertebrate families are filter-feeding larvae that adhere to rocks and are therefore less affected by changes in river flow than other prey types. Thus, Torrent Duck foraging time is independent of overall invertebrate abundance but responds strongly to changes in the density of the 4 most-consumed families. Foraging time is also affected by previous-day rainfall, suggesting that increases in water flow or turbidity have a direct effect on the ducks' foraging efficiency. We conclude that physical conditions of the river habitat impose restrictions on the ducks' activities, both directly and through an indirect interaction with food availability.

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