The Torrent Duck (Merganetta armata) is one of four species of waterfowl that live in fast flowing rivers. Torrent Ducks feed on benthic invertebrates in mountain rivers from Venezuela to Argentina. Prey selection by Torrent Ducks was investigated by comparing the proportion of prey taxa in their feces with the proportion available in river benthos. Feces and benthos were sampled during spring and autumn at sites with different precipitation levels in northwestern Argentine Patagonia. Energy values of the most important prey items were evaluated and the results adjusted for the amount of chitin body proportions. The average energy value of each individual prey item was 3.60 J in Gripopterygidae, 5.88 J in Simuliidae, 11.48 J in Atalophlebiinae and 12.25 J in Smicridea spp. These four invertebrate taxa represented approximately 80% of Torrent Duck diet. Prey availability and energy value alone could not explain the total Torrent Duck diet, and it was necessary to consider other factors such as aspects of prey natural history and their distribution in the benthos. Most consumed items were filter-feeding species (Simuliidae and Smicridea spp.) that inhabited rock surfaces that were more accessible to Torrent Ducks. Gripopterygidae and Atalophlebiinae were less accessible as they inhabit in the lower part of rocks. Diet information can be used to assess the availability of the most important prey items of Torrent Ducks in different rivers, and thus to estimate an important feature of habitat quality.
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Vol. 37 • No. 2