RESUMEN
Thousands of lesser scaup (Aythya affinis) die during spring and fall migrations through the upper Midwest, USA, from infections with Cyathocotyle bushiensis and Sphaeridiotrema spp. (Class: Trematoda) after ingesting infected intermediate hosts, such as non-native faucet snails (Bithynia tentaculata). The lesser scaup is a species of conservation concern and is highly susceptible to these infections. We collected female lesser scaup from spring migratory stopover locations throughout Illinois and Wisconsin and assessed biochemical and morphological indicators of health in relation to intestinal helminth loads. Helminth species diversity, total trematode abundance, and the infection intensities of the trematodes C. bushiensis and Sphaeridiotrema spp. were associated with percent body fat, blood metabolites, hematological measures, and an index of foraging habitat quality. Helminth diversity was negatively associated with percent body fat, albumin concentrations, and monocytes, whereas glucose concentrations displayed a slight, positive association. Total trematode abundance was negatively associated with blood concentrations of non-esterified fatty acids and albumin. Infections of C. bushiensis were positively related to basophil levels, whereas Sphaeridiotrema spp. infection intensity was negatively associated with packed cell volume and foraging habitat quality. Thus, commonly measured health metrics may indicate intestinal parasite infections and help waterfowl managers understand overall habitat quality. Intestinal parasitic loads offer another plausible mechanism underlying the spring condition hypothesis.