Food Habits and Availability for Lesser Scaup (Aythya affinis) During Spring

The Illinois and Mississippi River corridors provide critical stopover habitat for migrating waterfowl. However, recent evidence suggests that wetlands associated with these river systems provide limited seed and invertebrate biomass for waterfowl in spring. The “Spring Condition Hypothesis” states that these declines in midcontinent foraging habitat quality may explain extended declines in species like lesser scaup (Aythya affinis). We experimentally collected lesser scaup (n = 232) from foraging flocks throughout the Illinois and Upper Mississippi River valleys during February–April 2014–2015. We extracted upper digestive tracts and collected benthic food samples at collection sites to evaluate food use and selection. Further, we compared food items at collection sites to foods from randomly-collected samples throughout wetlands. Lesser scaup collected in 2014 contained plant material more frequently (92%) and at a greater percent aggregate mass than invertebrates (63%). Digestive tracts also frequently contained invertebrates (87%), but overall aggregate percent biomass (33%) was less than plant material. Overall food density was greater in the Upper Mississippi River Valley (321.6 kg/ha) than the Illinois River Valley (205.1 kg/ha). Food density at collection sites was greater than those from randomly-collected samples in 52.7% of wetlands. However, this frequency was much lower in the Upper Mississippi River (33.3%) than the Illinois River Valley (57.9%). The Upper Mississippi River appeared to be of greater foraging quality during spring than the Illinois River, perhaps contributing to lesser scaup’s lower frequency of selecting sites with greater foods than randomly available in this region. Moist-soil seeds and other plant material were important food items for lesser scaup, but overall food availability was low and likely a limiting factor of diving duck abundance.