Although it is one of the most abundant and widespread diving ducks in North America, the Lesser Scaup is relatively understudied and poorly known, due to the inaccessibility of most of its breeding grounds and its similarity to the Greater Scaup, with which it often associates. Widely known as "The Blue Bill”, the breeding male sports a black, purple glossed (and slightly peaked) head, a grey back, dark wings with a short white wing stripe and a distinctive blue bill, unlike the Greater Scaup which has a green gloss to its black, more rounded head and a considerably longer white wing stripe. The typically drab brown female also has the short white wing stripe, as well as a characteristic white face patch around her bill. The majority of Lesser Scaup breed in the boreal forest, and loss of boreal breeding grounds could drastically affect this species future. Recent environmental changes on its migration routes and wintering grounds also pose potential threats that have yet to be evaluated adequately.

Habitat:

The Lesser Scaup nests primarily in fresh, well-vegetated wetlands. Although it usually prefers permanent and semi-permanent water bodies, it makes more use of seasonal wetlands than do other diving ducks. Typical breeding habitat is shallow water with high volumes of emergent vegetation (cattail, bulrush, pondweed, water milfoil) and aquatic invertebrates; submerged vegetation is also generally present. After the breeding season, most birds shift to larger, more permanent, and sometimes less vegetated water bodies to moult. Migrants use a wide variety of aquatic and estuarine environments including lakes, rivers, bays, and marshes, the primary requirements being fairly shallow water and plentiful food; spring migrants often use smaller wetlands. Winter habitats are similarly varied and may also include saline and even hyper-saline habitats near sources of fresh drinking water.

Diet/Feeding Behavior:

The Lesser Scaup is more carnivorous than related species, although the diet shows extreme temporal and spatial variability. During most of the year, aquatic invertebrates such as amphipods, midges, leeches, clams and snails predominate. The introduced and often superabundant Zebra Mussel has become the species’ primary food (over 90%) during migration through the Great Lakes. Some fish (especially those already injured or dead) and fish eggs also are eaten opportunistically. Seeds of aquatic plants, such as pond lilies, often are selected preferentially and make up the bulk of the diet in some areas in late summer, perhaps reflecting their greater availability at that time. Nearly all foraging is under water; food is taken either from the water column or by shoveling in soft bottom substrates. Unlike other divers, this species dives at an angle rather than vertically, thus submerging and reemerging at different points. Most food is swallowed underwater, but some larger items are handled at the surface.

Breeding:

The Lesser Scaup is primarily a boreal breeding species, nesting from interior Alaska through much of sub-arctic Canada east at least to western Quebec and south in the Rockies to northern Colorado. There are isolated populations in the Klamath Basin region, Ruby Lake in Nevada, southern San Francisco Bay, central and southern Colorado, southern Ontario, and the St. Lawrence River. The Lesser Scaup remains unpaired much later than most ducks, forming pair bonds quickly just before reaching the breeding grounds from March to May. Males will court older, more experienced females, although the females make the final selection of a mate. The pair, led by the female, looks for a nest site, visiting several potential sites repeatedly before selecting one. Nests are typically built in dense vegetative cover, usually in marshy areas near open water. They are often found on islands and floating mats of vegetation, but also may be quite far from water. Egg-laying begins when the nest is a simple scrape; nest construction continues even during incubation as the female continues to add down and grasses to her nest. Clutch size ranges from 6-14 (usually 8-10), increasing an average of one/year through the female’s lifetime (though later clutches may be smaller). Like most ducks, female Lesser Scaup sometimes lay eggs in other ducks’ nests before their own. Males desert their mates roughly midway through incubation (which lasts 3-4 weeks), and form post-breeding flocks that are maintained through the molt. Once hatched, ducklings can walk, run, swim, and dive almost immediately, but are too buoyant initially to remain submerged, feeding at the surface for the first week. The female attends them for 2-5 weeks, by which time they can fly. Females and broods will sometimes amalgamate when they encounter one another, creating large groups with many ducklings and sometimes more than one female.

Migration/Winter Range:

The Lesser Scaup undergoes a short migration from nesting areas to boreal lakes after breeding for the purpose of molting. It departs the breeding grounds only when cold fronts and ice force it to do so, peaking in late October/early November in the north, late November in the south, with migrant flocks sometimes numbering in the hundreds of thousands. Migrants may appear virtually anywhere south of the breeding grounds, with its winter range extending farther south than that of other diving ducks, reaching the West Indies and Central America. Northernmost nesting populations appear to have the southernmost wintering grounds, flying over many birds in between. Migration may be over land or ocean, depending on the trajectory: most Alaskan and Canadian breeders travel through the Mississippi Valley to the Gulf of Mexico or east through the Great Lakes to the Atlantic, and birds wintering in California and western Mexico travel over the Pacific from Alaska. Spring migration is the most drawn out of all North American ducks, with some individuals moving north as soon as ice retreats and others not until several months later. It is assumed that most birds use the same routes in fall and spring.

Conservation Issues and Status:

Lesser Scaup population trends are difficult to assess because few surveys discriminate adequately between the two scaup species. However, overall scaup populations, to which Lesser Scaup contribute close to 90%, have declined roughly 50% since the mid 1950s, and these declines appear to be strongest in areas where Lesser Scaup predominate. The Boreal Forest and Prairie Parkland populations have declined markedly; the Christmas Bird Count shows a highly significant annual decline of 1.6%, with the strongest declines in the Gulf Coast region. The reasons for these declines are unclear, but prime suspects include prolonged drought on the Prairie Parkland breeding grounds (from which Lesser Scaup would be relatively slow to recover because of low productivity and high site fidelity compared to other ducks), contaminants (which they may be acquiring from eating Zebra Mussels), habitat changes in migration and wintering areas (caused by pollution, wetland drainage, siltation, exotic plants, etc.), and possibly disturbance on the breeding grounds. Widespread logging, fires, and acid rain in the boreal region probably threaten broad areas of breeding habitat, and recruitment in this region appears suppressed, for as yet unknown reasons. Without more-accurate species-specific data, it will be difficult to pinpoint declines in this species and their causes.

References:

• Austin, J. E., A. D. Afton, M. G. Anderson, R. G. Clark, C. M. Custer, J. S. Lawrence, J. B. Pollard, and J. K. Ringelman. 2000. Declining scaup populations: issues, hypotheses, and research needs. Wildl. Soc. Bull. 28:254-263.
• Austin, J. E., C. M. Custer, and A. D. Afton. 1998. Lesser Scaup (Aythya affinis). In The Birds of North America, No. 338 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, and The American Ornithologists’ Union, Washington, D.C.
• Bellrose, F. C. 1980. Ducks, geese, and swans of North America. 3d ed. Stackpole Books, Harrisburg, PA.
• Kantrud, H. A., and R. E. Stewart. 1977. Use of natural basin wetlands by breeding waterfowl in North Dakota. J. Wildl. Manage. 41:243-253.
• National Audubon Society. 2002. The Christmas Bird Count Historical Results [Online]. http://www.audubon.org/bird/cbc
• Sauer, J. R., J. E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, Results and Analysis 1966 - 2003. Version 2004.1. USGS Patuxent Wildlife Research Center, Laurel, MD.
• Tome, M. W., and D. A. Wrubleski. 1988. Underwater foraging behavior of Canvasbacks, Lesser Scaups, and Ruddy Ducks. Condor 90:168-172.