Editor's Note: Analysis of mitochondrial and nuclear DNA sequences in members of the shorebird tribe Tringini suggests that the genus Catoptrophorus is embedded within Tringa and should be merged into it. Thus the 47th Supplement to the American Ornithologists' Union's Checklist of North American Birds now recognizes Willet as Tringa semipalmata. Future revisions of this account will reflect this change.
The Willet loudly heralds the arrival of spring from aloft with its ringing “ pill-will-willet ” call, accompanied by flashing wing beats. A medium-sized, moderately abundant shorebird, the Willet remains brown and inconspicuous until it opens its wings, displaying an unusually broad white wing-stripe that runs across the primaries and secondaries, bordered in black. This species has one of the greatest latitudinal ranges of temperate breeding shorebirds in North America—from the Canadian Maritimes to Venezuela. It is the only North American sandpiper with a breeding range that extends south of the North-temperate region.
This species is composed of 2 disjunct breeding populations differing in ecology, in morphology, and subtly in vocalizations. Populations breeding in inland, primarily freshwater habitats of western states and provinces belong to the subspecies Tringa semipalmata inornata or Western Willet. Populations breeding in the marshes of the Atlantic coast, from New Brunswick to Tamaulipas, belong to the subspecies T. s. semipalmata or Eastern Willet. The two breeding environments differ in several characteristics. Eastern Willets often have abundant food resources but limited nesting habitat, while Western Willets often have abundant nesting habitat but unpredictable food resources, depending on wetland availability and drought. Ambient sound also differs between the breeding areas of the 2 subspecies, and this has resulted in a divergence in “song” characteristics. The song (“pill-will-willet”) of the Eastern Willet is emitted at a higher frequency and more rapid repetition rate than that of the Western Willet. Calls of both subspecies sound very similar to human ears, but Eastern Willets do discriminate between male songs of the 2 subspecies, responding preferentially to Eastern song. Western Willets tend to be larger and paler than Eastern Willets, with less ornate barring in their breeding plumage. These are average differences, and the 2 subspecies overlap in these morphological characteristics.
Research on Willets has been rather extensive in a variety of subject matter but scattered and often superficial in intensity. Several short-term studies have examined the breeding biology of the Eastern Willet (for example, Wilcox 1980, Howe 1982, Douglas III 1996), and a 6-year study was recently concluded on population biology of the Western Willet (CLG-T). Other studies of Willets have examined foraging methods and diet (Mendenhall 1970, Stenzel et al. 1976, Hansen 1979, Rompré and McNeil 1994, McNeil and Rompré 1995), parasites (Wong et al. 1989a) and habitat associations and ecology (Kelly and Cogswell 1979, Ryan and Renken 1987).
In this account, many sections will be partitioned with subheadings “Western” and “Eastern” to allocate observations geographically and taxonomically to the 2 subspecies.