Cameron Cox has traveled throughout the USA pursuing birds over the past eight years. Much of the ground work for this article was laid in the spring and summer of 2004 when he worked on a shorebird project in the Delaware Bay. An optics junky virtually since he first touched binoculars, it is no surprise that he now works as a birding specialist for Nikon Sport Optics. (Though those who know him may be surprised that he managed to obtain any kind of steady employment) When not attending birding festivals throughout the country for Nikon, he resides in Seattle Washington.
Standard Peeps (Semipalmated and Western Sandpipers) Figures 6-13
By far the most numerous of the peeps, both these species form huge flocks on tidal mudflats. They prefer more open habitat than does Least Sandpiper, often packing themselves shoulder-to-shoulder in huge feeding flocks. They feed by dropping their necks while keeping the chest well above the surface. Their tarsi are angled back only slightly when feeding, completely unlike the deep knee-bend of the Least. When feeding on a mudflat, they tend to plant their feet farther back and reach slightly forward with their bills. Unlike Least, there is a large gap between where the feet are placed and where the bill is probing. (Figs. 11- 12) The standard peeps are like a person bending at the waist and reaching forward to pick up something in front of them, while Least Sandpiper’s feeding style is more like someone crouching down to pick up something that is right by their feet. Standard peeps usually seem very focused while feeding, pausing to glance around far less frequently than Least. Though if a falcon is constantly harassing them, they may appear more cautious. Both Western and Semipalmated show very slight primary projection, slightly longer in juveniles. Their legs are clearly heavier than those of Least with more obvious knobby joints. ( Fig 6) These two species are the only peeps with “palmations”, small webs of skin between the toes, which can be seen if you look for them, especially on birds standing on sandy beaches.
Semipalmated is the dominant species of peep throughout most of eastern North America except during the late fall and winter, when virtually all depart for wintering areas in the West Indies and northern South America. Adult Semipalmateds molt to varying degrees during migration, rarely up to 95% of their body plumage, but will very rarely show full basic plumage in the US or Canada and never molt flight feathers until they reach their winter grounds. In late fall, first-year Semipalmated Sandpipers are instantly separated from Westerns because they maintain their brown juvenile plumage while Westerns are in gray formative (winter) plumage. It has been suggested that Semipalmated is significantly more aggressive than Western, often engaging in intense and drawn-out physical interactions, even among juveniles (C. Wright pers. comm.).
Semipalmated Sandpipers look compact, with even proportions. Both the breast and the belly are robust so the underparts are smoothly rounded and their heads are proportionally smaller than Western’s. ( Fig. 9) The species often appears bull-necked, unlike Western, which looks rangier. Their legs appear to be placed at the center of the body, so the distribution of weight looks even. Typical bill shape is short and straight with a blunt and slightly swollen tip. However, many do not have this classic shape. Both length and shape vary a great deal due to several variables. Females are substantially longer-billed than males. They also vary clinally, with birds breeding in the east having longer bills than western birds. Because of this, the greatest variability is seen along the East Coast where more typically individuals may be seen alongside extraordinarily long-billed females. In the fall, bills also vary somewhat by age, as many juvenile shorebirds attain their full bill length during their first winter, so some fall migrants will have abnormally short bills. On top of all this, add individual variation. Semipalmateds with longer bills also have finer bill tips and the longest billed females can even show a bit of a droop, closely approximating Western Sandpiper. Individuals with the classic bill shape can be identified by this feature alone, but more often several other characteristics must be used along with bill shape.
Western is by far the dominant peep on the West Coast, and Western along with Least are the only peeps likely to be seen in North America in winter. In fall, molt timing can be a quick way to separate Western from Semipalmated. Western Sandpipers molt much earlier, with some attaining full basic plumage by August. Large gaps appear in the wings of adult Western due to missing flight feathers, while Semipalmated Sandpiper is unlikely to be seen replacing flight feathers in North America. Any first-year standard peep with extensive gray, formative (winter) plumage is a Western. ( Fig. 8)
Western is lanky and long-legged compared to Semipalmated. Their head usually looks slightly too large for their bodies, while the reverse is true of Semipalmated. ( Fig. 7) Western appears to be carrying more weight in front of their legs which are placed slightly farther back on the body compared to Semipalmated. This creates a heavy-chested appearance that can be so pronounced that it seems surprising that they are able to stay upright. This is particularly apparent on roosting birds. (Fig. 13)
While bill length does not vary clinally in Western, as it does Semipalmated, the variation between the sexes is greater. Most, but not all, Westerns have a perceivable droop to the bill. At one extreme, many females have bills that recall Dunlin. While some juvenile males have short bills that are absolutely straight, like Semipalmated, but are slimmer with a finer tip.
I have yet to notice structural characteristics that allow consistent separation of the two standard peeps in flight. In fall, adult Westerns are instantly recognizable by the large gaps in the flight feather due to molt. Fortunately, both species are extremely vocal in flight and their calls are easily recognizable. Semipalmated gives a low-pitched, rolling “chrrk” while Western gives a high, “shik” that is both sharp and squeaky with a slightly whining quality. Feeding flocks may give these calls, but when feeding both species give a wide variety of short, sharp notes that are too variable to be consistently discernable.