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In juvenile plumage, Least and Pectoral Sandpipers are virtually carbon copies of one another in terms of plumage. In the absence of a comparison that provides meaningful size context, many birders struggle to tell these species apart. Generally speaking, I believe that the use of structure can only take you so far when identifying shorebirds, but for this species pair, one structural clue is not only extremely helpful, it is diagnostic on hatch-year birds seen away from the breeding grounds.
Least and Pectoral Sandpipers breed far to the north of where most North American birders live and do most of their shorebirding, so it's important to discuss what happens with these birds before they show up on the local mudflat. They hatch sometime in June or perhaps early July and remain on the breeding grounds until they grow their first complete set of feathers and become capable of flight and ultimately migration. This feather set is their juvenile (juvenal in some references) plumage. It will be retained, unaltered through the first 2-4 months of their life, during which time they will undertake their first southbound migration.
By the time they depart the breeding grounds on their first southbound journey, their flight feathers are fully developed, thus the comparative lengths of various feather sets is essentially static and predictable. Morphometric comparisons of feather sets–comparing the length of one set of feathers with another feather set–often provides tell-tale clues when it comes to sorting shorebirds with nearly identical plumages. In Britain and Europe, birders have long used this method for identifying the Old World warblers in the genus Phylloscopus.
Least Sandpipers are comparatively short-distance migrants that can be found wintering in at least small numbers as far north as the Mid-Atlantic states on the East Coast and north to Washington and perhaps southern British Columbia on the West Coast. A significant portion of their total population winters in Mexico and the southernmost United States, but some winter as far south as northern Brazil and northern Chile (O'Brien et al. 2006, Paulson 2005). Pectoral Sandpipers are extremely long-distance migrants. They vacate North America entirely during winter. They are rarely found away from South America during winter, with most birds wintering from southern Peru, Bolivia, and southern Brazil, south to central Chile and southern Argentina (O'Brien et al. 2006).
Among shorebirds, there tends to be a direct correlation between the distance a species migrates and its comparative wing length. When comparing Least and Pectoral Sandpipers, this holds true. Least Sandpipers have comparatively short primaries (outermost flight feathers), which at rest do not project beyond the tips of their longest tertials, the long innermost wing feathers that lay over the top of folded wings. On a feeding Least Sandpiper, the tips of the primaries are often completely hidden under the tertials.
By comparison, Pectoral Sandpipers have long primaries. On a feeding or resting Pectoral, the tips of multiple primaries, usually at least three, can be seen projecting beyond the tip of the longest tertials. The extent and number of primaries sticking out past the longest tertial is often referred to as "primary projection." Pectorals show significant primary projection, while Least Sandpipers show virtually none.
There are some secondary field marks that I look for on Pectorals. As noted in several of the captions above, the basal third of their bill is paler, ranging from brownish to more orangish or even yellowish at times. While some Leasts may show a very small amount of brown at the base of the lower mandible, in general they appear to have all-black bills and never show extensive paler coloration over the basal 0ne-third of their bill.
Secondly, while the streaking on the breast of a juvenile Least is quite variable, as a general rule they are most heavily streaked on the sides of the upper breast and the streaking thins out towards the middle. Additionally, the streaking doesn't form a strong bib across the breast, nor does it come to a widow's peak of sorts in the middle of the lower breast. Pectorals often look to have a bib with a slight widow's peak in the middle and the streaking ends abruptly across the lower breast.
Thirdly, Pectorals have sturdier, thicker legs that tend to be more yellow in juveniles, while juvenile Leasts tend to have dingier and greener legs. Leasts often bend their legs and squat as they feed, while Pectorals seem inclined to stand more straight-legged.
Finally, and this is a subtle mark, but to my eye Pectorals seem to look more plain faced with very little contrast between the auriculars and the rest of the lower face. This can make the eye look fairly large. Leasts seem a little darker in the face to me, with a more obvious auricular patch and a less conspicuous eye. The dark lores on a Least strike me as more noticeable and more connected to front of the eye, while the dark in the lores of a Pectoral is less conspicuous and often fades out a bit right in front of the eye.
Over time and with lots of study, the sum of the parts should allow you to more easily separate Leasts and Pectorals, even without a good size comparison. But, until you get there, look for the primary projection. This is a highly reliable field mark. Plumage wear and hue, bill color, leg color, and even posture can all vary from bird to bird, but the comparative lengths of the primaries and tertials are not variable and can be used to confidently tell these two species apart.
O'Brien, Michael, Richard Crossley, and Kevin Karlson. 2006. The Shorebird Guide. Houghton Mifflin Company, New York, NY.
Paulson, Dennis R. 2005. Shorebirds of North America: The Photographic Guide. Princeton University Press, Princeton, NJ.