Messy vs. Neat: Ageing Southbound Migrant Shorebirds

Unlike most species of birds, shorebirds (sandpipers and plovers) retain their juvenile plumages through much of their southbound (Fall) migration, thus in order to properly identify them it's important to first determine whether you are looking at an adult or a bird of the year (juvenile). If by chance you find and report a rare shorebird from a local mudflat, before asking anything else, your local field notes editor will want to know, "was it an adult or a juvenile." For many birders, even some with years of experience, this can be a stumper. Here's an easy first step to start learning how to make such determinations.

Since the majority of you reading this article live in the mid-latitudes of the Northern Hemisphere, let's first consider the timing of shorebird migrations where we live. Many of our familiar species of shorebirds are high-latitude breeders that nest near or inside the Arctic Circle. Their breeding season occupies a narrow window from very late May to early July. Once breeding duties are completed, adults don't linger long on the breeding grounds, thus southbound adults of many species can be seen back in the mid-latitudes (southern Canada and the Lower 48) by late June or early July and they will, in many cases, arrive on their wintering grounds by early August.

This adult Sanderling was photographed on 24 August. Notice how messy the plumage is and how there is no consistent color pattern to the upperparts. The dark centered feathers are remnants from the breeding or alternate plumage, while the paler gray feathers are part of the winter or basic plumage that is just growing in. This bird is in the process of molting. Also note the differences in the lengths of feathers that are side-by-side and how these feathers don't lay together smoothly. (Photo by Glenn Bartley)

This juvenile Sanderling was photographed on 4 September. The first thing you'll likely notice when comparing this bird to the adult in the image above, is how neat and orderly the plumage looks. All of the feathers on this bird were acquired during the same molt, thus they are consistent in color and size and they lay together smoothly. It shows no indications of molt in progress. (Photo by Glenn Bartley)

Their offspring are left behind on the breeding grounds to fatten up and complete the acquisition of their first complete set of feathers (juvenile or juvenal plumage). The first juveniles normally start  appearing Lower 48 and southern Canada at the end of July, about the time the last of the adults is moving through. After about the 10th of August or so, the bulk of the shorebirds one sees at mid-latitudes will be juveniles. There are exceptions to these generalities, most notably species like Black-bellied Plover, Dunlin, and Long-billed Dowitcher, all of which are later fall migrants.

Adults

By the time southbound adult shorebirds reach the mid-latitudes they will be in the midst of their prebasic molt, thus they are replacing feathers as they migrate. The basic or winter plumages of shorebirds are generally much duller (often cold gray tones) than the bright richly-colored alternate plumages they wear during the breeding season. So, as the new feathers grow in, the contrast between the old and new feathers is often quite conspicuous, creating a calico or mottled look. Since they are on the move, they can't replace all their feathers at once. Hence, this protracted molt may result in side-by-side feathers showing significant differences in  size and shape. 

This close-up view of the wing coverts of a molting adult Sanderling shows feathers that vary greatly in terms of length, overall size, and shape. As a general rule, the covert feathers on adult shorebirds are larger and generally more elongated than those seen on juveniles of the same species. They also don't seem to be arranged in the tract rows one expects to see when the molt has been completed. (Photo by Glenn Bartley)

Juveniles

When the first juvenile shorebirds show up at the local mudflats they will be in extremely fresh plumage that is likely no more than 4-6 weeks old. It will be several more weeks before they start transitioning from this juvenile plumage to their first basic or first-winter plumage. You won't see any of the color irregularities and inconsistency in pattern that is evident in adults at this time of year. Since the juvenile feathers are fully grown by the time they leave the breeding grounds, side-by-side feathers in the scapulars and coverts will be roughly the same size and shape and they will lay together in an orderly fashion. The overall plumage will look very smooth and uniform, with nothing out of place.

This close-up of the wing coverts of a juvenile Sanderling shows rows of feathers that are similar in size and shape and show consistent color pattern. Note the orderliness of the individual rows of feathers. As is clearly evident in this image, the covert feathers of juveniles are typically smaller and more rounded than those of adults. (Photo by Glenn Bartley)

Summary

Juvenile plumages are unique to hatch-year birds. In the majority of shorebird species the first complete feather sets are retained through most of the fall migration. The period of overlap when one is likely to see adult and juveniles together is fairly narrow for most species. While there are no absolutes, as a general rule if you see a southbound shorebird (July-September) whose plumage appears messy and inconsistently patterned, it is an adult. If, during this same time frame, you see a wader whose plumage is very neat and consistently patterned, then it is likely a juvenile.

In this image, taken at Madison, New Haven Co., Connecticut on 11 August 2011, we can directly compare a juvenile (left) with a transitioning adult (right) Short-billed Dowitcher (subspecies "griseus"). Defying conventional wisdom, juvenile shorebirds are typically brighter and more colorful than adults at this time of year. (Photo by Mark S. Szantyr)

To some, this may seem like an over-simplification of a complex identification issue, a point I would agree with on some level. At the same time I think it is important to keep in mind that the average birder may spend only a few hours per fall birding in habitats that allow them to study these differences. They may not grasp the difference between the scapulars and the coverts or know where to look to see a tertial feather. What is learned one year may well have to be relearned the following Fall. My hope in writing this piece was to offer one easy to remember nugget that can be retained. If we can at least help others learn to age these birds properly, we drastically reduce the number of plumage options that they must sort through as they endeavor to identify fall migrant shorebirds to species.