Shorebird "Fall" Migration

by Mike Stiles

It's September, and local birders are looking forward to the end of the "summer doldrums" and the arrival of our fall migrants. From mid-September onward, birders will be scouring the coastal migrant traps for southbound warblers, vireos, and others. Personally, I look forward to the return of the White-crowned Sparrows this month. September 18th is the earliest arrival date in my yard, since I've been keeping track.

For one group of birds though, the sandpipers and other shorebirds, the fall migration has been in full swing for months. Not only do they undertake incredible journeys, (see my November 2009 article), but the timing of their arrival along our Pacific coastal area is unique.

I will use the example of the Western Sandpiper, one of our most common shorebirds, to illustrate this point. They breed in Western Alaska in incredible numbers, estimated to be more than six million birds. The bulk of these birds will migrate down the Pacific coast. Their southbound migration starts in June when the adults, once the chicks are old enough to fend for themselves, leave, and they will arrive in our area by the end of the month.

June 23 is our earliest arrival date for the adult Western Sandpiper and numbers will quickly increase from individual birds being seen to several thousand by early July. Many of these birds will still be in alternate (breeding) plumage making this notoriously difficult group of birds somewhat "easier" to identify.

On the breeding grounds, the young will continue feeding and putting on weight and will leave Alaska about a month after their parents. July 21st is an early arrival date for a juvenile bird. Keep in mind that these birds are just a few months old, but nevertheless make the journey south with no direction from the adults. These birds in juvenal plumage will look bright and fresh, normally with the feathers on the back and wings edged in white, giving it a scaly appearance.  (Quick note: A bird in its first year is a juvenile bird, but the term for its plumage is "juvenal.")

Numbers of Western Sandpiper migrants will continue to increase through July, August, and even into September. By October, the birds we see on Morro Bay are here for the winter. By now they have molted into their basic (non-breeding) plumage, where gray and brown colors dominate. Many species do look alike at this point, and not only to the beginner birders among us. Wintering Westerns number in the thousands along Morro Bay. Christmas bird count numbers average 1,250 birds with a high count of 3,500.

Incredibly, there are early June records of alternate plumaged birds that are considered to be late Spring migrants. These very late northbound migrants are actually crossing paths with the early southbound birds. I can't imagine there is much breeding success with these birds. If they do make it to Alaska, short days and the winter cold will soon be setting in.

There are exceptions, of course, to shorebird arrival in our county. Dunlin are one of our latest arriving wintering shorebirds, where a late August or early September arrival would be considered early. Our rarer shorebirds typically arrive in September through October and are predominantly in juvenal plumages, and are transient and usually not in our area more than a few days…often to the chagrin of local "chasers."

Thanks to Tom Edell for arrival dates in this county, and to Paul Lehman and Don Roberson for information in their Santa Barbara County and Monterey County books, respectively.

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