Following is the latest in a monthly series of articles about birds and birding, celebrating The Year of the Bird, 2018-19, with authors from Kachemak Bay Birders.
Of the 52 species of shorebirds that occur in North America, 37 species regularly breed in Alaska. We know that after breeding in the Arctic and Subarctic during its usually bountiful summers, shorebirds start heading south, migrating as far south as Central and South America for the winter. Or do they all?
Not all species of Alaska’s shorebirds are snowbirds. Despite the ice, snow, and darkness, the contrarian rock Sandpiper spends its winter further north than an any other Pacific shorebird. It seems to thrive on Alaska coastal beaches that aren’t completely locked in with ice. A prime winter destination for these tough little birds is lower Cook Inlet. Flocks of rock sandpipers that number in the thousands congregate in places like Mud Bay in Homer and the mouth of rivers such as the Kasilof River. Smaller flocks are seen in Seward and other coastal areas.
Coastal Alaska’s extreme tides create wide intertidal beaches, good habitat for small marine invertebrates that bury in the mud and silt. During their non-breeding period, rock sandpipers feed mostly on marine mollusks, crustaceans, and larval flies. Rock sandpipers that overwinter in Cook Inlet feed mostly on Macoma baltica, an abundant, thumbnail-sized clam with a pinkish shell and lots of high-octane lipids.
Despite being nicknamed “Rockies,” these shorebirds spend most of their time foraging in intertidal areas, then roost on rocks during high tide when foraging opportunities are limited, whether that be 2 a.m. or 2 p.m. The diurnal cycle for shorebirds depends on tides, not the sun.
Taxonomically, rock sandpipers (Calidris ptilocnemis) are in the genus Calidris, commonly called sandpipers, which includes 14 naturally-occurring species in Alaska. Rock sandpipers are endemic to Beringia (Bering and Chukchi Seas) and have four subspecies. Each subspecies breeds in coastal tundra, but in separate regions of Beringia. Two subspecies overwinter in the Cook Inlet area: C.p. ptilocnemis which breed in the Pribilof Islands (migrating north for the winter) and C. p. tschuktschorum which breed in western Alaska and Siberia’s Chukotsky Peninsula. The latter comprise most of the Kachemak Bay flock.
Rock sandpipers are plump shorebirds with short legs that are yellow in the winter. In their non-breeding plumage, subspecies are similar: a nondescript gray head and back with streaks of gray spots on a whitish breast. However, a discerning eye can see differences. Pribilof birds are slightly larger and have paler gray plumage than the western Alaska subspecies.
The breeding plumage for rock sandpipers is noticeably different. They molt into a rufous-colored crown, mantle, and scapulars, similar to a Western Sandpiper. They also develop a black breast patch, like a Dunlin but scruffier. With Kachemak Bay “Rockies” some start developing their breeding plumage before they begin spring migration in early April.
The purple sandpiper, found on the east coast of the U.S. and Canada as well as northwestern Europe, is very similar to the rock sandpiper in all respects except range. Purple and rock sandpipers are considered sister species; they are geographically separated but descended from the same ancestor.
The population of rock sandpipers is difficult to estimate because of their remote habitat. The most recent estimate, based on a literature review, said the population was about 50,000 for C. p. tschuktschorum and 20,000 for C.p. ptilocnemis. Trends appear stable, but perhaps not in some Pacific Northwest overwintering sites.
According to eBird, the highest rock sandpiper count for Mud Bay was 10,000 on March 15, 2011. But their presence is quite variable, depending on ice conditions. Usually, in winter, a couple of thousand rock sandpipers can be seen somewhere on the Spit, often roosting in one big, cuddly group at high tide in the boat harbor. Notice that, when roosting, nearly all the birds will have their bill tucked under their wing except for a few that look alert. These are sentries that let out a warning call if danger lurks — like a predatory raptor.
The ability of rock sandpipers to survive icy Kachemak Bay winters is contrary to other shorebirds. They certainly deserve a drive out the Spit to observe and admire their tenacity — a good fit for Alaska.
George Matz is a longtime Homer birder and helps coordinate the annual shorebird monitoring project.
This article is brought to you by the Kachemak Bay Birders. For more information about Kachemak Bay Birders birding trips, meetings, and other activities and events, go to kachemakbaybirders.org.
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