by Bryce W. Robinson
Here in western Alaska I’ve been able to spend a bit of time pausing from a focus on Gyrfalcons to watch the Rock Sandpiper – Calidris ptilocnemis on its breeding grounds. My fascination with its appearance, behavior, and vocalizations caused me to look into what is known about their populations and distribution, a factor of bird biology that I’m growing evermore interested in studying. My investigation led me to understand that there occur three separate subspecific populations in Alaska, the nominate ptilocnemis of the Pribilofs, the tschuktschorum of the western mainland, and couesi of the Aleutian archipelago. As I am currently conducting research on the Seward Peninsula, I have been experiencing Calidris ptilocnemis tschuktschorum.
After some further reading, I learned that the nominate ptilocnemis has the most northern wintering region of any North American shorebird. Remarkably, these birds spend their winters in the Cook Inlet of coastal Alaska. How they manage to find enough food and weather the extreme temperatures and conditions is above my current comprehension, but I was able to find some literature on the subject. Apparently, their is no physiological characteristic or attribute that separates ptilocnemis from other Rock Sandpiper subspecies that would give it the edge to winter so much farther north. The other Rocks could physiologically manage a winter in the north as well, but for whatever reason ptilocnemis is the only ssp. to do so. This seems like a peculiarity, and that is precisely why I am so fascinated by their distribution.
There are other Calidris species that are polytypic, and have separate wintering grounds for each subspecific population. Learning about such cases is extremely enlightening and begs for some contemplation on the matter. Perhaps a polytypic species such as the Rock Sandpiper, or the Red Knot will have an edge on anthropogenic induced changes affecting key migratory sites and their ability to survive the great journey. Perhaps the ability to buffer such disruptions is already built into their distribution at the species level. They are certainly better off than a monotypic species that stops over or winters wholly in a small and localized area. If the lower Pacific Coast of North America becomes a dead zone, then perhaps the subspecies couesi and tschuktschorum will be hard hit, but the nominate ptilocnemis may fair well and maintain a presence for the Rock Sandpiper in North America. As we attempt to be aware of our ever increasing impact, it sure would be nice to know that some birds will be able to wear the disruption, even if it is to a minor degree, so we can then put more energy into those more sensitive.
Gill, Robert E., Pavel S. Tomkovich and Brian J. Mccaffery. 2002. Rock Sandpiper (Calidrisptilocnemis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:http://bna.birds.cornell.edu/bna/species/686
Ruthrauff, D. R., A. Dekinga, R.E. Gill, and T. Piersma. 2013. Identical Metabolic Rate and Thermal Conductance in Rock Sandpiper (Calidris ptilocnemis) Subspecies with Contrasting Nonbreeding Life Histories. The Auk 130 (1) pp. 60-68