The not-so-obvious reasons why Rock Sandpipers winter in Alaska
LINKED PAPER
Energetic solutions of Rock Sandpipers to harsh winter conditions rely on prey quality. Ruthrauff, D. R., Dekinga, A., Gill, R. E., & Piersma, T. 2018. IBIS. DOI: 10.1111/ibi.12534. VIEW
Nearly all shorebirds that breed at high northern latitudes are migratory. These birds travel long distances from nonbreeding areas to northern latitudes to breed, taking full advantage of the flush of food resources that spring and summer provides. As long, cold, dark winters loom, however, shorebirds typically depart northern regions for more benign environments that are predictably ice-free. A notable exception to this general trend is the Rock Sandpiper (Calidris p. ptilocnemis), individuals of which occupy the coldest-known nonbreeding range of any shorebird along the muddy shores of upper Cook Inlet in southcentral Alaska (Ruthrauff et al. 2013c; Figure 1).
Figure 1 Rock Sandpipers (Calidris p. ptilocnemis) roosting along shore-fast ice near the mouth of the Kasilof River, southcentral Alaska. These birds occupy the most-northerly nonbreeding site of any shorebird in the Pacific Basin © Daniel Ruthrauff
That these birds even existed in the region is a relatively recent discovery to science–who would have ever expected shorebirds to occupy a cold, mostly frozen inlet that receives just 5 hours of daylight at mid-winter? Once detected, and after some jaw-dropping observations of birds experiencing icing to their bodies and plumage (Figure 2), we began to build a body of evidence to attempt to answer a deceptively simple question: How do these birds do it?
Figure 2 The average temperature in upper Cook Inlet, Alaska, is below freezing from November–March, often exposing Rock Sandpipers to periods of extreme cold that can cause ice to form on plumage and limbs (Ruthrauff & Eskelin 2009) © Daniel Ruthrauff
Building this body of evidence was a bit like building a house. We first built the foundation upon which to base our inference: the who, what, where, and when of their occurrence in upper Cook Inlet. We answered these question via aerial surveys and, when possible, on-the-ground work (Ruthrauff et al. 2013c). We determined that upper Cook Inlet was the primary nonbreeding site for the nominate subspecies of Rock Sandpiper, a population with a breeding range restricted to a few small islands in the Bering Sea, and numbering ~20,000 individuals (Ruthrauff et al. 2012).
We next had to raise the walls, so to speak, to determine the physiological adaptations that supported this subspecies’ occurrence at such a cold site. We conducted compositional analyses on a series of specimens collected in upper Cook Inlet during winter and compared these to congeners collected at a migratory staging site during fall migration. We determined that Rock Sandpipers carry high fat loads during winter, presumably to buffer against periods when food is unavailable, and also greatly increase the size of breast muscles (Ruthrauff et al. 2013b), the organs by which birds generate heat via shivering thermogenesis. Rock Sandpipers also acquire a heavier winter plumage to improve their insulation, and augment stomach, liver, and kidney tissues to aid with increased digestive demands.
To better understand how Rock Sandpipers stoke their furnace, we also studied energetic (Ruthrauff et al. 2013a) and foraging-related behaviours (Ruthrauff et al. 2015) of Rock Sandpipers. With these studies, we were particularly interested in determining the degree to which aspects of their winter ecology reflected intrinsic, subspecific differences. To do this, we compared these traits in C. p. ptilocnemis to individuals of another Rock Sandpiper subspecies, C. p. tschuktschorum, individuals of which migrate relatively long distances to nonbreeding sites at more benign locations in the Pacific northwest. These studies indicated no subspecific differences in basic energetic capacities (Ruthrauff et al. 2013a), but demonstrated potential foraging-related differences that promoted higher intake rates for C. p. ptilocnemis (Figure 3; Ruthrauff et al. 2015).
Figure 3 Rock Sandpipers in upper Cook Inlet feed almost exclusively on the small bivalve Macoma balthica during winter. Like other molluscivorous shorebirds, Rock Sandpipers swallow their prey whole and crush the shells with their muscular gizzard © Daniel Ruthrauff
All of these lines of evidence were combined when trying to put the roof on the house. An obvious question arising from these studies was just how long Rock Sandpipers must forage to satisfy their high energetic demands. A seemingly simple question, but one made extremely challenging due to the very conditions these birds face. Rock Sandpipers occupy a large and mostly inaccessible stretch of Alaskan wilderness, making direct observations almost impossible. Further, we were prevented from tracking these birds using modern tracking technologies due to fear that such devices (especially their antennas) would accumulate ice and seriously impact the birds’ behaviour and survival.
We realized, however, that we had all the necessary components, from food quality to mudflat accessibility and long-term wind and temperature information, to properly model the population’s energetic demands. We knew that previous work by Wiersma & Piersma (1994) accurately predicted temperature-specific metabolic rates that we measured in a laboratory setting (Ruthrauff et al. 2013a), and so we applied this model as the basis for our exploration.
Figure 4 Macoma balthica constitute nearly the entirety of Rock Sandpiper diets in upper Cook Inlet during winter. The ratio of flesh:shell varies by size, with small clams having a higher flesh:shell ratio than large clams (Ruthrauff et al. 2018, figure 3). Rock Sandpipers generally prefer Macoma ≤10 mm in size, and these prey resources occur at densities ≥400 m-2 in upper Cook Inlet © Daniel Ruthrauff
As part of our aforementioned foraging experiments, we fortuitously had collected thousands of Macoma (Figure 4) from upper Cook Inlet to use in our controlled feeding trials. Unbeknownst to us, these samples proved to be the key that really opened the door to the house: when we analysed the quality of these prey items, we found that they had a very high ratio of flesh to shell compared to Macoma from other sites. These small clams are of such high quality, in fact, that they underlie this entire unusual story. We determined that there are simply not enough hours in the day to satisfy daily energetic demands with low-quality Macoma. Just exactly why upper Cook Inlet’s Macoma are so good remains to be seen—questions for a future project—but it is clear that in addition to exhibiting a unique suite of energetic and physiological responses to Cook Inlet’s winter environment, Rock Sandpipers ultimately rely on unusually high-quality prey resources to exploit high-latitude sites in Alaska during winter.
References
Ruthrauff, D.R., Dekinga, A., Gill, R.E., Jr., & Piersma, T. 2013a. Identical metabolic rate and thermal conductance in Rock Sandpiper (Calidris ptilocnemis) subspecies with contrasting nonbreeeding life histories. Auk 130: 60−68. VIEW
Ruthrauff, D.R., Dekinga, A., Gill, R.E., Jr., Summers, R.W., & Piersma, T. 2013b. Ecological correlates of variable organ sizes and fat loads in the most northerly-wintering shorebirds. Can. J. Zool. 91: 698−705. VIEW
Ruthrauff, D.R., Dekinga, A., Gill , R.E., van Gils, J.A., & Piersma, T. 2015. Ways to be different: foraging adaptations that facilitate higher intake rates in a northerly-wintering shorebird compared to a low-latitude conspecific. J. Exp. Biol. 218: 1188–1197. VIEW
Ruthrauff, D.R., & Eskelin, T. 2009. Observations of body-icing on Rock Sandpipers during winter in upper Cook Inlet, Alaska. Wader Study Group Bull. 116: 88–90. VIEW
Ruthrauff, D.R., Gill, R.E., Jr., & Tibbitts, T.L. 2013c. Coping with the cold: an ecological context for the abundance and distribution of Rock Sandpipers during winter in upper Cook Inlet, Alaska. Arctic 66: 269–278. VIEW
Ruthrauff, D.R., Tibbitts, T.L., Gill, R.E., Jr., Dementyev, M.N., & Handel, C.M. 2012. Small population size of the Pribilof Rock Sandpiper confirmed through distance-sampling surveys in Alaska. Condor 114: 544–551. VIEW
Wiersma, P., & Piersma, T. 1994. Effects of microhabitat, flocking, climate and migratory goal on energy expenditure in the annual cycle of Red Knots. Condor 96: 257–279. VIEW
Image credit
Featured image: Rock Sandpiper, Calidris p. ptilocnemis © Daniel Ruthrauff
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