Sharp-tailed Sandpipers have always held an allure for me. For starters, they are rare here in British Columbia (and absent from the rest of Canada). Although present along the coast every autumn, usually from mid-September into October, they are always exciting to find. Part of this rests on the fact that they are an Asian bird. This alone, in my eyes, sets them apart from other equally-rare species occurring locally that are otherwise found in North America, such as Hudsonian Godwit. In coastal BC they can be expected in places such as Reifel Bird Sanctuary and Iona, but they have the ability to show up anywhere, and even inland. Another intriguing fact is that virtually all the Sharp-tailed Sandpipers that turn up in North America are juveniles. In addition to their relative scarcity, another alluring factor is their appearance. They stand out from other shorebirds with their bright chestnut cap, buffy wash across the breast, and bright rufous edging to scaps, coverts and tertials.
In the February issue of The Condor, a paper by Lindstrom et al.1 helped explain why juvenile Sharp-tailed Sandpipers are as common as they are in North America, while adults so scarce. But before delving into specifics, let’s take a look at the broad view of shorebird migration. The general, well-known idea is for a bird to get from its breeding grounds to its wintering grounds, and vice versa. However, there are a number of different strategies a species/individual can employ. A bird can migrate using a time-minimizing strategy, or it could use an energy-minimizing strategy. The two are not necessarily exclusive. We can use the analogy of driving a car cross-country. In a time-minimizing strategy you would get in your car, fill the tank, and drive as fast, as long and as far as possible before filling up the tank again. In an energy minimizing strategy you would get in the car, fuel it up, drive at the most fuel-efficient speed, and probably put it in neutral downhill. For example, falcons may be good at time-minimizing while the soaring raptors are much better at energy-minimizing. Of course there is a third, equally important consideration, and that is predation. You can think of avoiding predators as a risk-minimizing strategy, but as the result of predation is death, it’s really a survival-maximizing strategy, and in that sense may actually be the greatest driver in the evolution of migration strategies. At any rate, now when you fill up the car there is a hungry monster potentially hiding behind the pumps, and the longer you spend fuelling the greater your chance of getting eaten! To add a greater complexity to the situation is that not all birds leave at the same time. In the world of arctic-breeding shorebirds, typically adult females migrate south first, followed by the adult males, and lastly the juveniles (though for Sharp-tailed Sandpiper adult males head south first).
In the case of Sharp-tailed Sandpipers, adults migrate from their breeding range in north-east Russia, through continental East Asia and then onwards to wintering areas in and near Australia. Many juveniles (possibly tens of thousands), on the other hand first make an ~2300 km detour eastwards to major shorebird stopover locations in western Alaska. On the surface, such behaviour seems neither time nor energy-minimizing. Lindstrom et al. found that Sharp-taileds generally arrived in their study site in early September and departed on average at the end of September (to be exact, departing on Sep. 27 and Sep. 29 for males and females respectively). Individual birds had a median minimum length of stay of 12 days, upwards to a maximum of 33 days. That is, most birds spent approximately the month of September in Alaska. An earlier paper covering more areas of Alaska found birds present from late August until early November.
When the birds first arrived in early September they were relatively lean, carrying just slightly higher fuel loads than their lean body mass, and their rate of fat deposition was low. By mid-September, however, the rate of fuel deposition increased markedly as birds began to fatten up for migration, often doubling their weight. With tanks full, it is estimated that these Sharp-tailed Sandpipers can then make a single flight in still air of nearly 10,000 km! It is postulated that these birds join the ranks of other trans-oceanic migrants, such as the Bar-tailed Godwit and Ruddy Turnstone, in making a single, non-stop flight from these staging grounds in Alaska to wintering grounds in Australia.
There are a couple of things to consider here. First is that the staging area is relatively free of predators. Such a high fueling rate would not be possible if under constant attack, especially as it’s been proven in other species that high fuel loads increase the risk of being captured. The second is that this predicted long-distance flight would not be possible if not for such a rich stopover location. We can then start to see that if other stopover locations along the East Asian-Australasian Flyway are (a) more dangerous and/or (b) less profitable, that a safe (from predators, pathogens, etc.) but long-distance trans-oceanic flight might actually be advantageous, despite that initial detour.
So how does the evolution of migration in Sharp-tailed Sandpipers impact birders in British Columbia? Well, we can see that the chronology of sightings in the province fits with the period of when birds show up in and migrate from Alaska. In addition, it explains why all of our birds are juveniles. And since most appear to leave Alaska directly for wintering grounds, it explains why we see only a fraction of the birds that occur in Alaska.
Of course, many additional unanswered questions are raised. For example, are our birds the ones that do not attempt a trans-oceanic flight? If not, where do they go? The lack of winter and spring records in North America indicates that they must either perish, make a different trans-oceanic flight to the wintering grounds, or winter at some unknown location in the New World and make an even more remarkable trans-ocean flight up to the Siberian breeding grounds from there. (I think they likely make their way to the wintering grounds eventually). Are our birds the ones that did not sufficiently fatten up, either due to disturbances or other unknown causes, to make the great flight? Are the birds we see simply lost, or is this an established alternative migratory pathway? Or is it that our birds are ones that actually started out over the ocean but got blown off-course or forced to seek land due to unfavourable weather conditions? (Personally I think if this were the case we’d see larger numbers of birds involved, rather than the 1 or 2 birds usually seen at any given location or time). And why don’t adults use this same strategy? For this last question, the answer may very well have to do with molt as we see in other adult sandpipers (the juveniles having just grown a new set of flight feathers don’t have this same constraint); at any rate, adults appear to be a time-minimizing migrant.
As you can see, we’ve just barely skimmed the surface of the evolution of migratory pathways and strategies. The different life-history strategies even within a species can help us birders predict when and where to look for birds. And if you are successful in catching a glimpse of a bird such as the Sharp-tailed Sandpiper, take a moment away from admiring its beauty or rarity, and consider just how improbable it is that it’s here at all, and just how far it has yet to go.
1Lindström, Å., R.E. Gill Jr., S.E. Jamieson, B. McCaffery, L. Wennerberg, M. Wikelski, and M. Klaasen. 2011. A Puzzling Migratory Detour: Are Fuelling Conditions in Alaska Driving the Movement of Juvenile Sharp-tailed Sandpipers?. Condor 113: 129-139.