Linked paper: Altitudinal bird migration in North America by W.A. Boyle, The Auk: Ornithological Advances 134:2, April 2017.
I became a birder in my early 20s when I moved to Costa Rica to play in the Orquesta Sinfónica Nacional. I didn’t know many people at first, and my Spanish was, shall we say, a work in progress. When I left Canada, I was given a pair of binoculars and the (then) newly published “A Guide to the Birds of Costa Rica” by Stiles and Skutch. Armed with these tools, I would get on a bus headed in a different direction every time I had a day off from my music job. At first I managed to identify only a few of the dozens of species that would pass in riotous mixed flocks. Pretty soon I realized that I had to focus on looking and taking notes, only later to pore over the book to figure out what I had seen. While those evening book sessions were occasionally frustrating (“Dang… I should have checked if that flycatcher had one or TWO wing bars!”), I also enjoyed reading the eloquent descriptions of bird behavior and soon found myself engrossed in tropical natural history. One of the things that puzzled me from the start were descriptions of the seasonal migrations of birds within that tiny, lovely, benign country. I grew up in a place where bird migration seemed not only logical, but frankly the ONLY sensible thing to do in winter. But why would some birds move up and down mountains each year in a place where the weather is always warm and food hangs from the trees wherever you go?
This question ultimately became the topic of my PhD many years later, and I did get some satisfying answers (full details here). But one unsatisfactory aspect of my chosen topic was that few other researchers were asking similar questions in other parts of the world. What common themes from my tropical work might hold true for other regions? What about North American birds? How common are these altitudinal migrations in our mountains? What else is known about them? Finally, in this article, I have attempted to summarize that knowledge. It turns out that we have LOTS of birds in North America that make similar types of movements. In fact, roughly the same proportion of the North American avifauna migrate up and down mountains as does the Costa Rican avifauna—20% to 30% depending on how you count it. With the exception of the Himalayas, reports from other avifaunas seem consistent with this figure. The higher latitude of North America makes things interesting, creating varied combinations of seasonal movements along both elevational and latitudinal gradients, and several of the North American species make movements that stretch our tidy migration terminology in complex ways. There is a reason I had trouble as a grad student finding this literature, however. Much of the information, now summarized in the Birds of North America life history series, was originally reported in bird atlases, Masters theses, or dated natural history accounts. Furthermore, despite early naturalists’ interest in the topic, few authors have cared to document patterns or tried to understand causes of these movements in recent years.
Why might this be so? Part of the reason might have to do with geography; there are more ornithologists in the flatter and more populated eastern portion of the continent compared to the topographically complex west, and this fact may have steered our collective research interest in some way. Part might have to do with the perception that these are not “real” migrations. Certainly the short distances many altitudinal migrants traverse are not the jaw-dropping feats of athleticism displayed by Red Knots, Arctic Terns, or Blackpoll Warblers. But I argue that they are real in many important respects: they involve seasonal return movements between breeding and non-breeding areas on predictable schedules. The fact that such movements are often partial (not all birds migrate), facultative (not genetically hard-wired), and short-distance actually makes them more attractive subjects for many types of migration research. We have far better chances of determining what ecological conditions tip the cost-benefit balance toward migrating in species that have built-in control groups in the form of resident individuals. Furthermore, the more “messy” movements are undeniably a part of the rich diversity of strategies that animals use to cope with a constantly shifting environment. If we are to protect our avifauna for future generations, understanding these movements will be as important as understanding the marathon flights of the migration poster children. Perhaps this review will inspire a blossoming of interest in the birds who make mountains their home.