Each morning for the past couple of weeks, I’ve stepped out onto my porch, stood and listened. Spring migration is just getting underway and I like to keep track of the new arrivals in my yard. The first migrant appeared ten or so days ago. It was a Ruby-crowned Kinglet, followed in the next days by American Robins, Dark-eyed Juncos, Merlin, and this morning, my first Varied Thrush of the season.
It’s all getting underway remarkably early this year. We are set to have the warmest spring on record which has led to the earliest ever breakup of the Yukon and the second earliest on the Tanana, and the earliest green-up in Fairbanks. The birds appear to be taking advantage of those extra days.
The arrival order of the birds is more or less the same year after year. One species might appear a few days ahead of another, but you can bet that the first Kinglet is going to arrive weeks ahead of the the first Swainson’s Thrush, and that Alder Flycatcher is going to be the last species of spring to arrive.
So what dictates what species arrive when? The answer to that question has a few parts, including food selection, migration distance, and seasonal weather.
Food is the obvious part. The species that arrive very early in the season are likely to have the toughest time foraging. Snow often covers at least large sections of the boreal forest, and flying insects have not yet emerged in good numbers. Thus early arriving species tend to be those that are either gleaners, like the Ruby-crowned Kinglets, that rely on dormant insects that have wintered on tree bark or branches, or they are generalists, able to eat a number of foods. These are species like American Robins, Juncos and other sparrows which can happily subsist on seeds, last year’s leftover berries, or emergent insects. These early arrivals also have to be tolerant of bad weather, they after all suffer the greatest chance of getting hit by a late season storm.
Species that arrive late in the season like most warblers and flycatchers (though notably Hammond’s Flycatchers manage an early arrival) wait to show up on the breeding grounds until the insects are well established.
Migration distance too plays a role, though it is one that may be tangled up in food. Migrants that head to the Neotropics, rather than the temperate zone, tend to arrive later. Migration distance is probably important, but as many of Alaska’s Nearctic-Neotropical migrants are insectivorous warblers and flycatchers, food may be just as important. If you’ve got a few thousand miles to fly, you want to be darn sure that you’ll have a meal waiting for you when you arrive.
Last, but very importantly, is weather. Weather can strongly impact migration times, and it doesn’t have to be weather at the destination, but anywhere along the route. A few years ago, here in the interior, a major late season snow storm swept through during the peak of the spring sparrow migration. Birds quite literally tumbled out of the air in eastern Alaska. Reports indicate that roadways, feeders, and fields were littered with birds stopped dead in their tracks (often literally). Though the weather was decent here in Fairbanks, there were no new birds arriving. The storm lasted days, and it took more time for the snow to melt and allow the birds to move on. Perhaps a week late, they finally arrived in town.
The length of the migratory period also varies by species. The paper from which I drew much of the information in this post relied on data from birds banded at Creamer’s Field here in Fairbanks. Very late arriving species like Alder Flycatchers all came through in a very short window about two weeks long. Early arriving species like the Hammond’s Flycatcher had a comparatively leisurely migratory period in interior Alaska, passing through over the course of more than 50 days.
I love it when my own observations are reflected in data. Makes me feel like I can predict the future with some accuracy, at least when it comes to what bird species are going to arrive next.
So let’s see, so far this year I’ve heard or seen the following songbird species here in Fairbanks: Ruby-crowned Kinglet, Dark-eyed Junco, American Robin, Yellow-rumped Warbler, Hammond’s Flycatcher, and Varied Thrush. Which means that the next species I see or hear should be…Fox Sparrow!
Guess we’ll find out soon enough.
Paper I used for some of the information here:
Benson, A.M. and K. Winker. 2001. Timing and breeding range occupancy among high-latitude passerine migrants. The Auk, 118(2): 513-519.
