Wednesday, August 28, 2013

So What's This So-Called "Fall" Migration?

The best part about Fall Migration is that it's totally not a Fall migration. The beginning of it comes bit by bit in mid-June, when failed breeding shorebirds and some songbirds show slight influxes. Amazingly, this happens as Spring Migration is just wrapping up.

Note the Fall peaks of these shorebird migrations. Data
from eBird
"Fall" Migration continues at a slow pace until mid-July, when things start picking up. With summer drying up lakes, ponds, and rivers into mudflats, we see shorebirds moving en masse southward. This has been the bulk of Illinois' "Fall" Migration so far. Birds like Semipalmated and Least Sandpiper, Semipalmated Plover, Lesser and Greater Yellowlegs, Short-billed Dowitcher, Solitary Sandpiper, and more have been coming south from their tundra or taiga breeding grounds since the end of their tiny window of breeding opportunity. And they, along with some vagrant relatives from Eurasia, have been the ones stealing the show in all their gray-brown splendor. These charadriiforms are peaking in their migration RIGHT NOW, and places like Chautauqua, the Lake Michigan shoreline, and various spots with the right conditions have certainly seen it, and the invertebrates that live there certainly have too.

But with the coming and going of peak shorebird migration, we first watch the post-breeding dispersal of songbirds and then, eventually, their southbound migration. As August hit its mid-point and began to wane, songbirds really started moving with a purpose, albeit not quite at peak levels...yet. Right now, we are at an epic transition period--in September, we will watch growing numbers of warblers, flycatchers, thrushes, vireos, and sparrows, many have whom have never migrated before, as they use our humble abodes as stopovers. As clocks in their heads tick away, as magnetite particles in their bills vibrate to the earth's magnetic field, as hormones rush in their veins telling them to push on, we will again be witness to one of nature's greatest phenomena.

Gradually, flycatchers, warblers, and various other passerine groups are making a showing in the prairie state, and some places have gotten fairly good species counts. If you're out birding now, you may be seeing Swainson's Thrushes, Veeries, Tennessee, Nashville, Magnolia, Chestnut-sided, Black-throated Green, Black-throated Blue, and Wilson’s Warblers, American Redstarts, and any early migrants from the Emberizidae, Cardinalidae, or Icteridae. Only expect these numbers to increase. And not just numbers of individual birds. Numbers of species too. This number will at least double before the end of September. And we never know what may show up in your neck of the woods...

Speaking now from personal experience, it seems that Spring Migration is much easier to tack a beginning and end onto than Fall Migration. My explanation for this is that in spring, birds are motivated northward by the will to breed, whereas in the fall, birds are motivated by survival. With fairly good consistency (see courtship rituals of almost any large mammal), survival, though important, doesn't motivate the sense of urgency that reproduction does. And we see just that in Spring Migration; birds seem to have a much greater sense of urgency to establish territories and mate, which all species will attempt to do. In Fall Migration, sometimes being early does give an advantage to those flying south to the nonbreeding grounds, but generally a set social hierarchy defines this regardless. Some species aren't even territorial on the breeding grounds. Some shorebirds never even make it to the final nonbreeding ground, and instead just stay over at some productive stopover site. And what about seabirds, who essentially just wander for food until the urgency of finding a territory and a mate returns in the Spring?

Understanding this we get to one of the best parts of "Fall" Migration from a birding perspective: vagrancy. Without this sense of urgency, as well as a huge crop of inexperienced new migrants, we get more individuals wandering brain-exploding-ly far off their typical migratory path, which makes vagrancy more frequent in the Fall. Some other migrants have elliptical migratory paths, where they take one path to the breeding grounds, and another--one closer to us--on the way back south. Overall, this beautiful chaos will take us all the way into winter, when we will arbitrarily decide that migration is over.

The line at the end of Fall migration--found somewhere in Mid-December--is as blurry as the beginning. Winter birding is characterized by many nomadic species, forced to be highly mobile by thin distribution of food in colder months. But this also is a cause of easy confusion, as migration southward turns into just plain wandering. Overall, this enigma that we call "Fall" Migration teaches us one thing (of many): though terms like Spring and Fall Migration are useful, they never really end, so to speak. Migration happens at the level of an individual bird, and that bird can be completely on its own schedule. With billions of their own schedules, of course migration is a little hard to define and categorize. But that's okay. I like it that way, in fact. It makes it exciting, and worth following. Complete order and uniformity is boring, and I'm glad nature knows that.

We'll still try to define and categorize it though. We humans just can't stop. Especially myself. Fall Migration is really an inaccurate term; maybe we should follow the Humphrey-Parkes System for naming molts, and name the migration after where the birds are going. Perhaps we'll call it the Pre-Nonbreeding Season Migration, or Pre-Wintering Migration. They're coming out pretty wordy for now...maybe you all could help me out with that.

Anyway, that's all for now, folks. Godspeed.

1 comment:

  1. Great writing, Nick!!

    Clear, concise explanations.

    Thanks for putting in all the effort to pull this all this information together, for birders in Illinois, in a real time manner.

    Tremendous contribution to the birding community.

    Pat Durkin