The Hidden Dimensions of Fall Migration

Hooded Merganser - Photo Paul Albertella

We’ve just come through another Fall migration, or so you’ve been told.  Fact is, if you’re reading this in late October, you’ve only come through the first 2 migrations of several.  Bird migration is not a single phenomenon, but includes several different ‘migrations’, each with its own motivations and cast of characters.  When you start to realize this, bird migrations become that much more interesting and precious.

First is shorebird migration.  It started in July, and was largely finished by mid-October.  Shorebirds migrate day and night; they’re trying to hit certain pit-stops that furnish good feeding grounds to help them fuel up for the next stage of their journey.  Most of these pit stops are famous for concentrations of shorebirds – think Copper River delta, Cheyenne Bottoms, or Chesapeake Bay.  Here in Ohio, the Western Basin of Lake Erie is our best shorebird pit stop.  No place in central Ohio yet offers reliable enough low-water levels for feeding to attract regular shorebird migrant flocks, but we always get a few hungry travelers at our reservoirs and wetlands.

After that comes the classic migration of Neotropical landbirds, mostly from late August to early October.  These are the warblers, tanagers, flycatchers, and orioles so beloved in the Spring, but not quite so pretty in the Fall, when the birds are in their duller basic plumage.  These are mostly insect-eating forest birds, and they’ve evolved to leave before insects get scarce.  They’re heading for tropical forests, and they migrate at night using complex star navigation, so when we see them in the morning, they’re feeding to ‘refill the tank’ after an evening of long hard flying.  After their morning feed, they hide and rest until the evening, when they become active again in preparation for their next evening flight.

Then, in mid-October, there’s a subtle shift to temperate migrants.  These are mostly fruit and seed eaters, like jays, robins, waxwings, and blackbirds, and they’re only heading to the southern U.S..  They migrate in the morning and evenings, often in long straggling flocks, and will sometimes roost in big flocks.  If you sit at particular spots, like Maumee Bay in northern Ohio, you can actually see the migration as the flocks stream by around that corner of Lake Erie.  Unlike nocturnal migrants, these morning migrants are averse to flying over open stretches of water.   There are still a few nocturnal migrants among these birds — think of kinglets and sparrows – but they are often active for longer periods during the day after a migration.  Perhaps they don’t fly quite so far in each movement.

Waterfowl and Waterbird migration follows this, usually starting in mid-October and running almost until mid-December.  This includes waterfowl and gulls, birds whose schedule depends more on water temperature and food availability than the yearly clock.  A few waterfowl, like Shovelers, Teal, and Franklin’s Gulls, migrate early, but most wait until the cold and declining food push them south.  For some waterfowl, these stimuli may not come until late in winter, if at all.  Goldeneye ducks and scoters feed on lake and river mussels, and can feed as long as they have open water.  We may not see them until hard freezes in January.

Finally, there are the winter irruptives.  These are the land bird counterparts to waterfowl migration, birds that only head south when increasing cold and decreasing food force them to do so.  Think of winter finches, Rough-legged hawks, or Snowy Owls.  It usually takes severe weather or food shortages to move them, so some years we never see them here in central Ohio.  Sometimes we see unusual species follow this pattern; for instance, an extended hard freeze often drives large numbers of Flickers into central Ohio, as they search for areas where they can dig in less frozen ground for winter insects.  In the same way, a bad oak acorn year can drive lots of Blue Jays and Red-headed Woodpeckers into central Ohio from wintering areas further north.

These are not all-inclusive categories; some birds defy easy placement in this scheme.  Thrushes can be both neotropical-type and temperate-type migrants, if they switch from bugs to fruit.  Waxwings can waver between temperate and irruptive.  Phoebes are flycatchers that have become temperate migrants.   Gnatcatchers seem to break all the rules, being diurnal temperate migrants during  August and September.  And don’t even get us started on swallows and swifts!  So the next time someone asks about Fall migration, you might want to ask them which birds they’re thinking about.