We are all accustomed to misleading or even paradoxical phrases in our conversations that we often use without thinking. Shouldn’t a “near miss” be a glancing collision? Have you ever gotten a “free gift”? Biology is not immune from such phrases and “fall bird migration” is at the top of my list.
Even though none of use wants to admit that the days are growing shorter, we are still in summer. Nonetheless, migration of birds is well underway. The southward movement will continue through autumn and into early winter. “Fall migration” is misleading; I prefer “post-breeding migration”. The majority of our swallows depart by the end of August and therefore never spend an autumn day in Maine.
Shorebirds (particularly sandpipers and plovers) are now migrating through Maine from their more northerly breeding grounds en route to more southerly wintering grounds. Shorebirds are some of our earliest and most remarkable migrants.
The majority of shorebirds that pass through Maine during migration breed at high latitudes on the Arctic tundra. For about six weeks (June to the middle of July), the arctic tundra is great habitat for sandpipers and plovers. Insect life is abundant and the sun never sets.
We describe the reproduction of shorebirds as precocial. The chicks hatch fully feathered, a striking contrast to the altricial development of most songbirds whose young hatch as naked, blind chicks. Within hours of hatching, young shorebirds are walking around, catching their first insect meals. The shorebird parents (sometimes only the mother in some species like the Buff-breasted Sandpiper and sometimes only the father in Red-necked Phalaropes and Red Phalaropes) guard the young but do not typically feed the young. The young do have cryptic coloration, blending into the tundra to avoid the eyes of predators like Arctic foxes and Snowy Owls.
The short breeding season in the arctic means that parents only have time for one brood per season. To increase the chances of surviving another year and reproducing again, sandpipers are under pressure to begin their southbound migration as soon as possible.
Because the nestlings can fend for themselves, the parents usually migrate from the breeding grounds before their young have even learned to fly! As a result, southbound shorebird migration occurs in two pulses. Along the Maine coast, the first adult shorebird migrants begin showing up by the middle of July. The first juveniles show up later; juvenile sandpipers and plovers will arrive mostly in August and September. In some species like the Semipalmated Sandpiper, the adult females leave the breeding grounds before the adult males producing a three-step migratory wave along the migration route: adult females, adult males and then juveniles.
After the parents have departed, the juvenile shorebirds learn to fly on their own and depart for wintering grounds they have never seen. With no adults to guide them, the juveniles must have the migratory route somehow encoded in their brains. We know that juvenile navigation is not as accurate as that of adults. During the southbound migration, shorebirds that occur as rarities are usually juveniles.
By ageing shorebirds, birders can increase their enjoyment and appreciation of the southbound migration. How does one go about determining if a shorebird is a juvenile or adult? Some adult migrants still have some of their breeding plumage. So a Black-bellied Plover or Dunlin with a black belly during southbound migration has to be an adult. However, adults often molt into their winter plumage along the migratory route and may resemble juveniles. A look at the wear of the feathers can usually allow one to distinguish such adults from juveniles. As a general rule, juveniles have brighter and crisper plumages. The feathers of the juvenile are only a few weeks old and the adult feathers are a couple of months older and hence more worn.
[First published on August 5, 2012]