Fall migration has special challenges in identifying birds
As well-known birder Coen Dexter of Nucla noted in a recent email, fall birding is a real challenge for several reasons.
“No. 1 is all those juveniles which look, for the most part, like females,” he wrote. “Many of the juveniles look strange because feather growth many not be complete, bill size, flight patterns, etc.”
He said adult males, if they have not molted, are quite easy to identify because they “look just like spring males but the plumage is worn.”
Plus, early migrants do not travel symmetrically, often through and gone by mid-August, and tend to be males.
Females, Dexter noted, may look about the same at all times of the year.
Melissa Mayntz, on Birding.com, listed some of the factors affecting fall migration.
Light: As in the spring, changing light levels spur different behavior in all animals and shorter days tell birds it’s time to head south.
Temperature: A similar thing happens when temperatures turn cool, indicating a change of season.
Plus, Mayntz writes, “in some climates, the onset of more rains is another factor” influencing the timing of migration.
Food: As food supplies grow short, birds know it’s time to move to areas of more abundance. Mayntz said years of severe drought or other factors that have reduced food sources may cause birds to migrate earlier than normal.
Offspring: Family first. Birds, according to Mayntz and other avian specialists, will not migrate until their offspring are mature enough to care for themselves or to begin their first migration journey.
“Birds that breed later in summer will also typically migrate later, but as the baby birds mature, the time for fall migration is at hand,” she writes.
Location: A bird’s location can “dramatically” affect its fall migration timings, Mayntz wrote.
“Shorebirds that nest in the Arctic may begin their autumn journey as early as July, while passerines in areas closer to the equator may not start migrating until late September.”