Heavy gray clouds settled in for the afternoon, giving off the aura of November. During my drive home from work, though, a break had opened to the west, and focused beams of gold snuck under the clouds. The last, clinging, yellow leaves of aspen and maple, and the rich red-browns of the oaks, lit up like they were on fire. A stiff breeze whisked their fallen comrades into whirlwinds of color. Leaves stirred up from the ground met leaves just now fleeing the leaden sky.
From that whirling chaos of leaves and light and wind also emerged swirling flocks of small, gray birds, with white belly and tail feathers flashing. Snowbirds, some folks call them, because their plumage imitates the winter color scheme of dark skies above and white snow below. Here, in their overwintering habitat of the lower 48, they also seem to bring the snow with them as they move south each fall.
Dark-eyed juncos breed across Canada, with some subspecies staying year-round in the Appalachian Mountains, the West, and the Northeast. The northern forests of Wisconsin and Minnesota are at the southern edge of their mid-continent breeding habitat, and we sometimes see a few juncos through the summer. You can identify them by the flash of white on their outer tail feathers as they fly away. The real influx comes when the leaves begin to fall, as juncos head south to their winter range.
This year at Hawk Ridge Bird Observatory in Duluth, MN, their migration began on September 10, with a single junco, and peaked in early October with counts of 1593 on October 7, and 1224 on October 13. While I don’t count the dancing flocks that rise from the roadsides and fields, I, too, have noticed the wave of junco migration.
Females tend to migrate the farthest south, and comprise 70% of winter flocks in the southern United States. Males will risk harsher weather farther north in order to get a jump start on spring migration and arrive first at prime breeding territories.
Their impressive and easily observed movements have taught us much about migration in general. Prior to 1924, the best hypotheses about how birds know when to migrate and breed centered on them responding to changes in temperature and barometric pressure.
William Rowan, who founded the Department of Zoology at the University of Alberta, suspected that day length was more important. Without the support of the University president, Rowan conducted an experiment to confirm his theory – in secret. With two outdoor aviaries full of juncos – one with lights and one without – Rowan spent a very cold winter in Alberta simulating the increasing daylight of spring.
After three months of increasing “day length” by five minutes per day, Rowan’s juncos began to sing. It seems obvious now, as I gather eggs by a lamp’s artificial sunlight in the chicken coop, but Rowan’s experimental confirmation of “photoperiodism” was a major advance for science.
Being easily caught, able to thrive in captivity, and adaptable to new habitats, juncos make excellent experimental subjects. Rowan’s was just the first of many junco-based experiments. Currently, scientists are comparing the various subspecies of juncos that live across North America to study evolution and speciation. One population of juncos on the campus of UC-San Diego seems to have evolved new behavioral and physical traits in just 30 years.
Juncos make wonderful winter friends for citizen scientists, too. The Cornell Lab of Ornithology’s Project Feeder Watch notes that “juncos are sighted at more feeding areas across North America than any other bird. Over 80% of those responding report juncos at their feeders.”
This abundance is likely due to the fact that juncos are primarily seed-eaters who forage in flocks. Under the feeders, along roadsides, and in open areas, juncos will forage by hopping, scratching, and pecking at the leaf litter, and flying up to glean food from low twigs and grasses. These sparrow-sized birds will even land on the top of a grass stem and use their 25 gram bodyweight (the equivalent of 25 paperclips) to “ride” it to the ground. From there, they can stand on the seed head and feed more easily.
According to Don and Lillian Stokes of the Stokes Field Guide to Birds, watching the social hierarchy at work in a junco flock can be quite entertaining. The same birds tend to return each year, and the earlier arrivals tend to rank higher in the flock. When asserting dominance, a junco will face the offending subordinate bird and fan his tail to reveal the white outer tail feathers. Sometimes a chase, dance or, pecking fight will ensue.
Juncos were already scratching under my feeder when I stepped outside the next morning. The gray skies had lifted, but in their place was a light dusting of white flakes on the car. The snowbirds are back.