For most birds, autumn is a time of migration. As is the case in spring, not all species engage in their bouts of long distance travel at the same time; some are known for heading out early while others linger in the region for several additional months before starting their journey.
Among the birds that are quick to depart the North Country are the warblers, a large group of small, delicate creatures that abound in the vast expanses of forests when daylight is at a maximum and bugs are at their peak.
Even though periodic stretches of pleasant, summer-like weather continue to occur the warblers will have mostly vacated the Adirondacks by the middle of October. Since these songsters are heavily dependent on insects and other forest-dwelling invertebrates for food, their departure precedes the decline in insect activity during the final days of summer. In order to survive winter’s cold, all lower forms of life must seek out a sheltered location and enter into some dormant stage of their life cycle. By the time the equinox arrives, few bugs are available on which warblers, vireos, swallows and other insectivores can feast. Those feathered bug eaters that remain have adapted their diet to target the various types of insects that continue to remain active into the first part of fall.
Like all migrants, warblers ingest large quantities of food in the weeks prior to their departure. This allows these birds to develop enough temporary fat to fuel their flight. Even though birds do not appear to increase their girth with the approach of autumn, all birds must develop substantial deposits of fat in summer either to provide the energy for a sustained period of travel, or to help insulate them against the cold.
While nearly all birds are primarily diurnal, most migratory species are known to travel at night. This allows individuals to gorge themselves during the day they depart, so their digestive system and blood stream contain a high concentration of nutrients to fuel the initial portion of their journey. Flying all night and landing at dawn also provides a migrant with the ability to begin feeding immediately after it lands. Because the air is less turbulent, especially within a few hundred feet of the ground following sunset, a night flight places far less stress on these small birds than traveling through the more agitated air currents that typically develop during the day.
When journeying southward, warblers, along with most other smaller birds, prefer to fly only a short distance above surface obstacles. Maintaining a low altitude throughout the course of their travel reduces the impact of the higher speed winds that ordinarily occur at greater heights. Following the passage of a strong cold front, such as the one that swept through the State this past Saturday afternoon, the wind speed only a thousand feet above the ground can reach 30 mph or more. The westward component of these more powerful higher elevation winds can easily push small birds well to the east and away from their targeted area. Even though the wind from a mass of cool, Canadian air has a favorable tail-wind direction, it appears that small birds prefer to remain close to the tops of trees and out of the influence of these stronger winds rather than incur the push southwards while having to struggle to maintain a specific course.
The flyways used by our various species of warblers are gradually becoming known to ornithological researchers, and there is evidence that suggests that each type of warbler has its own preferred corridors of travel. The notion that birds simply begin to fly in a southerly direction for a long period of time without regard to their location is incorrect. Warblers, flying in the dark, are able to navigate their way from their breeding grounds in northern latitudes to specific resting spots along the route and then eventually to the tropics, particularly the Amazon. Several sensory adaptations, such as their ability to detect small features of the Earth’s magnetic field, enable these birds to travel thousands of miles with uncanny accuracy.
Over the past several decades, the Earth’s magnetic field has changed its position measurably. Additionally, each year more obstacles are being placed above the tree tops, such as wind mills, and high voltage power lines that are nearly impossible for these birds to detect when flying in near total darkness. Yet, despite these adversities, the warblers always seem to arrive at their wintering grounds, and then return during the latter part of spring here in the Adirondacks.
Photo: Male Yellow Warbler, courtesy Wikimedia user Mdf.
A version of this story was first published in September, 2012. Read more about Adirondack Birding here.