Fall migration is an exciting time for birding. With migrants on the move your local birding site can become dynamic overnight, with an abundance of new species to identify. We realize some of our best birding days center on being at the right place at the right time. But these fantastic interactions with bird migration can be reliably predicted with a basic understanding of birds and weather. In this article we’ll discuss the basics of fall bird migration as it relates to weather, timing and locations so you can get on the right track to finding more birds.
The Basics Of Fall Migration
Fall migration starts earlier than most people realize, with many shorebirds on the move by late June, and the first land birds heading south soon thereafter. August through October are peak months, but migration continues into December for some species, especially shorter-distance land birds (e.g., sparrows, blackbirds), raptors, waterfowl, and seabirds. Birds are often on the move every day during this entire period, but there are reasons that some days are better than others for observing them. The volume of migration depends on the weather, with many birds waiting until conditions are favorable before initiating migratory flights.
Birds use varying migratory strategies depending on, among other attributes, mode of flight (e.g., soaring vs. flapping), migration distance, and the species’ natural history. When it comes to migration, we can break birds down into two basic groups: diurnal and nocturnal migrants. Soaring birds that rely on thermals to generate lift migrate during the day (e.g., raptors, American White Pelican), whereas most long-distance migratory land birds (e.g., warblers, sparrows, thrushes) migrate at night — or at least they initiate their flights under the cover of darkness! Relatively few species do both, unless faced with some geographic barrier. For some species, migration is a nightly process, with birds ‘dropping-out’ in the morning to spend the day refueling for the next flight; but for others migration is essentially non-stop once initiated, and certain species travel from the Canadian Maritimes all the way to South America in a single non-stop flight!
Factors That Affect Fall Migration
There are many different factors that affect when birds migrate in the fall, and they use different clues to determine when the time is right to move from their breeding range to their wintering range. Factors that affect birds’ fall migration include:
Light: Light levels and the overall daily angle of the sun are strong clues for the change of seasons. As the days grow shorter and light angles shallower birds know the time is right to begin migration.
Temperature and Climate: Cooler temperatures are another indication of the change of seasons. In some climates, the onset of more rains (the rainy season) is another factor that influences the timing of migration.
Food: As summer crops are consumed and less food is regionally available, birds know it is time to move on to areas that have more resources available. In years of severe drought or when other factors have reduced food sources, birds may migrate earlier than normal.
Offspring: Birds 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.
Location: Where birds are located dramatically impacts when they begin fall migration. 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.
Weather Basics That Effect Migration
During the peak of fall migration, birds are on the move every day, particularly as the days become increasingly short. But the volume of birds is greatly affected by local and regional scale weather patterns. In addition to precipitation, wind direction plays a major role in creating favorable migration conditions. Think about how it feels when you go on a bike ride and the wind is at your back—easy sailing right? But returning against the wind requires a lot more of your energy to cover the same distance, and you get tired quickly. The same is true of migrating birds, and many species await favorable tail winds – tail winds that are not too strong – before undertaking long migratory flights. When weather systems create these winds, birds move en masse.
So much of bird migration is also based on the passage of strong cold fronts, associated with movements of low pressure centers, which in turn produces big flights of migratory birds in fall. The conditions associated with these weather systems often produce what are known as ‘fall-outs’, when thousands of birds concentrate in a relatively small geographic area. In some cases these fall-outs will be local, and in others they may extend across entire regions. But don’t spend so much time watching the Weather Channel trying to predict the movements of these systems that you don’t actually look for birds!
Tracking Your Local Fall Migration
With so many different factors affecting the timing of migration, how is it possible to know when your local migration is underway? As birds travel from breeding ranges to wintering ranges, they can cover great distances in a single day, and birders who aren’t aware of migration patterns can miss spectacular opportunities as the birds move through their region. Fortunately, there are many clues you can use to determine exactly when birds are migrating in your area.
Fall Festivals: Birding festivals in autumn are often planned to coincide with peak migration periods with the greatest diversity of visiting migrants. If you know of local festivals, start watching for migrating birds several weeks before the festival dates to see what new arrivals are in the area as the seasons change.
Backyard Species: Watching your familiar backyard birds carefully year after year can teach you about their seasonal migration habits. Use a calendar or birding journal to note when you last see the birds each autumn, and within a few years, you will be able to reliably predict when their seasonal migration will begin.
Flocks: Many migratory birds, particularly swifts and swallows, will form tremendous flocks in the fall just before leaving on their migration journey. When these flocks begin to form on wires or at popular roosts, migration is about to begin.
Plumage: Many male songbirds, such as American goldfinches, molt into non-breeding plumages that are duller and more camouflaged than their familiar breeding brilliance. Even if those birds do not migrate themselves, their change of attire can indicate the change of seasons and the onset of fall migration. Similarly, watch juvenile birds as they mature, and the more they begin to resemble their parents, the closer fall migration will be.
Genders: In dimorphic species, watch each gender carefully to see when one leaves the other behind. In rufous hummingbirds, for example, mature males migrate a week or two before females, and their disappearance can be a clue that migration has begun.
Make the Most of Migrating Fall Birds
No matter when your fall migration takes place, you need to be ready to take advantage of its great birding opportunities.
Plan your bird-friendly landscaping with migration in mind and opt for flowers that bloom in late summer and early fall to help attract migrating birds.
Leave berries, fruits and seed-bearing flowers intact, rather than dead-heading, late in fall to provide a refueling stop for migrants. These foods will also be welcome for winter visitors.
Avoid pruning trees and shrubs in autumn if possible to provide additional shelter for migrating birds. If the pruning is necessary, add the cuttings to a brush pile for easy shelter.
Attract birds using leaf litter that you leave on your lawn or underneath shrubs to provide a rich foraging area for ground-feeding birds such as sparrows, doves and quail.
Winterize your bird houses in late fall to convert them to roosting boxes for late season migrants and winter residents.
Go birding frequently, particularly in areas that cater to fall birds’ needs for food and shelter, to note any new arrivals and to enjoy the last glimpse of departing summer species. By learning what to look for to time fall migration, it's easy for every birder to enjoy this rich, productive birding season.
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