Factors Affecting Migration
A number of factors determine when birds migrate from their breeding range to their wintering range in the fall. These include:
Light: Light levels and the overall daily angle of the sun indicate a change in season. As the days grow shorter and light angles become shallower, birds know the time is right to begin migration. Temperature and climate: Cooler temperatures are another sign of the change of seasons. In some climates, the onset of 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. As the baby birds mature, the time for fall migration is at hand. Location: The location of birds will dramatically impact 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.
Local Tracking
As birds travel from breeding ranges to wintering ranges, they can cover great distances in a single day. Birders who aren’t aware of migration patterns can miss spectacular opportunities as birds move through their region. Fortunately, there are many ways you can know 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 a few weeks before the festival dates to see what new arrivals are in the area as the seasons change. Hunting season: Game bird hunting is often timed during peak migration. Being familiar with local hunting seasons for ducks, geese, quail, doves, and other game birds can help birders recognize the most productive birding times. 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 certain bird species 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. Their disappearance can be a clue that migration has begun.
Birding Opportunities
No matter when your fall migration takes place, 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 and nourish migrating birds. Leave berries, fruits, and seed-bearing flowers intact, rather than dead-heading the plants, late in the fall to provide a refueling stop for migrants. These foods will also be welcome for winter visitors. Avoid pruning trees and shrubs in autumn 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, thrushes, and quail. Winterize your birdhouses in late fall by converting 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.
