The sight of large flocks of blackbirds signals a shift in seasons. The term “blackbird” includes several distinct species, each with unique migratory behaviors that influence their presence and departure times across North America. Understanding these patterns helps to clarify their seasonal movements.
Common Blackbird Species and Their Habits
The term “blackbird” refers to several distinct species. In North America, common species include the Red-winged Blackbird, Common Grackle, and Brown-headed Cowbird. Each exhibits different migratory tendencies.
Red-winged Blackbirds are widespread across North America; northern populations undertake significant migrations, while southern populations are year-round residents. Common Grackles are prevalent, especially east of the Rocky Mountains, and are short-distance migrants. Brown-headed Cowbirds’ northern populations migrate south for winter, while southern populations remain resident.
General Departure Timelines
Departure times for these “blackbird” species vary by geographic location. Northern Red-winged Blackbirds typically begin their southward migration from late August through October. Common Grackles from northern regions generally migrate in late fall, peaking around October or November.
Brown-headed Cowbirds in northern areas may start departing as early as July or August, continuing into the fall. Rusty Blackbirds migrate from northern forests in early to mid-September. These timelines can fluctuate based on environmental conditions, with birds in more northern latitudes tending to depart earlier.
Environmental Cues for Departure
Bird migration is triggered by predictable environmental cues. The most important is decreasing day length, known as photoperiod. As days shorten, this cue signals winter’s approach, inducing hormonal shifts for long journeys.
Falling temperatures also influence migration. A sudden cold snap can prompt birds to move south in search of warmer weather and more abundant food sources. Diminishing food supplies, such as insects and ripe seeds, reinforce the need to migrate. Birds rely on these combined environmental signals to time their departure accurately.
Pre-Migration Behaviors
Before migration, many blackbird species exhibit distinct behaviors. Large flocks, often containing thousands of individuals, form. These mixed-species flocks, which can include Red-winged Blackbirds, Common Grackles, and Brown-headed Cowbirds, gather in communal roosts from late summer through winter.
Birds within these flocks increase their foraging activity to build up fat reserves. This increased feeding, known as hyperphagia, is essential for fueling their long migratory flights. Observers might also notice increased restlessness or agitation, a behavior known as Zugunruhe, indicating physiological preparation for migration. These collective behaviors prepare them for their journey.