Chimney Swifts are small, sooty-gray migratory birds that spend nearly their entire day on the wing. These aerial insectivores rely heavily on vertical structures for resting and reproduction. Historically, they used hollow trees and caves, but today they primarily utilize masonry chimneys for their summer residence, timing their stay in North America with breeding and the availability of flying insects.
The Annual Migration Timeline
The seasonal stay of the Chimney Swift in North America is a well-defined period beginning in spring and ending in the fall. They spend the winter months in the upper Amazon basin of South America, timing their northward journey with the emergence of flying insects, their sole food source. The exact arrival and departure dates depend significantly on latitude. Swifts arrive earliest in the Southeast United States (late March or early April) and later in northern breeding grounds like the Northeast and Midwest (late April or early May).
The swifts’ stay spans approximately six to seven months. Fall departure from the northern parts of their range begins in late August or early September, while those in the Mid-Atlantic and Southwest may linger until mid-October. This synchronized timing ensures they are present during peak insect abundance before making their return journey south.
The Duration of the Nesting Cycle
The most noticeable period of the swifts’ stay is the duration of their nesting cycle within a chimney structure. Chimney Swifts are monogamous for the breeding season and raise only a single brood each summer. The pair builds a nest of twigs, which they glue to the vertical chimney wall, and the construction and egg-laying phase takes about two weeks.
The female lays three to five eggs, and both parents share the responsibility of incubation. The incubation period lasts between 16 and 21 days, averaging around 19 days.
Once the eggs hatch, the nestling period begins. The young swifts will cling to the nest and nearby wall for approximately 14 to 19 days. They then fledge about 28 to 30 days after hatching.
The total time a single pair and their young occupy a chimney, from the start of nest building to the fledglings leaving, is approximately seven to eight weeks. After fledging, the young birds may return to the chimney at night to roost with the parents for a short time. In some cases, a pair may attempt a second nesting if the first one fails early in the season.
Pre-Migration Roosting Behavior
Following the completion of the nesting cycle, swifts transition into communal roosting, which marks the final stage before migration. This behavior begins in late July or early August and involves swifts moving from their solitary nesting chimneys to communal roost sites. These gatherings can be immense, with hundreds or even thousands of birds congregating nightly in large masonry chimneys.
The swifts arrive at the roost site in the evening, flying in a swirling pattern above the chimney. They descend in groups just before or after sunset, dropping into the shaft to spend the night together. This communal roosting primarily acts as a staging behavior for the long migration ahead.
The large groups allow for social cohesion, and the shared space provides a safe, sheltered environment for both the adults and the newly fledged young as they prepare for the journey south. This spectacle is often the most visible evidence of the swifts’ presence to the general public.
The communal roosting period lasts for several weeks, continuing until the final departure of the local population and transient migrants. As the season progresses into early fall, the number of birds at these roosts gradually diminishes. Their final migration south occurs by mid-October at the latest, leaving North America quiet until their return the following spring.