Tips for Atlassing for Chimney Swifts

Winnifred Wake, Nature London Chimney Swift Liaison

April 30, 2024

At a Glance: Atlassing for Chimney Swifts 

  • Find swifts as they fly overhead to establish general presence in an area.
  • Identify likely locations (close by or within a few km) for potential nesting (usually chimneys). 
  • Verify swift use by watching for entries at dusk, and/or observe potential sites during the day to establish whether swifts are nesting or are using the chimney only for overnight roosting.
  • There is only one swift nest active at a breeding site at a time.
  • After finding a likely nesting site, observe in the daytime and apply breeding codes.

Arrival on breeding grounds and migration timing

Chimney Swifts begin returning to southwestern Ontario in late April (and later farther north). Some early arriving swifts start nesting in early-to-mid-May, while others delay until mid-June when migration is largely over. Breeding sites were historically located in tree cavities, hollow trees, and caves, but are now primarily located in uncapped and unlined stone or brick chimneys, as well as other human-made structures like barns, silos, pipes, and old buildings.  Swifts take approximately two weeks for nest construction and egg laying, about three weeks for incubation and another three weeks until fledging. In southwestern Ontario, fledging often occurs in July, but late nests may not fledge until August, and nests in central Ontario are often slightly later. 

Large numbers of non-breeding swifts may spend their nights in communal roosts during spring and fall migration, although some non-breeding or failed-breeding individuals will continue to roost communally throughout the breeding season. Especially during May and June, courtship activity and identifiable pairs can be seen among groups of socializing swifts around communal roosts, active nest chimneys, other chimneys and elsewhere. As swifts gather at a roost site at dusk, they often spend many minutes chattering and repeatedly circling above the chimney before descending into the roost. 

A communal roost is usually easier to find than a chimney occupied only by a nesting pair, but nesting pairs are sometimes resident at the same time as an overnight roost. Sites used for breeding have only one active nest at a time.

Nesting behaviour

Most Chimney Swifts nest in chimneys but they also regularly use barns or other abandoned or little-used structures, so don’t discount these as possible nest sites. Because most swift nests are placed inside chimneys or other human-made structures, observations of swift behaviour around probable breeding sites are often used as indicators of nesting. In the spring, when swifts first return, they may be quite visible, chasing and chattering, coming and going from the chimney. During incubation, they become secretive, seemingly absenting themselves from the general area, though they may be seen and heard high overhead, sometimes in the company of other swifts. About once an hour, a swift may appear out of nowhere and drop silently into the chimney. The arriving bird replaces the incubating bird, which emerges within a few minutes.

After eggs hatch, the rate of these visitations may increase as parents and additional non-breeding helpers feed the young throughout the day, although timings of feeding intervals can vary throughout the rearing process. Such visitations can happen within the blink of an eye, so patience and concentration are needed. 

Breeding evidence

Swifts may forage and socialize near their nest site, or up to a few kilometres away. In order to be confident a nest is present, the goal should be to see a daytime visitation at a probable breeding site (often indicators of incubation exchanges or food deliveries). Even better are multiple entry/exit observations during a single day, to rule out swift pairs inhabiting a site but not engaged in an actual nesting attempt.  However, if you observe swifts entering the probable breeding site with nesting material (i.e., twigs) or carrying food during the safe dates, this qualifies as breeding evidence (breeding evidence code: NB – nest building, CF – carrying food). Note: Swifts don’t normally carry food in their bills, but hold foraged material in their gular pouch. Close observation is needed to discern the distended pouch. Nest building often occurs in the afternoon and early-evening.

The Chimney Swift is a Threatened species, both federally and provincially. During the nesting season, avoid disturbances such as flying a drone over a chimney, or peering up or down the shaft. 

Identifying areas where swifts have been reported or are most likely to be found

  • Check eBird and NatureCounts records for the Chimney Swift observations in your area.
  • Review maps and satellite images for potential nesting and foraging habitat. 
  • Contact birders, nature clubs, and others who might have local knowledge of swift whereabouts in a particular atlas square.
  • Cities, towns, and villages are usually the best places to start.

Cities, towns, and villages

In the oldest parts of urban centres, look for chimneys on <1960s buildings, including schools, churches, institutions, houses, apartments, and other large buildings. Back alleys may reveal chimneys not visible from the street.

  • From late May to July, walk, bicycle or drive slowly through older business districts, and older residential areas that have mature trees.
  • Swifts often fly above treetops and buildings (>20 m up). Learn to recognize swift vocalizations:
  • If swifts are seen in areas without suitable chimneys, look for chimneys up to several kilometres away.
  • Detection of swifts in an area is a first step; finding an actual nest site is more challenging!

Rural and agricultural landscapes 

  • Swifts are unlikely to be present in vast agricultural landscapes, so look for areas with less-intensive farming.
  • Watch around farmsteads especially those that have older houses, wooden barns, silos, and other suitable structures.
  • Watch for swifts feeding over natural habitat or other insect-rich human-made water bodies such as sewage lagoons, drainage ditches, farm ponds, and water-filled gravel pits. 
  • If swifts are found, look within a few kilometres for possible nesting habitat (e.g., chimneys, old barns/sheds, hollow trees, silos, old cottages, old wells, houses, other buildings). 

Northern and remote habitats

  • If swifts are seen in a remote forest, wetland or cottage country, check big trees/snags (>30 cm inside diameter).
  • A trunk may be open topped or not; swifts can enter through openings much smaller than 30 cm diameter; these could be at the top, on the side of a trunk, or in a large branch.
  • Watch for swifts around abandoned mine shafts, mining infrastructure, old well shafts, old shacks and other structures, and in unexpected places (e.g., caves, cracks in cliffs, and outcrops).

Characteristics of chimneys occupied by swifts

Chimney may be made of stone, brick, concrete, stucco,but not metal)

  • May be any shape as long as a swift has room to enter; usually most ‘suitable’ chimneys were built before the
  • Top of chimney must be open; may have one or more open, protruding “tile liners.”

Exterior dimensions of nesting chimneys

Nesting chimneys are most often found on houses. Suitable chimneys are usually >2.5 x 2.5 bricks wide along a horizontal course as viewed from the outside (equates to minimum interior dimensions of 25 to 30 cm per side), but

  • If a chimney is wider on one side it may be narrower on the other (e.g., 2 bricks x >3 bricks).
  • Swifts may enter a much smaller top opening if the shaft broadens out 1 or 2 metres down.
  • In a two-shafted chimney, swifts occupy only one shaft (okay for other to be capped).
  • Very occasionally swifts occupy very small-diameter chimneys (2 x 2 bricks).

Larger chimneys (e.g., >4 x 4 bricks can be used for nesting and/or communal roosting (at same or different times).

In choosing nesting sites, swifts sometimes think outside the box; be sure you do too; some chimneys may appear closed at first glance, but actually have enough of a small opening to allow swift entry.

Characteristics of other structures occupied by swifts

Older wooden barns, sheds, outbuildings, silo, cottages can host swifts if there is an opening to allow entry (e.g., broken or open window or door, gap in boards), and textured interiors to allow for clinging vertically to the wall. Watch for swifts in the area and/or entering the building. Inside, use a flashlight/binoculars to check high, dimly lit areas for a nest glued to the wall or young clinging to the wall (e.g., upper gable peak, vertical wall, on or beneath beams). 

Distinguishing between a chimney used for nesting and one used only by non-breeders

Communal roosts usually occupy large-diameter chimneys, but nests may be in either small or large chimneys. A nesting pair may occupy a chimney at the same time as a roost of non-breeders. It is relatively easy to detect roost chimneys, as swifts arriving for the night are vocal and visible. This makes roosts a good starting point in the search for nesting activity.

Tips for evening viewing (to verify roosting activity and to possibly detect indicators of nesting)

  • For a full picture, observe for an hour starting 30 mins before sunset: watch for entries/exits.
  • If 1 or 2 (up to 3 or 4) swifts enter for night, check during daytime to verify nesting.
  • If a big roost, look for entries/exits by 1 or 2 swifts well before non-breeders arrive.
  • If early entries are detected, check during daytime to verify nesting activity.

Tips for daytime viewing (to verify nesting activity)

  • Watch carefully for up to one hour or more, eyes fixed on the chimney rim.
  • Watch for quick, silent, often paired, entry-then-exit; adults don’t linger in the area.
  • Rarely, non-breeders roost in chimneys during the day; may be mistaken for nesting swifts. 
    • To determine if birds are non-breeders, watch for unusual numbers or patterns of entering or exiting swifts. 
  • Swifts with very poor flying skills may indicate newly fledged young (mid-July or later). They may be flying unusually slowly, flapping harder, staying mostly on one plane, executing only very broad turns, practising clumsy daytime entries, etc.
  • Most productive daytime viewing to detect nesting activity is late June to late July, but can be detected until mid-August.

NOTE: Most chimneys used by swifts are located on private property; either view from a public space or ask for permission.

  • Avoid directing binoculars at buildings with people inside. Generally, swift entries and exits can be observed by the naked eye. 
  • Always carry information about the Atlas and swifts and present it to landowners whenever appropriate.
  • If a building owner/occupant learns of swifts using their chimney, take time to inform them about swifts and the purpose of the Atlas, try to shape any conversations positively. Unfortunately, some owners block access to chimneys after learning swifts use them so positive conversations and education is important.

Identification and conservation of Chimney Swift habitat

Chimney Swifts are protected by Ontario’s Endangered Species Act and Canada’s Species at Risk Act. Chimney Swift nest and roost sites may be protected under one or both of these acts, particularly if it meets the threshold as critical habitat as set out in the Recovery Strategy for the Chimney Swift (Chaetura pelagica) in Canada. That is, 1) confirmed breeding records and 2) evidence of at least one swift entering a suitable roost or nest site on at least two different days.

Since many sites may be proposed to be developed, or in construction during the breeding season, it’s useful for Ontario SwiftWatch to know in real-time if there’s a concern for imminent demolition. If you have concerns for a site being used by Chimney Swifts, please reach out to Ontario SwiftWatch by visiting: and emailing


Significant contributions to this article were made by Emily Rondel, Natasha Barlow, and the Atlas-3 Significant Species Committee.

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