Cliff Swallows

The full version of this article, written by Pam Sinclair, first appeared in the Yukon Bird Club’s newsletter, “Yukon Warbler,” in the spring 2020 edition, and can be found online at

When travelling the Yukon highways this summer, did you notice a swarm of small birds as you crossed a bridge over a river? These acrobatic birds dart and swoop and disappear under the bridge. They are Cliff Swallows, but instead of a cliff, they have made the bridge their home. Their round mud nests are sheltered on the underside of the structure. Their strategy is safety in numbers and in building an almost predator-proof nest.

The natural nesting habitats of the Cliff Swallow are cliffs, especially rocky cliffs with overhanging ledges that protect the mud nests from rain. This swallow was originally a bird of Western North America, but over the past 150 years, the Cliff Swallow has expanded eastward, across the continent, as increasing numbers of bridges and buildings have provided abundant nesting habitat. Cliff Swallow populations are still increasing in the central and southeastern United States. However, they appear to be declining in much of Canada, including in the Yukon.

Each nest is made of about 1,000 beak-sized pellets of mud and may take two weeks to construct. Each mud pellet is carried, one at a time, to the nest from patches of mud of a suitably sticky consistency. It’s not surprising that many Cliff Swallows choose to repair a nest from last year, rather than to start from scratch. Like most aspects of a Cliff Swallow’s life, collecting mud is a very social activity. Many birds work at the same patch of mud, looking like oversized butterflies as they flutter their wings to avoid sinking in.

There are pros and cons of living with a lot of close neighbours. Being able to cue in to their neighbours’ discoveries of insect swarms is one benefit. Also, in a dense colony, many pairs of eyes can lookout for predators, and colony members warn each other when they notice a Sharp-shinned Hawk, a Merlin or other predator. On the negative side, large colonies can become infested with parasites, such as swallow bugs and bird blow flies, which feed on nestlings. The sooner the birds can raise their young and leave, the better! Here in the North, cold winter weather may knock back numbers of these bugs. Indeed, the birds at a Haines Junction colony reuse old nests more frequently than the well-studied Cliff Swallows in Nebraska, suggesting that nest parasites may be less of an issue here. More study is needed to determine if warmer winters are allowing the bugs to tip the balance against Yukon Cliff Swallows.

There are many threats to Cliff Swallows, including declining insect populations and habitat loss. Also, the availability of flying insects, at crucial times, can be jeopardized by bad weather and storms, which are becoming increasingly frequent and extreme, and the warming climate is causing insects to emerge earlier. Cliff Swallows spend the winter in South America. All year long, they rely on abundant flying insects that provide the energy required for nesting, for migration and for growing a whole new set of feathers each winter.

In the Yukon, Cliff Swallow numbers are going down. In the 1970s and 1980s, huge numbers were reported nesting on some of the larger bridges, such as the one at Johnson’s Crossing; and the Lewes Bridge across the Yukon River, south of Whitehorse. Even at more-northerly locations, such as the culvert at Willow Creek, on the North Klondike Highway, flocks of 300 to 600 were once reported.

Next summer, when you see a swirling flock of Cliff Swallows, take the opportunity to watch them for a while at their nesting colony. Even in mid-summer they may still be adding mud pellets to the tunnel-like entrances of their round mud nests, and catching enough flying insects to raise a brood of youngsters healthy enough for the upcoming 10,000-kilometre journey to South America.

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