Cliff Swallows: A Social Call

Swooping, soaring and flying in beautiful patterns in the sky. These social birds capture the attention and interest of many by swarming around bridges, overpasses and fields in the summer. Yet, rarely are Cliff Swallows seen without the company of another. Feeding off of each others’ energy, I don’t think this bird quite knows the definition of “alone time”.

Description + Identification

Scientific Name:
Petreochelidon pyrrhonota

Order:
Passeriformes

Family:
Hirundinidae

Adult Cliff Swallow in flight by Bryan Calk
Adult Cliff Swallow in flight by Matt Davis
Juvenile Cliff Swallow by Mary Rumple
Adult Cliff Swallow by Caleb Putnam
Under ideal lighting conditions, adults have dark-blue metallic backs and pale brown rumps. Their head is rufous with a white forehead.

Weighing in at 19-34 grams, the Cliff Swallow is a small bird with both adult females and males measuring 13 cm in length and flying through the sky with a wingspan of 28-30 centimeters – comparable to the size of a sparrow (All About Birds). As with most birds, the light conditions in which you see it will affect the colours and patterns the human eye can identify. Under poor light, Cliff Swallows appear brown with dark-coloured throats and a white underbelly. When in good light, the observer can see that their backs are metallic, dark-blue with pale brown-orange rumps. Their faces are a rusty red with a bright white patch above their bill. There are no distinctively different identifying features between the female and male Cliff Swallow. However, the juveniles appear as messy adults with pale, rusty red faces and a dingy-looking breast.

The best time to find this bird is in the summer when they are swarming to hunt insects. They will likely be foraging in flocks close to fresh water, such as lakes, rivers, or wetlands. Key features which differentiate the Cliff Swallow from other common swallows in North America are their square tail and the pale brown-orange rump on their dark upper body (Sibley, 2016). They also tend to forage higher than other species, so turn your gaze high to the sky.

Vocalizations

Cliff Swallows have a limited vocal repertoire with five distinctive vocalizations for signalling actions such as feeding, raising the alarm and mating (Loesche, Beecher & Stoddard, 1992). Songs are reserved for activities related to breeding like courting and nesting. The Cliff Swallow sings a combination of squeaks and gutteral grating sounds in six second intervals. The most common Cliff Swallow call sounds like a soft chur. Calls during foraging sound like a squeak, while calls alarming the rest colony about danger sound like a purr.

Habitat + Distribution

Like its namesake, the Cliff Swallow habitat orginates in the western mountains nesting on cliff sides and rock ledges in large colonies. Presently, however, their nests are more likely to be found on human-made structures such as bridges and house eaves (Blair, 1996). Cliff Swallows live in grasslands, towns, and river edges while avoiding areas of heavy forest and deserts (All About Birds). It’s likely to find most nesting colonies close to a water source and open fields which are optimal for foraging (All About Birds).

As long-distance migrants, Cliff Swallows spend months migrating through Mexico and Central America before reaching their overwintering grounds in South America (Audubon). These birds migrate during the day at a leisurely pace, taking time to forage insects along the way (All About Birds). The map on the left illustrates the movement of Cliff Swallows (Migratory birds, Government of Canada).

Nesting

Although Cliff Swallows can nest in solitary, they are far more inclined to nest in colonies. The farther west in the Cliff Swallows’ breeding distribution, the larger the colony can be. Colonies can reach up to 3,700 nests in one distinctive habitat (All About Birds)! You can spot nests on a variety of structures such as cliffsides, building eaves, bridges, and large trees. Each nest is built between a vertical wall and a horizontal overhang. Cliff Swallows gather mud in their bills and bring these mud pellets back to mold them in place. Each gourd-shaped nest contains upwards of 9,000-12,000 mud pellets costing an abundance of energy and time to build this nest (Gauthier & Thomas, 1993)! Even though both the female and male nesting pair contribute to making the nest, that is still 4,500-6,000 trips back to the mud source.

An unlikely partnership which aids the Cliff Swallows in building their nests.

Behaviour

As aforementioned, the Cliff Swallows are an extremely social bird. They feed, bathe, preen, nest, and migrate in groups – talk about a commune! Sightings of Cliff Swallows are common as they forage for a variety of swarming insects during the day. On a clear, warm spring or summer day, they can be found over grassy meadows and other open areas feeding in groups from 2 to over 1,000 birds. Cliff Swallows take advantage of thermal air currents which bring together the swarmming insects causing less energy expenditures during foraging. If out birding on a rainy or cool day, it is more likely to find the Cliff Swallows foraging over fresh water (All About Birds).

These social birds turn to their neighbours in the colony if they find themselves unsuccessful during a hunting trip. Individuals who have returned to the colony unable to provide for their family, or hungry themselves wait in their nests until they notice another come back with food. Often, the successful individual will alert the rest of the colony by giving a specific call. The unsuccessful individuals can then follow the successful neighbour ensuring they are all well fed (Brown & Brown, 1997). It is truly a community effort to ensure the benefit of all.

Cliff Swallows foraging over a river in Bakersfield, California

Conservation Status

The Cliff Swallow is showing widespread decline of population in Canada (Migratory birds, Government of Canada). During the 1980s, there has been an overall decrease in species known as avian aerial insectivores (i.e. swallows, swifts, nightjars, and flycatchers) across North America (Smith et al., 2015). While the causes of the decline in these species remains uncertain there have been changes in swarming insect populations resulting from the use of insecticides (Hallmann et al., 2014). Another factor which contributes to the population decline is the invasive species of House Sparrow which steal Cliff Sparrow nests, contributing to their overall breeding success (Leasure, Kannan & James, 2010). Even though the Cliff Swallow population is declining in Canada, the conservation concern remains low as their abundance in other parts of the continent balance out these numbers (All About Birds).

Cliff Swallows are rated as Least Concern by the International Union for Conservation of Nature (IUCN) Red List (Bird Life International)

Human Impact on Cliff Swallows

As previously discussed, the urbanization of once natural landscapes have altered the historical habitat of Cliff Swallows. This swallow population has been observed to take advantage of human constructed structures such as highway overpasses, bridges and buildings for nesting areas. What repercussions do Cliff Swallows face when having to adapt away from their standard and natural nesting areas?

A 2021 article by Lauryn Benedict et al. published in The Journal of Ornithology studied the affect urbanization had on the presence of an ectoparasite, commonly known as the “swallow bug” (Oeciacus vicarius), found in the nests of Cliff Swallows. Swallow bugs severely impact Cliff Swallows as they are associated with nesting failures and shortening lifespans by making the swallows more susceptible to viruses.They hypothesized that swallow bug populations would increase in highly urban areas because of the elevated presence of the invasive House Sparrow in cities. House Sparrows are another host to swallow bugs. They take over Cliff Swallow nests throughout the year, especially when the swallows migrate south.

As seen on this graph, Benedict et al. predicted correctly in that swallow bug presence would be higher among urban settings. These results suggest that Cliff Swallows residing in urban colonies experience negative impacts from the boosted swallow bug population.

With higher parasite numbers in urban areas, why would the Cliff Swallows continue choosing to habitate in human-altered areas? While living among a parasite infestation might seem unlivable for you and I, for the Cliff Swallows, this cost does not seem to outweigh the benefits of forming large colonies at urban sites. As the researchers suggest, benefits might include street lights attracting insects for easier foraging, provide protection from predators and concrete providing ideal adhesion for mud-nests. Further research into these, among many other factors, are suggested by Benedict et al. to help explain why Cliff Swallows continue to nest in large and highly parasitized colonies.

Cited Literature

11 thoughts on “Cliff Swallows: A Social Call

  1. Guy real grating indeed Bree! Thanks for including that sound snippet. These were very clear descriptions of the bird that were very vivid. Not only did I learn about the Cliff Swallow but I learned that I had an assumptions I hadn’t even fully expressed about birds. I guess I had always through the male and female birds were dichotomous. I will keep a more keen eye now! The parasites in these populations. I’d like to read more about cases of bird populations that have migrated because of parasite presence. This seems like it would be potentially problematic for the mating cycle and migratory patterns and I am curious about how birds and other species have adapted to these changes when they are ousted due to environmental threats. Lots to work with. Thanks!

    1. Thanks for reading and for your insights to the post, Lola! The implication of ectoparasite spread is definitely huge, especially in relation to their migration and even in day-to-day travels. It’s likely that the swallow bug is being spread into new ecosystems too! I’m also hoping to read more about natural factors such as these along with more about human impact.

  2. Wow, thank you so much for the detailed description on how to identy differences from other birds! Beautifully written description of a miraculous bird. Their migration distance blew my mind, what a journey for such incredible creatures. Well done.

    1. Thank you for reading, Tim! Something that stuck me about their migration pattern was how they feed and migrate during the day. Most birds will feed during the day and migrate overnight, but it seems like these social birds like to do it all in the sunshine.

  3. Really wish I read this before doing quiz 4, you did a great job describing what makes cliff swallows unique which will definitely help me not get them mixed up with barn swallows in the future. Do you know of any hotspots on the island? I’d love to see some in person.

    1. Thanks for reading, Codey! I’m glad you’ll never forget these key identifyers now! According to eBird, there are quite a few hot spots here in Nanaimo like at Buttertubs Marsh and Westwood lake. It’s more likely you’ll spot them in the spring and summer though as they are in the midst of southern migration. It would be awesome to observe a nesting colony though! For your interest, here is the eBird link that you can peruse: https://ebird.org/map/cliswa?env.minX=-178.203369424671&env.minY=-54.969919623211&env.maxX=177.529483086659&env.maxY=71.9081724700314

  4. Hi Bree! Such a good read! I know you said their decline was a bit mysterious, and curiously that their population in Canada is specifically declining- can you make any guesses as to why the decline in Canada and not other countries? I find that really interesting! And do they often favour urban areas at both the overwintering and breeding sites?

    1. Thanks for reading, Chloe! Your questions are absolutely thought-provoking. The overall population of Cliff Swallows declining in Canada and not as much in other countries is definitely quite puzzling. Other than the suggestions I’ve included in the blog, I found that a recent study in the Maritimes suggests that the breeding success of Cliff Swallow was not related to insect abundance. It should be noted though that other factors, such as parental compensation or the fact that fewer individuals require fewer prey items to survive, may be obscuring any effect on nesting survival. The paper can be found here (https://www.ace-eco.org/vol12/iss2/art19/).

      From my research, it seems that there are certain populations that do prefer their nesting sites to be in urban areas in both cases!

  5. Loved the blog Bree! I am wondering, do these birds often congregate in areas that have herd animals? For example, cows that could create mud similar to the bison? Would a farm be a good place to see these birds? Thank you!

    1. Thanks for reading this post and watching the video of the unlikely relationship between bison and Cliff Swallows. It definitely would be beneficial for the birds to have nesting sites close to where herd animals congregate or migrate because of how they upheave dirt and mud. I infer that Cliff Swallows are less likely to nest close to farms however because of the similar niches that the Cliff Swallow and the Barn Swallow occupy.

  6. Great blog Bree! Their nest building is fascinating. I really enjoyed learning about their interaction with bison!

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