PART I: INTRODUCTION TO VIOLET-GREEN SWALLOWS

DESCRIPTION AND IDENTIFICATION

The Violet-green Swallow (Tachycineta thalassina) is a relatively small passerine that belongs to the swallow family Hirundinidae. As their name would suggest, these beautiful aerial acrobats have a magnificent purple and green plumage. Starting at the back of their head and the superior portion of the wings, T. thalassina has a gleaning iridescent green that travels down the back where it meets and blends into a brilliant violet colour. They have a bright white wrapping around the eye that trails to the cheeks and the birds’ underside down to the rump. When in flight and occasionally seen while perching, white saddlebags are seen (eBird). The brilliant purple and green colours are not well seen in low lighting; they show up as relatively dark coloured birds.

Both the females and juveniles show a duller, almost dusty version of the described above; females tend to have a drab brown cap and lack the bright white cheeks (All About Birds). 

T. thalassina can be identified while in flight by its long wings, short and square tail, small rounded body and head, and tiny bill (All About Birds). Both sexes weigh in at around 0.5 oz (14 g), have a wingspan of approximately 10.6 in (27 cm), and a body length of 4.7 in (12 cm) (All About Birds).

VOCALIZATIONS

Violet-green Swallows obviously live up to the name when it comes to plumage; however, when it comes to being members of songbird grouping, they lack the elaborate flair that many of its relatives possess. Instead, they remain relatively quiet throughout the day and reserve their calls for the day’s early and pre-dawn hours. Both their songs and calls are relatively simple; a series of chirps and twittering.

RANGE AND DISTRIBUTION

Violet-green Swallows are distributed throughout the mid-west to the west coast of North America. They can range from as far North as Alaska during the breeding season to as far South as Costa Rica during the non-breeding season (All About Birds).

These vibrant coloured birds are known to breed throughout the western United States and Canada. They are quite adaptable birds and are found at sea level and as high an elevation as 11,500 ft (All About Birds). They are known to migrate to Mexico or to Central America, making them a moderate to long-distance migrator. In some cases, Violet-green Swallows are known to stay year round in the central parts of Mexico (All About Birds). Overall, Violet-green Swallows are one of the more adaptable migratory passerine species in their range.

In British Columbia, Violet-green Swallows are most commonly found in the Southern interior and coastal regions; however, there is evidence of them breeding in Northern and central BC. Highest probability of observing these vibrant birds is of course highest in the Southern and coastal regions.

HABITAT

Violet-green Swallows are known to breed in open woodlands, this can include evergreen, deciduous, or a mixed forest (All About Birds). However, they are not limited to open woodlands; they are also frequently found around streams and lakes hunting for insects. Violet-green Swallows nest in cavities, most often in cavities of old trees located within open woodlands. They often nest in cavities and holes found on cliffsides. Due to the adaptability of this species, as well as the ever-growing human-made landscape, Violet-green Swallows are found to nest in man-made nest boxes near settlements (All About Birds). Violet-green Swallows are the swallow species that are most commonly seen in urban areas due to their adaptability (Bird Web).

DIET

Violet-green Swallows are aerial insectivores. They feed almost exclusively on insects that fly (Bird Web). Common insects hunted are wasps, leafhoppers, leaf bugs, ants, bees, and different beetle species.

They feed, as well as drink almost exclusively while in flight (Sialis), which has lead the to evolve in to incredible aerial acrobats, performing marvelous routines while catching insects in flight.

NESTING

Violet-green Swallows are known to nest in cavities such as old trees with woodpecker holes, natural rock crevices, nest boxes, and bird houses. When they find a suitable cavity to raise their offspring both the male and female bird go off to gather a variety of nest materials. Such as grass, small twigs, fallen feathers, and roots (All About Birds).

Once they gather the materials they create a bowl-like structure with a shallow dip; the size of the nest varies depending on the size of the cavity, however, often measures to approximately 3 inches in diameter (All About Birds). Their nests are often seen as “sloppy” when compared to other songbird species; since they create the nest within cavities there is less pressure to assure strong structural integrity of the nest (Sialis). They pay particular focus on lining the nest with feathers to insulate and cushion the eggs. The process of building the nest can vary in days, it can take anywhere from 3 days up to 20 days.

Violet-green Swallows have a usual clutch size of 4-6 eggs. The eggs are solid white in colour and around 1.6-2.1 cm in length (All About Birds). Often there is only one brood a year, but it is not uncommon for two to occur (Audubon).

The incubation period is roughly 13-18 days and is almost entirely done by the female (Audubon). Once the young hatch, both the male and female will fed the nestlings; however, the females often give more parental care to the nestlings than the males.

Around 23-24 days after hatching, the juveniles are ready to leave the nest. The parents still continue to feed their offspring even after they have left the nest (Audubon).

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BEHAVIOUR

Violet-green Swallows are gregarious birds, they are often found in either single species or mixed swallow species flocks (Bird Web). These tiny sky dancers are often found to fly at high altitudes, relative to other swallow species. They will fly low in poor weather conditions, or when foraging over open lakes and streams (Bird Web). Their social behaviour is seen year-round; often seen in large groups when foraging in both breeding and non-breeding grounds (All About Birds).

Fun fact: a group of Swallows can be referred to as a “gulp”, “kettle”, “herd”, “sord”, or “richness” of Swallows!

During the breeding season, depending on available resources and cavity availability Violet-green Swallows may breed in groups or solitarily (All About Birds). Violet-green Swallows are known to be opportunistic when it comes to nesting cavities. There is even an observed account of a pair of Violet-green Swallows that assisted a pair of Western Bluebirds raise their young. They guarded the nest, as well as tended to the nestlings of the bluebirds. Once the bluebirds had fledged, the swallow pair took over the nest site and used it as their own (What Bird).

Violet-green Swallows are most often found in monogamous pairs, and they share parental care once the young hatch from their eggs.

Swallows are often known for their brilliant aerial routines. Violet-green Swallows definitely live up to the stereotype. However, the wingbeat and flight pattern is slightly different than its relatives. Violet-green Swallows follow a rapid, shallow, and long gliding wingbeat pattern; this creates a more fluttery pattern than other swallows.

Their wingbeat pattern can cause their flying to look slightly off balance; nonetheless, they still possess the same elegance and poise.

Because of the cavity nesting behaviour Violet-green Swallows exhibit, they are often prone to more external parasites within the feathers (All About Birds). As a result, it is not uncommon to see these swallows preening their feathers while resting or perching.

CONSERVATION STATUS

Violet-green Swallows are listed in the low risk category at a rating of least concern. Their current global population trend is increasing. Violet-green Swallows have a very wide range of habitat and are very adaptable birds. They have estimated 6,100,000 square kilometers of global range (What Bird). As of right now, there are little signs of species decline, therefore they are currently not on the IUCN Red List (What Bird).

Fun fact: Violet-green Swallows are of least concern on the conservation list, they are important sentinels in determining ecosystem health! They have been used to measure environmental contaminants, assess population structure changes due to increased agriculture, and they have even been used to determine links between physical abnormalities and nuclear radiation (Garlick et al., 2014).

Although the overall population trend for the species is increasing, there are certain areas that are currently facing regional population decline.

Other than natural habitat decline due to urbanization, another human activity that has lead to slight population declines in Violet-green Swallows is the introduction of non-native bird species. The Violet-green Swallow’s biggest concern would be either the introduction of the House Sparrow or the European Starling (National Geographic). These bird species will destroy nests, destroy eggs, and even kill young. These introduced birds would have had the greatest affects to the Violet-green Swallows in Southern Canada. These species are known to attack not only the Violet-green Swallow, but other swallows species as well. Below are examples of a massacre that happened within a Tree Swallow nest box.

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NEST BOXES

With decline of natural resources and natural cavities due to human activity, Violet-green Swallows (ironically) will nest in human made nest boxes. These nest boxes have become increasingly common for bird lovers, there are hundreds of nest box blueprints online so you too can build a next box! There are plenty of websites such as Nest Watch that give great tips on how and where to place a nest box.

The most successful nest boxes will be monitored and made difficult to access by the merciless House Sparrows. Many members of the bird community would agree with the statement that it is much better to have no nest box than have a nest box that raises House Sparrows (Tree Swallow Project).

A responsibility that a nest box owner must take is assuring that nest boxes are cleared out once the young have fledged and the nest has become empty. This gives chances for other birds to use the box.

PART II: THE VAST REPRODUCTIVE ADAPTABILITY OF VIOLET-GREEN SWALLOWS

Violet-green Swallows are known to be very adaptable in the sense of where they choose to nest; they will nest anywhere from a natural tree cavity to an old drain pipe. However, their adaptability is not only limited to where they choose to raise their young.

Interspecific pairing

In a 2017 observation, there was evidence of a male Tree Swallow and female Violet-green Swallow sharing a nest in Ontario. The Tree Swallow had been the original owner of the nest; however, when the Violet-green Swallow arrived it was seen to move into the nest with the Tree Swallow (Armstrong & Johnston, 2018). There was observed copulation attempts by the Tree Swallow and Violet-green Swallow. The male was seen delivering feathers to the female within the nest box, as well as the female delivering feathers to the male (Armstrong & Johnston, 2018). The initial nest attempt was abandoned due to a red squirrel that had previously created a home in the nest box; however, a new nest attempt was made by the same (presumably) two interspecies birds. The second attempt at a nest was unfortunately abandoned as well, yet this nest there laid two abandoned eggs (Armstrong & Johnston, 2018). Violet-green Swallows are not commonly thought to copulate with other species; however, this instance of an interspecific pairing brings up interesting questions regarding the reproductive adaptability of these magnificent birds. An interesting point to note is that Violet-green Swallows are not commonly found in Ontario. Perhaps the mating attempt with the Tree Swallow was a chance to try and increase off spring survival in an unfamiliar breeding ground.

Metabolic rate and glucocorticoid hormone response adaptation to periods of stress as a predictor to reproductive output in females and growth time in juveniles

Studies have found that Violet-green Swallows increase specific stress and steroid hormone levels in unfavourable conditions or in periods of increased stress as a way to increase fitness. Glucocorticoids are essential hormone sin regulating metabolic state, especially in response to unprecedented environmental changes.

In rearing female Violet-green Swallows it is seen that females that experience some form of physiological stress will increase glucocorticoid levels to keep up with parental care (Rivers et al, 2016). With the raising of these stress response hormones, female Violet-green Swallows are able to adjust to the physiological stress in order to maintain reproductive output (Rivers et al, 2016). Despite the stress the adult female may face, she is able to continue feeding her nestlings at a rate the is comparable to females unaffected by stress. There is evidence of elevated glucocorticoid levels in the nestlings of the stress affected females, the reason behind this is unstudied. In females, levels of these hormones were measured using both the baseline and the stress-induced hormones (cortisol and corticosterone, “CORT” from hereafter). Studies have found that baseline CORT levels are associated with baseline metabolic investment, more specifically a relation between reproductive performance and metabolic rates (Jimeno et al., 2020). A higher metabolic rate, and higher baseline CORT levels are associated with a higher reproductive output (Jimeno et al., 2020).

On the other side of the nest there are the nestlings, within the nest the growing birds can face stress through sibling-competition in larger broods. There is record of elevated glucocorticoid levels in nestlings in large broods  (Hardt et al., 2018). When there is increased sibling-competition, the nestling Violet-green Swallow adapts to the stress by elevating hormone levels to increase personal fitness. The elevation of glucocorticoids in juvenile birds shows no evidence of stunting growth, however, growth rate is slightly slower (Hardt et al., 2018). When  the reverse was looked at, a very small brood showed no change to hormone levels compared to a normal brood size (Hardt et al., 2018).

These hormone responses seem to work in a trade-off system, the adult female will increase hormone levels, but lose some mass in order to keep up with reproductive output (Rivers et al, 2016). While the juveniles increase hormone levels to assure that they grow enough to successfully fledge the nest, they extend the growth time within the nest to slightly longer (Hardt et al., 2018). The mechanism behind the hormone changes remains unknown, but they are fascinating adaptions to external stress.

CLOSING REMARKS

Violet-green Swallows are whimsical birds with a keen ability to dart around the sky in magnificent patterns. They show us just how amazing they are at not only dancing in the sky, but through their strong adaptions to ensure reproductive success, their ability to show ecosystem distress, and so much more. I hope that after reading this you have acquired a similar level of love for these exquisite birds.

References

All About Violet-green Swallows (Tachycineta thalassina). (n.d.). Retrieved from http://www.sialis.org/vgswbio.htm

Armstrong, T., Johnston, C. (2018). Observations and interspecific pairing of a Violet-green Swallow and Tree Swallow in northwestern Ontario. Ontario Birds, 36(1), 42-55. http://www.ditsy.ca/library /serve/ob-36-1/index.html?page=43

BC Breeding Bird Atlas. (n.d.). Retrieved from https://www.birdatlas.bc.ca/accounts/speciesaccount.jsp?lang=en&sp=VGSW

Creating Successful Tree Swallow Nest Box Projects. (n.d.). Retrieved from https://www.treeswallowprojects.com/creproj.html

First sighting of swallow nestling in one of the nest boxes. (n.d.). Retrieved from http://www.cosmicbirding.com/bird-blog/first-sighting-of-swallow-nestling-in-one-of-the-nest-boxes

Garlick, N.W., Newberry, & Rivers, J.W. (2014). An Assessment of Nestling Diet Composition in the Violet-green Swallow (Tachycineta thalassina). Northwest Scientific Association, 88(1):49-54. http://dx.doi.org/10.3955/046.088.0109

Jimeno, B., Prichard, M.R., Landry, D., Wolf, C., Larkin, B., Cheviron, Z., & Breuner, C. (2020). Metabolic rates predict baseline corticosterone and reproductive output in a free-living passerine. Integrative Organismal Biology. https://doi.org/10.1093/iob/obaa030

Hardt, B. M., Ardia, D. R., Bashaw, M. J., & Rivers, J. W. (2018). Experimental brood enlargement differentially influences the magnitude of the corticosterone stress response in closely related, co‐occurring songbirds. Functional Ecology,32(8), 2008-2018. doi:10.1111/1365-2435.13116

Rivers, J. W., Newberry, G. N., Schwarz, C. J., & Ardia, D. R. (2016). Success despite the stress: Violet‐green swallows increase glucocorticoids and maintain reproductive output despite experimental increases in flight costs. Functional Ecology,31(1), 235-244. doi:10.1111/1365-2435.12719

Violet-green Swallow Life History, All About Birds, Cornell Lab of Ornithology. (n.d.). Retrieved from https://www.allaboutbirds.org/guide/Violet-green_Swallow/lifehistory#

Violet-green Swallow Range Map, All About Birds, Cornell Lab of Ornithology. (n.d.). Retrieved from https://www.allaboutbirds.org/guide/Violet-green_Swallow/maps-range#

Violet-green Swallow. (n.d.). Retrieved from https://ebird.org/species/vigswa

Violet-green Swallow. (n.d.). Retrieved from http://www.birdweb.org/birdweb/bird/violet-green_swallow

Violet-green Swallow. (n.d.). Retrieved from http://identify.whatbird.com/obj/195/_/violet-green_swallow.aspx

Violet-green Swallow. (2019, August 27). Retrieved from https://nestwatch.org/learn/all-about-birdhouses/birds/violet-green-swallow/

Violet-green Swallow. (2020, February 04). Retrieved from https://www.audubon.org/field-guide/bird/violet-green-swallow