The violet-green swallow (Tachycineta thalassina) is a small North American passerine bird in the swallow family. These aerial insectivores are distributed along the west coast from Alaska to Mexico, extending as far east as Montana and Texas. With an appearance very similar to the tree swallow, these individuals can be identified by the white rump side-patches that appear to separate their green back and purple tail. Violet-green swallows are secondary cavity nesters, found in a number of habitats including deciduous and coniferous forest. In addition to nesting in tree holes within these habitats, they are also widely observed nesting in the cracks of large cliffs.

Colours

thumb|270x270px|Male in California, U.S.The violet-green swallow gets its name from the iridescent colours on its back and rump. The species is best identified by the glossy green on the top of the head and back as well as hints of purple on the nape, rump and upper tail. Below the green back, the remainder of the wing is a greyish-bronze. Similar to many other swallows, it is white below, but differs in that the white continues onto rump side-patches that almost meet at the base of the tail. These white rump side-patches sometimes look like one continuous stripe when in flight. White also extends onto the cheeks, partially surrounding the eye. Juveniles are not as easily identifiable compared to adults due to their duller colours. Similar to other juvenile swallows, especially the tree swallow, the juvenile violet-green swallow is white below and greyish-brown above. However, the more gradual change in colour and visible (but slightly reduced) white cheek patches can help identify juvenile individuals of the species. Violet-green swallows are one of nine species of New World tree swallows within the genus Tachycineta. This genus is a monophyletic group and is considered the sister taxon of a group of other swallow genera restricted to the New World.

Violet-green swallows are important members of the genus Tachycineta due to their large geographic range and because they are one of the two species that breed within North America. It is generally agreed that Tachycineta is split into two clades, one in North America and one in South America, but the phylogenetic placement within the genus depends on the type of analysis used. The violet-green swallow is a member of the North American grouping, along with T. bicolor (the North American tree swallow), T. euchrysea (the golden swallow of Hispaniola and Jamaica) and T. cyaneoviridis (the Bahama swallow). T. t. thalassina and T. t. brachyptera, differing in breeding ranges and wing length. T. t. thalassina, which occupies most of the species range, has slightly longer wings; it has been known to breed as far north as Alaska. The shorter winged T. t. brachyptera breeds further south in western Mexico in Baja California and Sonora. A third subspecies T. t. lepida formerly sometimes accepted is no longer considered distinct.

Hybridization

Hybridization has been known to occur among the swallows, with a few cases involving violet-green swallows being reported. Breeding between violet-green swallows and both tree and barn swallows has been recorded on separate occasions, but resulting offspring were not observed. In 2000, the strange plumage of a swallow seen in British Columbia, Canada, suggested the possible successful hybridization of a violet-green and a cliff swallow. Nonetheless, these observations are rare, which accounts for the minimal research into swallow hybridization.thumb|270x270px|A violet-green swallow in flight

Habitat

The type of habitat occupied by violet-green swallows varies depending on their geographical location and elevation. Across their range, these birds can be found in deciduous, coniferous and mixed forests, as well as within canyons and in close proximity to large cliffs.

Behaviour

Vocalizations

There are many knowledge gaps surrounding the vocalization behaviour of violet-green swallows. These gaps include vocalizations made during early development, alarm calls and the daily pattern of calls. The little information available suggests two main classes of vocalizations, the chee-chee calls and twitter calls.

Reproduction

Violet-green swallows will usually arrive at their breeding area weeks prior to the start of reproduction. Location dependent, copulation takes place mid-May with the eggs being laid by the start of June, followed by a couple weeks of incubation before hatching. The average clutch size is 5 eggs, with females laying approximately 1 per day. During the weeks post-hatching, the parents feed their young as they slowly begin to grow their feathers. After approximately three weeks the young are ready to leave the nest. It is more common for pairs to reproduce once per breeding season, but a second brooding has been observed on occasion.

Nesting sites

Violet-green swallows are secondary cavity nesters, meaning they typically nest in natural holes or previously occupied nests. Some breeding pairs will even go as far as usurping nests from other species. Pairs typically breed in independent nests, except on cliffs where large colonies can be found nesting together.

Interspecific cooperation

Multiple accounts of violet-green swallows forming cooperative relationships with other species have been recorded. From 1981 to 1982, three independent swallow pairs were observed co-occupying the nest boxes of western bluebirds. These violet-green swallows protected the nesting site, removed fecal sacs from the boxes and fed the bluebird fledglings with no resistance from the adult western bluebirds. Despite too small of a sample size to hold statistical significance, fledgling survival was greater when violet-green swallows provided active care. It is believed that the swallows benefit from the cooperative relationship by inheriting the nest box after the bluebird parents and fledglings have left.

Interspecific competition

Similar to the occasional observation of violet-green swallows building interspecific cooperative relationships, they have also been involved in interspecific competition for nesting sites. One example from 1974 noted the competitive behaviour between mountain chickadees and violet-green swallows. The physical altercation began when a pair of swallows attempted to overtake a nesting site occupied by a pair of chickadees and their fledglings. After multiple aerial battles, the chickadees regained control of their nest and successfully defended against future violet-green swallow attacks. The same field observer noted a similar altercation between violet-green swallows and hairy woodpeckers, which resulted in the same outcome. These species are permanent residents, allowing them first choice of nesting sites, while the violet-green swallows are summer residents with late breeding habits. Thus, a scarcity of viable nesting sites may lead to this interspecific competitive behaviour in violet-green swallows. A similar observation of competitive behaviour for potential nesting sites has been observed between acorn woodpeckers and violet-green swallows.

Torpor

It has been speculated that certain birds, including swallows, enter a state of torpidity during harsh winter conditions. Few observations of this decrease in body temperature and metabolic rate have been recorded in violet-green swallows, the first case occurring in 1965 near Death Valley National Monument in Saratoga Springs, California. Field observers noted three "dead" individuals on the east-facing slope of a dune the morning after a cold and windy night. Upon closer observation, the violet-green swallows made minor movements, such as opening their eyes, but did not struggle while being handled. The physical appearance of the swallows, including the position of their head in relation to the slope and the fluffing of their feathers was similar to that of other birds in torpor. Despite the lack of thermometers, it was apparent these three individuals were in a state of hypothermia for about 45 minutes after the sun began to rise. Eventually the three swallows awkwardly flew off, signifying the end of their torpor bout. violet-green swallows have been observed behaving similar to hyperthermic individuals while sunbathing. Indications of this activity include the orientation of their wings and tail, their trance-like state and the occasional loss of balance. Research has shown that increasing feather temperatures during basking can control ectoparasites. The period of sunbathing in violet-green swallows is short and dependent on ambient temperature and wind velocity, but typically occurs after an increase in parasite loads near the end of nesting.

References

  • Violet-green swallow videos on the Internet Bird Collection
  • Violet-green swallow species account - Cornell Lab of Ornithology
  • Violet-green swallow - Tachycineta thalassina - USGS Patuxent Bird Identification InfoCenter
  • Violet-green swallow photo gallery VIREO