THE SWALLOWS OF THE AUTONOMOUS CITY OF BUENOS AIRES

The belief that there is only one species of swallow that completes a magnificent migration between North America and South America is quite common among porteños (the inhabitants of the Autonomous City of Buenos Aires). In my case, this was my assumption until adulthood, when I began to seriously research the world of birds. However, specimens of 11 species of swallows have been detected in the Autonomous City of Buenos Aires. Moreover, although there are swallow species that migrate from North America, most swallows in the City of Buenos Aires come from regions of the rest of South America, including Argentina.

EcoRegistros holds records of specimens of 11 species of swallows taken in the Autonomous City of Buenos Aires (considered on the site as a jurisdiction separate from the Province of Buenos Aires) since its inception in 2011. The species, ordered from the one with the highest number of detections, are as follows:

• Brown-chested Martin (Progne tapera)
• Blue-and-white Swallow (Pygochelidon cyanoleuca)
• Grey-breasted Martin (Progne chalybea)
• White-rumped Swallow (Tachycineta leucorrhoa)
• Barn Swallow (Hirundo rustica)
• Chilean Swallow (Tachycineta meyeni)
• Southern Martin (Progne elegans)
• Southern Rough-winged Swallow (Stelgidopteryx ruficollis)
• Collared Sand Martin (Riparia riparia)
• Cliff Swallow (Petrochelidon pyrrhonota)
• Tawny-headed Swallow (Alopochelidon fucata)

It is pertinent to emphasize the obvious fact that urban swallows are a very small portion of the mass of birds that inhabit and migrate throughout the entire region. Evidently, thanks to their remarkable vitality, swallows appear in parts of the city.

The figure of the swallow against the sky is iconic, with dark and pointed wings forming an arc; in some species the tail is forked and in others it ends with an angle. The swallow species of the Autonomous City of Buenos Aires are obviously distinct, but they share a dark back (black, bluish-black, blue, greenish-blue, or brown) and a lighter or white belly; the exception is the Southern Martin, the belly of which is also dark. According to the Digital Field Guide of the Birds of Argentina by the organization Aves Argentinas, the length of the specimens ranges between 11 and 19 cm. The Blue-and-white Swallow is the smallest, with a length of 11 cm; the Southern Martin is the largest, with a length of 19 cm. The three species of the genus Progne, the Brown-chested Martin, Grey-breasted Martin, and Southern Martin, are the largest.

On the EcoRegistros website, you can consult the statistics of the records for each species in the Autonomous City of Buenos Aires. These data change over time with the inclusion of new records. The number of records for each of the 11 species up to the date 05/16/2025 is detailed as follows: Brown-chested Martin, 1371 records; Blue-and-white Swallow, 1265; Grey-breasted Martin, 1211; White-rumped Swallow, 1133; Barn Swallow, 270; Chilean Swallow, 70; Southern Martin, 57; Southern Rough-winged Swallow, 31; Collared Sand Martin, 16; Cliff Swallow, 10; and finally the Tawny-headed Swallow, with only 1 record.

According to EcoRegistros data, the most common swallow species in the Autonomous City of Buenos Aires are clearly the Brown-chested Martin, Blue-and-white Swallow, Grey-breasted Martin, and White-rumped Swallow. The Brown-chested Martin stands out the most in the records; this species is abundant in summer along the riverside. The Barn Swallow ranks fifth with just under a quarter of the number of detections of the White-rumped Swallow. There is only one record of the Tawny-headed Swallow documented in the city by EcoRegistros since 2011, indicating that it is a very rare species in the city.

EcoRegistros also records the number of detections of individuals of each swallow species within the areas of the city that are most important for birdwatching. The Costanera Sur Ecological Reserve is the prominent location of the general detections. Exceptions are the fact that the Southern Rough-winged Swallow has not been detected there — but has been detected in the Ciudad Universitaria-Costanera Norte Ecological Reserve — as well as the single historical record of the Tawny-headed Swallow.

The swallows’ preference for habitats combining abundant water and lush vegetation is evident not only from the statistics of the Costanera Sur Ecological Reserve but also from the abundant detections in the Ciudad Universitaria-Costanera Norte Ecological Reserve, the Lago de Regatas area, Parque Tres de Febrero, and Lago Lugano Ecological Reserve. Among the statistics of the four species that are clearly the most numerous, those of the Grey-breasted Martin (the third most abundant species in the city) are remarkable. It is worth noting that although detections of this species show a significant presence in the Costanera Sur Ecological Reserve, the numbers lean more towards the rest of the city, including a substantial number of detections in the Belgrano neighborhood, than those of the other three species.

Additionally, in EcoRegistros, you can find dynamic migration maps that show their movements across the different months, marking the various seasons. These maps include the American migrations of the five most common swallow species in the Autonomous City of Buenos Aires: the Brown-chested Martin, Blue-and-white Swallow, Grey-breasted Martin, White-rumped Swallow, and Barn Swallow; there are no dynamic maps on EcoRegistros for the other six species. From the dynamic maps in EcoRegistros of the most common species, it is clear that the population of these swallows in the region reaches its peak every year in the December-January two-month period and its lowest in the June-July two-month period.

It is a conventional rule that swallows migrating from North America breed there and spend their non-breeding season in South America during the southern spring and summer only to feed. It is also assumed that species native to South America tend to breed during warm climates in the southern hemisphere and that their migratory vector points north when temperatures drop. However, these conventional rules include exceptions. It is also notable from migration maps on all information platforms (which include the 11 species of the Autonomous City of Buenos Aires) that many individuals of the species that breed in North America manage to connect regions at the extremes of the American continent in their migrations, while South American species restrict their movements within South and Central America.

While investigating migrations in detail, many nuances are discovered. For example, many individuals of the Blue-and-white Swallow are detected year-round in the Autonomous City of Buenos Aires. While some people assume these birds are permanent residents, it is possible that the species’ presence is mainly due to individuals migrating from Patagonia in winter and from the north in summer. Regarding swallows supposedly breeding only in North America, in 1980, the nesting and reproduction of Barn Swallows were documented in the summer at Mar Chiquita, Buenos Aires Province. Subsequent studies found a growing population in the Pampas region, where the birds used bridges and other human constructions for nesting. A similar process may be occurring with some individuals of the Cliff Swallow detected breeding in Villa María, Córdoba, in 2015. These very rare events, called “latitudinal trans-hemispheric reproductive colonization,” represent cases in which a small population suddenly begins to reproduce in an anomalous hemisphere. Ornithologists watch here for sudden evolutionary changes with fascination.

The following section presents representative photos of specimens of the 11 swallow species detected by EcoRegistros in the Autonomous City of Buenos Aires. Each photo is followed by a paragraph providing information about the geographic distribution and migratory activity of each species. The geographic distribution was verified with detection maps from eBird, a platform of the Cornell University Laboratory of Ornithology, USA. Migratory summaries were taken from the Digital Field Guide to the Birds of Argentina by Aves Argentinas; in the case of the White-rumped Swallow, the summary is from Birds of the World, Cornell University.

Brown-chested Martin. It inhabits all of South America, with few detections in Chile and the far south of Central America. Some surprising detections have been recorded in the northeastern USA and the states of Louisiana and Arizona. In Argentina, it is concentrated in the center and north; in Patagonia and Tierra del Fuego, it is has been detected in small coastal areas. It breeds from November to January and migrates north in autumn, reaching northern South America and the far south of Central America.

Blue-and-white Swallow. It inhabits all of South America except large regions of Brazil around the Amazon and the forests of Chaco and Paraguay. It is also found in southern Central America. Some surprising detections have occured on the Gulf of Mexico coast at the US-Mexico border. In Argentina, it is found throughout the country except the Chaco region. It breeds from September to January. In autumn, it migrates north to northern Argentina, Chile, and Central America. It is one of the few species detected during winter in northern Argentina and the City of Buenos Aires.

Grey-breasted Martin. It inhabits all South American countries, with few detections in Chile. It is also found throughout Central America and southern Mexico. Some surprising detections have occured in New York, USA. In Argentina, it is found in Buenos Aires Province and nearly all regions to the north. It breeds in spring and summer. In autumn, it migrates north reaching Central America and Mexico.

White-rumped Swallow. It inhabits Argentina, Uruguay, Paraguay, Bolivia, and southern Brazil. There have been detections in Peru, Colombia, and Venezuela. In Argentina, it is found throughout the country except Patagonia and much of Mendoza. It breeds in Argentina between October and February. Most of the southern population migrates north with the onset of winter and the species is almost absent in Argentina from May to August. However, many individuals remain residents and can withstand the austral winter.

Barn Swallow. It covers the entire American continent, including the South Atlantic Islands, except the depths of the Amazon. It inhabits almost all of Argentina, although it is absent in some Patagonian regions. It has always been assumed that it breeds only in North America. However, in 1980 a flock was detected breeding in Argentina, and later a growing population with nests was recorded in several places in Buenos Aires Province and northeastern Patagonia. The predominant North American population breeds there and migrates south when temperatures drop. Studies using geolocator tags documented that some of the anomalous swallows that bred in warm Argentine climates changed migratory habits by not returning to North America and decided to spread throughout South America to feed.

Chilean Swallow. It inhabits along the length of Chile and in Argentina including the South Atlantic Islands; also Uruguay, Paraguay, and southern Brazil. There have been some detections in Bolivia. In Argentina, it is only absent in Chaco regions and the northeast. It breeds in Patagonia, Tierra del Fuego, and South Atlantic Islands in spring and summer. Northern Patagonian populations are residents year-round. Southern populations are migratory and in winter migrate to central and northern Argentina, Uruguay, southern Brazil, Paraguay, and Bolivia. These birds surprise the residents of the city of Buenos Aires by appearing in winter from Patagonia.

Southern Martin. It mainly inhabits Argentina, but there have been detections in the rest of South America, Panama, and Costa Rica. It is found throughout Argentina except in Chaco regions and Mesopotamia. It breeds between October and February. It migrates north to northern South America and Central America in autumn; some individuals remain in northern Argentina.

Southern Rough-winged Swallow. It inhabits all South American countries except Chile and is also found in Central America. In Argentina, it is found only in northern Buenos Aires Province, all of Mesopotamia and its surroundings, and the northeast. It breeds from October to November. Southern populations migrate north in autumn and winter.

Collared Sand Martin. It covers the entire American continent, including the South Atlantic Islands, except for vast regions of Brazil. In Argentina, it is concentrated in the center and north and absent in large Patagonian regions. It breeds in North America and migrates south when temperatures drop. In spring and summer, it appears throughout northern Argentina and the region of the city of Buenos Aires.

Cliff Swallow. It covers the entire American continent, including the South Atlantic Islands, except vast regions of the Amazon, Patagonia, and Chile. It breeds in warm climates in North America. Just like the Barn Swallow, it was historically thought to breed only in North America. However, in 2015, nests with chicks were detected in Villa María, Córdoba. In the same way as with the anomalous Barn Swallows, studies using geolocator tags were conducted. Ornithologists found that the birds changed their migratory habits to spend winter in various parts of America. They confirmed that the birds stopped breeding in the northern hemisphere but documented that some individuals reached their former breeding sites to overwinter.

Tawny-headed Swallow. It is found in Argentina, southern Brazil, Uruguay, Paraguay, and Bolivia. There have been detections in Chile, Colombia, Venezuela, and Guyana. In Argentina, it is absent in almost all of Patagonia and some northern regions. It breeds from September to December. It is found in spring and summer in central and northern Argentina and in northern Patagonia. It migrates north in autumn and winter.

According to current migratory maps, besides the Barn Swallow, two other species arrive in the Autonomous City of Buenos Aires from faraway North America: the Cliff Swallow and the Collared Sand Martin. The Chilean, White-rumped, and Tawny-headed Swallow restrict their movements within South America. The Grey-breasted Martin, Blue-and-white Swallow, Brown-chested Martin, Southern Martin, and Souther Rough-winged Swallow are found in South America but can reach Central America. The Barn Swallow and the Collared Sand Martin do not restrict their habitats within the Americas and are truly cosmopolitan. It is no surprise that these species (besides the Cliff Swallow) also complete the longest migrations linking the continent’s extremes. The Barn Swallow and Collared Sand Martin from different continents have been classified by taxonomists into subspecies. The scientific name of the American Barn Swallow subspecies is Hirundo rustica erythrogaster. Regarding the Collared Sand Martin, the subspecies is Riparia riparia riparia, which is also found in Europe and western Asia; different subspecies exist in even more distant regions of the globe.

In conclusion, it is important to emphasize that the inhabitants of the Autonomous City of Buenos Aires will continue to associate swallows with spring. This is partly due to the countless individuals of the Brown-chested Martin that appear along the Buenos Aires waterfront and the rest of the Río de la Plata coast during September. In other areas with leafy trees and especially near bodies of water, the number of individuals of all species except the Chilean Swallow also increases considerably along with the rising temperatures. The Blue-and-white Swallow maintains its presence during winter, and the Chilean Swallow is peculiarly winter-active. Overall, the summer swallows are by far the most abundant and will continue to justify the saying "one swallow does not make a summer."