Progne subis
purple martin

Progne subis, commonly known as purple martins, inhabit the Nearctic region and can be found across North and South America. Progne subis is a migratory species that breeds in North and Central America and overwinters in South America. The northern extent of the breeding range includes the Canadian Territories of Alberta, Saskatchewan, and southern Manitoba. Purple martins breed across the eastern half of the United States, and also may be found along the Pacific coastline including the entire Baja Peninsula. (Brauning, 1992; Brown, 1997; Johnston, 1966)

Purple martins overwinter across most of South America including the countries of Colombia, Venezuela, Guyana, Suriname, Ecuador, Peru, Brazil, Bolivia, Paraguay and Argentina. Their winter range has been recorded to be South American lowlands anywhere east of the Andes Mountains. Concentrated populations have been found to winter in Bolivia and some provinces of Brazil. There have been some records of purple martin populations found in the British Isles, but these birds rarely migrate outside of the Americas. (Brown, 1997)

Progne subis prefer open spaces that are situated close to any water source, as they are insectivores and are attracted to the large populations of insects near wetlands, swamps, and wet meadows. Purple martins also seem to avoid high elevations, for instance the Appalachian Mountains, but may be found at elevations from less than 100 m to 4,000 m. Due to colonization and human interactions in their natural habitats, purple martins are now accustomed to human interaction and live in close proximity with humans today. They tend to find shelter in urban settlements, often living in specially made birdhouses called "martin houses". Historically, this species inhabited forest edges, montane forests, and deserts and nested in abandoned woodpecker cavities. Some populations that breed in the western United States continue to live in these natural settings, however most utilize man-made martin houses.

During migration, these birds stopover in a variety of habitats. They usually fly over coastal lines and cross the Gulf of Mexico. They have been recorded in lowlands and the high mountain ranges of Venezuela and Columbia. They are often seen in cities and open areas while migrating south.

Wintering habitats include rainforests, agricultural areas, and clearings of South America. They may also reside in urban plazas. (Brauning, 1992; Brown, 1997)

Purple martins have an average body mass of 56 g. They have a wingspan of 45.7 cm long and an average body length of 20.3 cm.

Progne subis exhibits distinct sexual dimorphism. Males and females differ slightly in size, where the males tend to be a bit larger than the females. Males are entirely shiny, deep purple or almost black in coloration. Purple martin males have less shine on their blackish wings and tails compared to their heads and backs. Females are overall gray or gray-blue with darker wings and crown feathers, and feature a white breast smudged with varying degrees of gray. Western females are overall paler than eastern.

Juvenile purple martins are overall gray to black with a white belly and gray-streaked breast. In juveniles, there is a much clearer line between their gray throats and white bellies compared to females which will have a primarily gray belly. In flight, a juvenile's tail may have a narrow, slight fork whereas adult tails are distinctly forked and wider.

Purple martins have a dark, black-brown bill and the average length is 8.2 mm for males and 8.5 mm for females. Their gape tends to be yellow in young birds and a dull orangey-brown color in adults. Their legs and feet have a black brown coloration and their eyes are dark brown. (Brown, 1997; Layton, 1972)

Populations residing in the southwest United States exhibit lighter coloration than purple martins in other regions. This lighter coloration is hypothesized to be an adaptation to the desert climate and serves to absorb less heat. (Layton, 1972; Wade, 1966)

Purple martins are socially monogamous, but form a pair bond that rarely extends to subsequent seasons. In Texas, 5% of purple martins that occupied more than one nest practiced polygamy. Males arrive at the breeding grounds first and will select nest cavities where he will display to potential mates. The male often selects two nest sites for females to choose from. Males are very territorial and will aggressively defend nest sites from other males. Unpaired males will perform an aerial display to any nearby female. This display begins with the male flying out from his nest in a wide arc, then swooping back into the cavity, popping out his head and singing. Research has suggested that despite these efforts, females are more interested in the quality of a nest site than a male's displaying ability. Once a pair has formed, the male defends the nest cavity as well as his mate. Although the pair is aggressive towards foreign intruders, they tolerate each other and will continue to tolerate each other in subsequent years even when they are paired with other mates. (Brown, 1979; Brown, 1997; Johnston and Hardy, 1962)

Generally purple martins spend their winters in South America just past the Andes, and start returning to their northern breeding grounds as early as January 1. They slowly move northwards progressing generally 3 to 5 degrees latitude (330 to to 550 km) every half a month and they reach their northern limits around May 1. Adult males often return first, followed shortly by adult females while sub-adults return a couple weeks after. This general breeding pattern has been found to be true for a generalized population, except the non-colonial Saguaro desert broods. These purple martins arrive early May, which is about two and a half months later than any other colony at the same latitude. Reasons for the difference in arrival patterns have not yet been discovered. (Johnston and Hardy, 1962; Morton and Derrickson, 1990)

After a pair bond is formed, the martins can start building a nest. Nest building starts about a month before the pair intends to lay the eggs. Common materials used to build nests are green leaves, grass, sticks, paper, mud, and feathers. The use of green leaves as nesting materials is poorly understood, but there are currently many hypotheses. The female performs most of the construction, while the male gathers materials and defends the cavity from other martins. Historically, purple martins used natural cavities to nest in but due to deforestation and the removal of dead snags, these birds mostly nest within man-made "martin houses" that support their colonial lifestyle. (Alsop III, 2002)

The breeding season for Progne subis starts in May and will last until June. A purple martin nest can have anywhere from 3 to 8 white, oval eggs but the average amount of eggs laid is 5. These eggs are usually about 2.4 by 1.7 cm in size and are then incubated for 15 to 18 days. The chicks fledge after 26 to 31 days and travel in a family group. The group returns to sleep at the nest for several days. At 7 to 10 days after fledging the young are able to survive on their own and will disperse. Young purple martins can reproduce in the first subsequent breeding season. (Alsop III, 2002; Brauning, 1992; Brown, 1979; Brown, 1997; Layton, 1972)

After the eggs are laid, the female is the primary incubator. Incubation lasts from 15 to 18 days. When the female leaves the nest, the male will proceed in incubating the eggs himself but this happens infrequently. As soon as the chicks hatch, brooding begins and usually lasts until the tenth day. The female alone broods the young. Purple martin chicks are altricial and are completely dependent upon the parents for survival.

The young are fed within hours of hatching and will continue to be fed for 5 to 7 days after the young fledge. Both the male and female feed the brood. Feeding occurs by regurgitating food and transferring it into the mouths of the young one by one. As the brood gets older the feeding sessions become more frequent and reaches its peak when the young start gaining the most weight, which occurs around days 17 to 21. The feeding becomes so regular that it may occur every 30 seconds. Parents ensure that the food is the proper size to be swallowed and if the pieces are too large and swallowing does not occur instantly, the food is removed from their mouths. After two weeks of development the female purple martin will cease to sleep in the same compartment as the nestlings, because less frequent night brooding is necessary.

Parents also play an important role in keeping the nest clean. The parents will eat the fecal sacs and will encourage the young to defecate by poking at their cloacal region. (Alsop III, 2002; Brown, 1980; Brown, 1997; Layton, 1972)

The longevity of purple martins range from 0 to 13 years and nine months. Purple martin mortality is often the result of severe weather. Three to four days of severe weather can lead to insect numbers drastically declining. If there is a lack of food, purple martins cannot survive and this often results in population decline. Another hindrance to long life expectancy is often body parasites. Purple martins host a protozoan blood parasite Haemoproteus prognei. This parasite can have disastrous effects on the surviving rate of first year birds during the winter and migration period. (Brown, 1997; Layton, 1972)

Progne subis are colonial birds that live together in large colonies. They either choose to fly around in pairs or stay in their colonial groups. At the end of the summer the whole colony commutes back to South America. These communal flights home can include tens of thousands of birds.

Purple martins are known to act aggressively when they are threatened. For instance, they will defend themselves by pecking and flapping their wings trying to encourage the unwanted visitor to leave. When purple martins are threatened they take on a “Horizontal Threat Posture”, orientating their bodies so that it runs parallel with the surface and pressing their feathers down close to their bodies. Often when threatened, purple martins alert other martins by singing. Another defense mechanism purple martins resort to is gaping. Gaping is when a purple martin opens its mouth wide and directs it to the potential threat. Purple martins resort to lunging as an intimidation effect and will also try to intimidate a threat by bill snapping.

Purple martins have shown distinct group behavior. Martin pairs are monogamous and very territorial, but when they join a colony they can show strong group-orientation and protectiveness of the colony's territory. The colony socializes together by exploring the environment around them. This includes visiting other colonies, scoping out potential food sources, and visiting nearby streams and swamps. The socializing groups consist of 3 or more purple martins. Purple martins forage together by hunting in large flocks. When the flocks do find food they call the rest of the colony to come feed by making distinct sounds.

Purple martins also display synchrony when they live in large colonies. Colonial purple martins work together in an effort to increase the reproductive success of the colony. Larger colonies have more success at achieving synchrony, because they are usually made up of mature birds, where as smaller colonies are made up of younger adults with less experience. Synchrony is related to age and is reflected better in birds of similar age. A colony of purple martins will achieve synchrony better if the birds are similar in age. Colonial living does carry the risks of increased disease transmission and resource competition.

Purple martins are migratory birds that travel from central North America to northern South America to breed and overwinter, respectively. They are active during the day and are most active at dawn and dusk. (Alsop III, 2002; Johnston and Hardy, 1962)

Purple martins often return to the same nesting site where they had lived the previous year. There is no information on how far these birds will travel while foraging. Purple martins have the ability to fly far distances to get to their nests. This suggests that these birds must have a large home range. Purple martins are known to travel from 2.8 to 664 km to get back to their nests. (Brown, 1997)

Progne subis communicates vocally and visually. Purple martins have eleven identified vocalizations that they use for different occasions like mating, warning, and teaching the young during fledging. Male purple martins use singing and visual displays to attract potential female mates. Juvenile purple martins resort to ‘choo-choo’ calls to attract their parents’ attention if protection is needed. Parents use juvenile 'choo-choo' calls to assemble their broods and return them safely to the nest. Female purple martins use the ‘choo’ call to lead their young to and from groupings areas during the fledging period. Purple martins only resort to ‘zwarck’ calls when they need to send a high intensity alarm, and it is often accompanied by the birds diving straight down towards the invader. Male purple martins use ‘hee-hee’ vocalizations to fight off intruders. ‘Zweet’ calls are used to show intraspecific excitement, as well as send an alarm to warn other purple martins of a potential threat and to encourage them to fly away. Purple martins use ‘cher’ calls to communicate daily and will use ‘chortle’ calls in high excitement situations. Males attract females by singing ‘croak songs’ as well as to warn off unmated males from entering their territory. During courtship males make a clicking sound by snapping their lower and upper mandibles together. The last vocalization male purple martins use are ‘subsongs’ and are heard during feeding and pre-migratory periods. ‘Subsongs’ are used to communicate with other purple martins while socializing together. Purple martin males that are part of a stable colony often perform a 'dawnsong' which include a variety of sounds early in the morning. (Brown, 1978; Brown, 1984; Brown, 1997; Johnston and Hardy, 1962; Morton, 1985)

Purple martins are primarily insectivores. They capture food in flight and rarely glean insects from foliage or the ground. Purple martins prefer eating fruit flies Ceratitis, mosquitoes Culicidae, wasps Polistes, beetles Coleoptera, ants Formicidae, grasshoppers Orthoptera, cicadas Cicadidae as well as dragonflies Anisoptera. Purple martins may consume 400 flies or 2000 mosquitoes in a day. Young purple martins prefer eating dragonflies over other insects while adults show no specific preference to dragonflies.

Purple martins rarely eat spiders Araneida and prefer any other insect instead. Generally purple martin diets consists of 23% wasps and bees (Hymenoptera), 16% flies, 15% assorted bugs like stink bugs (Pentatomidae) and black bugs (Thyreocoridae), and 12% were beetles (Onthophagus). Purple martins also eat butterflies and moths (Lepidoptera) but dragonflies seem to be preferred.

Purple martins are greatly dependent on the weather, since it has a profound effect on insect populations. At low temperatures insect food sources tend to decrease, where as at high temperature purple martins have an abundance of food. High velocity winds also decrease food availability. Purple martins tend to eat beetles throughout all the seasons, but flies tend to disappear from their diet in late August. Insect populations tend to be at its highest during August which coincides with greater nutritional need in preparation for fall migration. It is during August that purple martins must hoard up food and nutrients for the long flight back south. These birds consume water in flight by skimming any water source. (Brown, 1997; Campbell Finlay, 1976; Layton, 1972)

The most common predators for purple martins are owls and snakes which prey on both adults and juveniles. Owls attack while the birds are inside the nest. Owls grab the nest and shake it, which disorientates the purple martin. The bird then tries to slip out of the entrance hole where the owl grabs it with its claw. Humans can help prevent these owl attacks by attaching curved rods over nest entrances so that owls cannot perch atop the martin houses. Owls have also been known to reach their claws into martin houses to grab any unfortunate purple martin.

Predators like snakes or raccoons are able to climb the bird house poles and make their way to the entrance cavities. They pull out any adult birds and then proceed to eat the eggs. The snakes that tend to prey on purple martins are usually non-poisonous and often climb up the poles and eat both the eggs and young. Rat snakes are the most common snake predators. Hawks and blue herons are the only two predators that prey on purple martins in the air. Domestic cats prey on purple martins when they are on the ground in search of nesting material. Squirrels also prey on purple martins by climbing up the nest and entering the cavity. The squirrel then kills the young, breaks up all the eggs and can even occupy the nest to raise its own young.

One anti-predation behavior shown by purple martins is vigilant nest cleaning. Purple martin parents will eat fecal sacs and encourage juvenile birds to defecate by poking at their cloacal region. The feces will be either consumed or removed from the nest by dropping them outside. The elimination of feces and fecal sacs allows for protection, since the scent trails would be removed. (Brown, 1997; Kostka, 2000; Layton, 1972)

Purple martins respond to predator attacks by sending the 'zweet' call. 'Zweet' calls are used to warn other purple martins of the threat or to encourage them to fly away. Purple martins often dive bomb their attacker. Purple martin colonies have no coordinated response to predators. They do not all attack the predator but do assemble as a crowd to confuse predators and make it difficult to focus on one bird. The only birds that do attack the predator are the owners of a threatened nest. Purple martins will generally stay out of the predator’s way unless the predator comes within a few meters of their nest and young. Then purple martin adults have been known to attack the predator. Purple martins benefit from living in large colonies, because it adds to their protection and stability. Large colonies are able to detect predators faster, thereby decreasing predation. (Brown, 1979; Brown, 1984; Brown, 1997; Davis and Brown, 1999)

Purple martins play roles as predator, prey, competitor, and host. Purple martins are insectivorous and are known to be effective pest controllers. Purple martins are preyed upon by many species and also serve as a host to several species of parasites. Purple martin populations have been greatly affected by mite parasites, specifically Dermanyssus prognephilus that live inside their nests. These blood-feeding parasites are able to decrease clutch size and an outbreak may lead to colony abandonment. Other parasites include ticks, beetles, louseflies, fleas, and bowflies.

Purple martins have to compete for nesting sites with house sparrows and European starlings. Starlings often corner purple martins in their own nest cavities where fighting results and often ends in death. This competition is particularly unfortunate, as both house sparrows and European starlings are invasive species in the United States and often out-compete native purple martins for nesting habitat. (Brown, 1997; Kostka, 2000; Layton, 1972; Moss and Camin, 1970; Wade, 1966)

Humans benefit largely from the insectivorous food habits of purple martins. Purple martins consume large quantities of pest species including flies, stink bugs, clover weevil beetles, and mosquitoes. (Wade, 1966)

There are no known adverse effects of purple martins on humans.

The IUCN Red List has listed Progne subis as least concern. The United States Federal list has enlisted Progne Subis as being a species of concern. Currently several groups are working on conserving the natural habitat of purple martins. Under the Michigan Special Animal list, Progne subis is under no danger. Overall, purple martins have stable population numbers and inhabit a wide geographical range. In Canada, purple martin populations have seen some decline, and are currently considered at risk in British Columbia. Local populations have suffered greatly from weather related mortalities in the northern edges of the breeding range. Purple martins are also declining due to the competition for nesting sites. Purple martins compete with invasive house sparrows and European starlings for nesting sites. Currently in British Columbia they are setting up special nesting boxes in the hope of sustaining the remaining population. (Brown, 1997; Fraser, et al., 1997)