SPECIES

Purple Martin Progne subis Scientific name definitions

Charles R. Brown, Daniel A. Airola, and Scott Tarof
Version: 2.0 — Published September 10, 2021

Demography and Populations

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Measures of Breeding Activity

Age at First Breeding

Both yearling males and females can breed. Floating populations of males and females exist (267, 209, 13), most of which are yearlings; the number of floaters that never find a territory and never breed is unknown. Some yearling males who hold territories with appropriate nest cavities may go unmated the entire season (266): 12.9% of yearling males in a Maryland colony went unmated (n = 179), versus only 3.6% of adult males (n = 167) and 0.6% of females (n = 302) (222). Some yearling male Progne subis hesperia held territories, but did not attract mates (13). These data suggest that males predominate among nonbreeders and that females may be limiting, although in a northern Texas colony no male went unmated the entire season, and there was an apparent floating population of both sexes (267, CRB).

Intervals Between Breeding

If nest sites can be found, birds apparently will breed each year.

Clutch Size and Number of Clutches per Season

Yearlings produce smaller clutches than adults. In northern Texas, nests attended by yearling males (who were assumed to be paired to yearling females; 274) contained a mean of 4.11 eggs per nest ± 0.71 SD (n = 121), compared to 4.97 ± 0.58 SD for adults (n = 214 300); in Alberta, yearling pairs had 4.29 eggs per nest ± 0.67 SD (n = 14), and adult pairs had 5.00 ± 0.63 SD (n = 40; 274); in northwestern Pennsylvania, yearling females had 4.43 ± 0.10 SD eggs per nest, compared to 4.92 ± 0.06 SD eggs for adult females (128). Older males with older females as mates have earlier first egg dates (31 May) compared to older males paired to young females (5 June). Similar pattern for young males (7 June versus 12 June) (128). No information on how clutch size may vary among older age classes.

Typically lays only one clutch per season. Replacement clutches are usually produced if nests fail during the first part of the nesting season.

Annual and Lifetime Reproductive Success

Annual reproductive success (number of young fledged) in northern Texas was 4.24 young/nest for adults (85.3% of clutch, n = 214) and 3.83 young/nest for yearlings (82.3% of clutch, n = 121; 300); in North Carolina, 3.42 young/nest for adults (n = 31) and 2.00 young/nest for yearlings (n = 10; 315); and in eastern Massachusetts ranged from 1.3 to 3.4 young/nest in different years for the parental ages combined (301). In northwestern Pennsylvania, overall brood size averaged 4.17 nestlings ± 0.05 SD (n = 905 nests; 128). In Alberta, overall average for all pairs was 3.4 young/nest (70.7% of clutch, n = 55; 274); in Missouri, 2.4 young/nest (50.0% of clutch, n = 45; 224); in Michigan, 3.0 young/nest (58.2% of clutch, n = 13) in one year and 1.2 young/nest (17.5% of clutch, n = 12) in another year with cold and wet weather (20); in Pennsylvania, 2.3 young/nest (49.8% of clutch, n = 1,215; 316); in central California, 2.3 young/nest (n = 63; 94); and in southern British Columbia, 3.1 young/nest (annual averages ranging from 2.1-4.0; n = 5,027; 52).

These figures approximate annual success for most pairs, since second broods are rare. For 6 pairs raising second broods in northern Texas (221), annual success was 8.3 ± 1.4 SD young fledged/pair. None of these estimates takes into account extra-pair fertilizations, which may produce substantial variance in success among males within a colony (197; see Behavior: Sexual Behavior). Intraspecific brood parasitism may also cause variance in female success.

No information on lifetime reproductive success.

Number of Broods Normally Reared per Season

Typically one brood reared per season. Reported to be double or triple-brooded, but these reports are not credible in the absence of banded birds (20). True double-broodedness (second laying by birds that successfully fledged young earlier in the same season) occurs rarely in the southern United States; reliably reported only in northern Texas (220, 221).

Proportion of Total Females that Rear ≥ One Brood to Nest-Leaving

Not known.

Life Span and Survivorship

The maximum recorded life span is 13 yr, 9 mo for a bird banded in Texas (317). In Pennsylvania the oldest known male was 9 yr, 2 mo, 6 d; oldest female, 9 yr, 1 mo, 14 d (P. Kramer, personal communication). The two oldest individuals in British Columbia were 11-yr-old females (52).

The apparent annual adult survival probability, determined using a 14-year banding data set for known-age birds (585 males, 392 females) in Pennsylvania, was best predicted by age and an age-and-sex interaction. Survival probability of one-year-old females (0.48 ± 0.03 SE) was lower than that of 1-yr-old males (0.59 ± 0.02 SE). Apparent annual survival did not differ for males (0.62 ± 0.02 SE) versus females (0.64 ± 0.02 SE) in birds 2–4 years of age (318). Variation in apparent annual adult survival probability was not explained by year, prevalence of West Nile virus, or the El Niño Southern Oscillation index. Birds at least 5 yr old showed about 20% lower survival compared with those 2–4 years of age, indicating possible senescence in survival (318). Annual survivorship, based on band recovery data from across eastern North America, was estimated at 60.9% ± 2.2 SE for adults and 32.2% ± 4.5 SE for first-year birds using program SURVIV (319), and estimated survivorship seemed to be constant across years for the period 1960–1984. At a northern Texas site, 3.7% of breeding males and 0.8% of breeding females (n = 241 birds of each sex) disappeared while nesting, presumably killed (214), and in North Carolina over 21-year period, an average 2.3% of birds (n = 506 birds) disappeared while breeding (320). These studies suggest that survivorship during the breeding season is high in the southern United States. Re-sighting probability for color-banded birds in a rapidly declining population of western Purple Martin in central California over a 6-yr span was 58%, with no difference between adults and yearlings (94). Annual survival in a growing population in southern British Columbia averaged 0.49 ± 0.025 SE over a 14-year period (52).

Apparent annual juvenile survival (n = 3,990) from 1998–2002 in Pennsylvania was 0.27 ± 0.027 (128). Likelihood of survival for fledglings to the onset of fall migration was estimated at 0.87 ± 0.03 SE. Juvenile survival during migration and overwintering periods was 0.32 ± 0.04 SE, suggesting that most mortality for juveniles occurs after they begin migration; fledgling survival probability decreased with later fledging date and increasing brood size (128).

Disease and Body Parasites

Diseases

Reported to suffer from viral avian pox (Avipoxvirus) in Arkansas (321), sometimes resulting in nestling mortality. In an outbreak of type C avian botulism in a colony in Indiana, most of the adults and nestlings died, and in Kansas, hundreds of birds were killed almost overnight by fungal infections of Aspergillus fumigatus (322). In central California, shortly after arrival of West Nile virus, 1 bird showed antibodies to West Nile virus and 22 birds did not (323, 94).

Body Parasites

The most common flea in eastern North America is Ceratophyllus idius; Ceratophyllus styx was reported in Quebec (324) and Ceratophyllus niger in western North America (325). Blowflies include Protocalliphora hirundo in Oregon and commonly Protocalliphora sialia (= splendida) in eastern North America (326). An unidentified lousefly (Diptera: Hippoboscidae) was reported on a Purple Martin in Pennsylvania (325). Hesperocimex sonorensis (Hemiptera: Cimicidae), an ectoparasite, was described from the nests of Progne subis hesperia in Sonora, Mexico (327). Dermestid beetles, especially Attagenus piceus, are often found in nests (328). Mallophaga include the amblycerans (Menoponidae), Machaerilaemus americanus and Myrsidea dissimilis, and the ischnocerans (Philopteridae), Brueelia subis, Brueelia gracilis, Brueelia longa, Brueelia brevipes, and Philopterus excisus (329, 330, 324).

Principal mite is Dermanyssus prognephilus (Dermanyssidae; 234), infestations of which can reach several thousand per nest. These blood-feeding ectoparasites can have serious effects on Purple Martin, causing reduced clutch sizes, mortality among nestlings, weight reductions in surviving birds, and colony abandonment (331, 332). The feather mites Scutulanyssus prognei and Scutulanyssus subis (333) and a new species of cheyletoid mite in Manitoba, Canada, also parasitize Purple Martin (334). The nematode Diplotriaena obtusa (Filariidae) and the trematode Plagiorchis maculosus (Plagiorchiidae) were found in the same bird from Ontario (335).

Purple Martin is hosts to the protozoan blood parasite Haemoproteus prognei (336, 123, 337) and have also been associated with Leucocytozoon sp. (338), Trypanosoma avium (339), and one unidentified filaria nematode species (337). An average 28% of birds in a Maryland colony were infected with Haemoproteus prognei (123). Haemoproteus parasites may cause substantial mortality among birds during their first migration and first winter, but among martins surviving to 1 year of age, the more heavily infested birds do not seem to be at any disadvantage (123). Haemoproteus prognei were found not to affect extra-pair paternity, mate-guarding, and feeding performance, except that feeding performance was reduced in subadult females (340). Nematode infections are also known to occur in Maryland, with average infection rate of 22%. Infection incidence of filaria is higher in birds at least two years old. Infected yearling martins exhibit reduced return rates (337). Adults with multiple infections (8%; Haemoproteus prognei and filaria nematodes) rarely survive (337). The Dermanyssus prognephilus mite is a vector for an Alphaproteobacterium, Bartonella, that is blood-borne and transmitted to martins; multiple variants of Bartonella have been identified in the eastern subspecies subis (341).

Except for mites and Haemoproteus, relatively little is known about the effects of any of these parasites on the Purple Martin. Prevalence and severity of Haemoproteus infection varies with the age of the host, the parasite type, and whether a bird is infected by single or multiple endoparasite species (337). When all ectoparasites were removed from nesting houses in Pennsylvania, parasite-free birds fledged 1.98 young/nest, versus 1.08 young/nest for naturally parasitized pairs breeding at the same time (342). However, in western New York when mites were reduced by replacing nesting material, nestling condition and survival of martins were unchanged (343). In northeastern Oklahoma mite abundance did not affect nestling body mass except at the largest brood sizes, and nestling survival was unaffected by the number of mites in a nest (234). The lack of a cost of parasitism may partly reflect a compensatory response by parents that increase food provisioning of parasitized nestlings (306).

Causes of Mortality

Exposure

Adverse weather may kill more Purple Martins than all other sources of mortality combined. Birds cannot find insects in cold weather, and when such conditions extend > 3–4 days, mortality can be substantial. Die-offs are regular in northern North America, especially in New England, the Ohio River valley, the Appalachians, and the Puget Sound region of Washington and British Columbia. A large die-off in eastern North America occurred in 1972, when a stalled hurricane in late June, producing continual and torrential rains, wiped out populations from Virginia to Massachusetts and west to the western Great Lakes (344). Between 90–100% of nestlings and thousands of adults died, and survivors deserted their colonies (345). Birds were absent from some areas for up to 10 years afterward, and martins still remain absent from some of the affected areas. A similarly massive mortality event occurred in New England in 1903 (269). Annual reproductive success can be sensitive to El Nino-related climatic variation during the breeding season, with fewer young produced in years with cooler nighttime temperatures and more rainfall (346). Weather-related kills occasionally extend as far south as northern Texas (CRB). Thunderstorms during migration and winter can lead to both minor and major mortality of birds at roosts, with regular storm-related kills documented at pre-migratory roosts in northeastern Oklahoma (206) and at roosts in Brazil during the overwintering period (347).

Predation

Predation by owls and snakes (see Behavior: Predation) can be locally significant; in some colonies, all nests are lost. Domestic cats can sometimes have large effects on Purple Martins when gathering nesting materials on the ground. Cooper's Hawk (Accipiter cooperii) can take most of the adults at a colony. As Cooper's Hawks have become more urbanized in the southern Great Plains in the last 15–20 years (348), they have reduced populations of Purple Martins nesting in cities such as Tulsa, Oklahoma, and may have selected for changes in martin behavior to be less conspicuous to predators (CRB). American Kestrel (Falco sparverius) have reduced colony sizes in Sacramento, California, perhaps in part because of landscape alterations that have made colonies more conspicuous to kestrels and removed alternate prey species (263, 93, 94).

Parasitism and Disease

Parasitism by mites (see Disease and Body Parasites) can lower fledging success (331, 342), but it is unclear how pervasive this effect is across the species' range. Nestlings are often found on the ground, apparently having jumped out of nests (or, more rarely, having accidentally fallen when parents enter the nest). In some cases jumping out seems to be a response to levels of ectoparasites in nests; in other cases it may be a response to hot weather that increases the temperature inside the poorly designed (metal) birdhouses to intolerable levels; and sometimes it may reflect predation on adults or lack of insect food that leads to nestling starvation. Disease outbreaks (see Disease and Body Parasites) seem localized and do not apparently affect a large fraction of the total population.

Competition with Other Species

Eggs are often lost to House Sparrow (Passer domesticus) and European Starling (Sturnus vulgaris) that compete with Purple Martins for nest cavities (see Behavior: Social and Interspecific Behavior). Nestlings can be killed or wounded by wandering House Sparrows and especially European Starlings that attempt to usurp nests later in the summer. European Starling (and rarely House Sparrow) kill adult Purple Martin by cornering them in nest cavities (240). The European Starling indirectly causes additional egg losses when martins inadvertently kick their own eggs out of a nest (224, CRB) in responding to alarm calls elicited by starlings (or other potential nest predators). Almost all martin nest sites in eastern North America are at potential risk of losing eggs or nestlings to House Sparrows or European Starlings unless the housing is equipped with modified entrance hole shapes to exclude starlings. The European Starling and House Sparrow also constitute a threat to most western populations, except those nesting in habitat types that support no or low starling and sparrow populations, such as dense forests, shrublands, open water, urban bridge sites, and coastal locations (101, 51, 52). The largest effect of these species, however, is usurpation of nest sites and prevention of breeding by Purple Martins (see Conservation and Management: Management).

Other species, such as House Wren (Troglodytes aedon), occasionally destroy eggs of Purple Martin in competition for nest sites (see Behavior: Social and Interspecific Behavior), but the overall effect of these species on Purple Martin populations is likely negligible. Tree Swallow (Tachycineta bicolor) and Violet-green Swallow (Tachycineta thalassina) return to breeding areas in the Pacific Northwest before the Purple Martin and can defend occupied boxes from martins, but because Tree Swallow and Violet-green Swallow are territorial, their effects are minimal (52). Western Purple Martin is dominant over Northern Rough-winged Swallow (Stelgidopteryx serripennis) and White-throated Swift (Aeronautes saxatalis) at California bridge sites and thus rarely suffer negative effects from these species (250, 255, 94).

Direct Human Impacts

In areas with high use of neonicotinoid insecticides, such as Sacramento and the Lava Beds National Monument in northern California, Purple Martins have declined dramatically and face extirpation (92, 94). Disturbance of nest sites during urban transportation and development projects in the bridge-nesting population in Sacramento poses threats to remnant colonies there (93, 94).

The effects of climate change on the Purple Martin are not known. However, population models for the eastern subspecies under different climate scenarios suggest a possible increase in abundance of 2.4–12.3%, depending on the model used (349). An increase in abundance in the Gulf states, Arkansas, and the Ohio and Tennessee river valleys, and some loss of abundance in the Upper Midwest and Missouri, Kentucky, and Indiana, are predicted. However, since one of the predictors used in the model was the distribution of a particular oak (Quercus muehlenbergii), a species that would seem unimportant to eastern martins, the methods behind these models (349) may be suspect.

Population Spatial Metrics

Individual Distance

Lowest for nonbreeding birds, and increases on average during the nesting period. In pre-migratory and overwintering roosts, individuals perch as close as 5–6 cm apart (72, CRB). When perching on wires near nesting houses, males defend space around females up to a 1-m radius while mate-guarding. In saguaros, martins defend regions around the nest and keep all intruders out of this zone (13). After nesting is completed, individuals often perch together closely on birdhouses with little aggression; exhausted migrants sometimes huddle together, touching one another upon making landfall after a presumed trans-Gulf crossing (137). Eastern and western Purple Martins will tightly pack themselves into compartments or nest boxes with other martins during adverse cool or wet weather when the risk of starvation is high.

This species exhibits relatively little aggression toward conspecifics in all contexts, except when intruders enter nest compartments and when older birds attack recent fledglings (310); residents sometimes perch within a few centimeters of an intruder outside a nest compartment without attacking it. Even displacements of birds from perches at roost sites are nonviolent and do not involve the high level of physical confrontation seen in smaller swallows such as Cliff Swallow (CRB).

Territory Size

In eastern populations, individuals defend territories that often consist of multiple compartments within a birdhouse, and occasionally multiple birdhouses or gourds (208). Subspecies arboricola nesting in clustered single nest boxes < 0.5 m apart in British Columbia may defend one to several (2–3) nest boxes at the start of the season, but only rarely more than one by the time they are feeding young (B. Cousens, personal communication). In study areas supporting dense saguaro cacti in Arizona, subspecies hesperia males defended much larger areas, in some cases controlling extra nest cavities up to 115 m away; territories have an average radius of 20–30 m from the chosen nest cavity (13). Recent studies by Tucson Audubon Society, however, in areas with sparser saguaros have identified 2 pairs nesting in 9 of the 110 saguaros used for nesting (i.e., 15% of the nesting population), while no multi-pair nest saguaros were found in the areas previously studied by B. Stutchbury (J. MacFarland, personal communication) These results suggest that territory size may decrease when nest sites are limited. For further details, see Agonistic Behavior: Territorial Behavior.

Home Range Size

No information on how far birds range looking for nest sites in the spring or how far breeders typically travel from nest sites while foraging. Homing ability of breeders is well developed over moderate to long distances, suggesting that birds may be relatively familiar with a large area surrounding their breeding site. In experiments in Michigan, 80% of displaced birds (n = 108) returned to their nest sites from distances ranging from 2.8 to 664 km (350). One bird released 374 km away returned to its nest in 8.6 h, flying mostly at night (199). The percentage of birds returning declined with the distance at which they were released from the nest site: for birds released ≤ 40 km away, 100% returned; 41–240 km, 94.1% returned; 241–400 km, 76.7% returned; 242–480 km, 66.7% returned; and for 243–950 km, 60% returned (350).

Population Status

Numbers

Using data from the North American Breeding Bird Survey (BBS), Partners in Flight (351) estimated the mean annual population (2006–2015) for Purple Martin at 8,700,000 individuals (95% Confidence Interval: 7,800,000, 9,800,000) in the United States and Canada, with 8,400,000 for the United States and 320,000 in Canada. Extrapolating the United States/Canada estimate to Mexico, the global population was estimated at 9,300,000 (351).

Highest population estimates for Bird Conservation Regions (with percent of global population) include: 2,000,000 individuals (22.0%) in the Southeastern Coastal Plain, 840,000 (9.0%) in Central Hardwoods, 760,000 (8.2%) in Mississippi Alluvial Valley, 730,000 (7.8%) in Oaks and Prairies, and 730,000 (7.8%) in West Gulf Coastal Plain/Ouachitas (351). Abundance maps based on eBird data can be found here.

Trends

Data from the Breeding Bird Survey from 1966 to 2019 (n = 2,441 routes; 352) suggested that populations in the United States and Canada have experienced a significant long-term decline of 0.54% per year (95% Confidence Interval: –0.87, –0.26). Over a 45-year period (1970–2014), BBS data indicated that the population declined by an estimated 23% (353). Before 1980, populations in much of the continent had been stable or increasing, but significant declines are now evident in the following Bird Conservation Regions: Boreal Hardwood Transition, Lower Great Lakes, Atlantic Northern Forest, Prairie Hardwood Transition, West Gulf Coastal Plain/Ouachitas, Southeastern Coastal Plain, and Appalachian Mountains (352). Significant increases have occurred only in the Central Hardwoods, Edwards Plateau, Tamaulipan Brushlands, Prairie Potholes, and Northern Pacific Rainforest (352).

The historical population status of the western Purple Martin is incompletely known. Available accounts suggest that populations were localized but more widespread and abundant than at present (100, 102, 50, 51). Purple Martins were nesting in buildings in Sacramento, California, as early as 1867 (98, 99). Populations in west coast states and British Columbia likely increased in response to availability of nest sites in buildings and bridges until the 1970s, before European Starling breeding populations established and increased rapidly. Populations subsequently declined dramatically in many areas, presumably due to starling competition (90, 101, 50, 51). BBS data show significant declines for western martins in the Sonoran desert and Sierra Madre Occidental and significant increases in the Northern Pacific Rainforest and the Southern Rocky Mountains (352). The population in coastal British Columbia increased from 5 nesting pairs in 1985 to about 1,200 pairs in 2016–2018 (52; B. Cousens in 354). The breeding population in California was estimated as 950–1,850 nesting pairs in the mid-2000s (51). The species has been extirpated from Lava Beds National Monument, and populations in Sacramento have declined by > 80% (94). No comprehensive estimates exist for the Rocky Mountain and Intermountain regions, but 250–500 pairs were estimated to nest in Colorado over 20 years ago (355, 356).

Little is known about causes of population changes in this species, although weather probably plays a role at least in northern North America. Anecdotes suggest that the Purple Martin was more abundant in the 1800s (357); present populations may be lower because of nest-site competition from introduced House Sparrows and European Starlings. Overall availability of nest sites (birdhouses) now may be greater than in the 1800s. There is no evidence that birds from the declining more northerly breeding populations overwinter in different habitats than those from the more southerly breeding populations, and thus habitat on the overwintering range does not seem to be driving population declines (28). Population modelling of the 2006–2009 Sacramento, California, population with known adult survival rates suggested that low reproduction and subsequent low survival of juveniles contributed to rapid population decline there (94).

Population Regulation

Few specific data on broad-scale factors that regulate populations, but see Causes of Mortality. Periodic weather-related kills clearly regulate Purple Martin populations in northern North America, these events occurring frequently enough to prevent major population increases and to cause local extinction. As a secondary-cavity nester, the species historically was probably limited by availability of nest cavities; the presence of floaters (see Measures of Breeding Activity) suggests that breeding sites are still limiting. Competition with the European Starling for nest sites has caused localized extirpation in the East and over large areas in lowlands of the West and continues to prevent population recovery.

Recommended Citation

Brown, C. R., D. A. Airola, and S. Tarof (2021). Purple Martin (Progne subis), version 2.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.purmar.02