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Almost every country tavern has a martin box on the upper part of its sign-board; and I have observed that the handsomer the box, the better does the inn generally prove to be. — John James Audubon (1831)
Audubon’s observation illustrates the long and close association between humans and the Purple Martin, a relationship that distinguishes this bird from nearly all others on the North American continent. Extremely popular and well known to much of the public, this species in eastern North America now breeds almost entirely in backyard birdhouses. Its conversion to human-made martin houses from ancestral nest sites—abandoned woodpecker holes in dead snags—was almost complete before 1900; only a few records of natural nestings east of the Rocky Mountains have been reported during the twentieth century. Yet in the mountain forests, deserts, and coastal areas of western North America, where the species is less common, it still nests almost exclusively in woodpecker holes or natural cavities. Few other species show such a marked or abrupt geographic difference in use of nest sites.
The Purple Martin’s popularity as a backyard bird has spawned a flood of literature on the species, a profitable industry in birdhouse manufacturing, and two national organizations in which martin enthusiasts regularly communicate their observations via newsletters.
This interest means that it is sometimes difficult to separate fact from fiction in the enormous volume of published material about the Purple Martin, much of which uncritically repeats earlier misstatements. This species is probably second only to the more widely distributed Barn Swallow (Hirundo rustica) as the most thoroughly studied swallow in the world.
The largest swallow in North America and among the largest in the world, the Purple Martin is the northernmost species of a group of closely related species whose systematics remain unclear. The genus Progne is widely distributed throughout the Americas, and all of its members share similar morphology and behavior. At least eight different species have been described at various times, but their relationships remain uncertain. The difficulty in identifying the different species in the field, and the varying taxonomic treatments, have confused the status of each in regions of Central and South America where they overlap during at least part of the year. Consequently, the Purple Martin’s migratory routes, the southern edge of its breeding range, the extent of its wintering range, and its extralimital occurrence are not known with certainty.
Surviving on a diet consisting exclusively of flying insects, the Purple Martin is not well suited to the climatic regime of middle and northern North America. The species has been recorded as far north as northern Yukon, northern Alaska, and central Labrador, but the more northerly populations are small and ephemeral. Martins are highly vulnerable to spells of cold and rainy weather during spring and early summer, conditions that temporarily reduce their insect food supply. Periodically, regional martin populations as far south as the mid-Atlantic states may be eliminated or reduced by cold weather.
As a secondary-cavity nester, the Purple Martin has also suffered from the introduction into North America of European Starlings (Sturnus vulgaris) and House Sparrows (Passer domesticus), which compete with it for nest sites throughout much of the eastern half of the continent. Without human intervention and management of colony sites, starlings and sparrows can cause local extinction of martins by appropriating their nest cavities and making them permanently unsuitable for martin use.
The relatively recent conversion of Purple Martins to artificial nest sites in most of its range has probably affected its social behavior. The species is often considered “colonial” because multiple pairs nest in the same or adjacent birdhouses, but the western and Mexican populations frequently nest solitarily, and the Purple Martin’s behavior is in many respects similar to that of swallows that nest solitarily. The advantages and disadvantages of different colony sizes in Purple Martins have yet to be studied. Such research would likely reveal whether these birds experience a net cost or benefit from colonial nesting.