The purple martin (Progne subis) is the largest member of the swallow family in North America. These aerial acrobats have some unique nesting habits compared to other bird species. Purple martins do not build their own nests in nature but instead rely on cavities for nesting sites. Their nesting preferences have adapted them to be dependent on human-provided housing in many areas. Understanding where purple martins nest naturally gives insight into their biology and ecology.
What kind of nests do purple martins use?
Purple martins nest in enclosed cavities, either natural or artificial. They do not build an open cup nest like many other birds. The enclosed cavity provides protection from predators and shelter from the elements. Martins line the interior of the cavity with bits of leaves, grass, twigs, paper, or pine needles, but these materials are not structured into an elaborate nest.
Why are cavities important for purple martins?
Cavities offer key benefits for purple martins:
- Protection – The enclosed space helps keep predators like raccoons, owls, and snakes from reaching eggs, nestlings, and even adult martins.
- Temperature control – Cavities provide insulation to help maintain stable temperatures inside the nest.
- Shelter from elements – The cavity shields the martins from sun, wind, and rain.
Cavity nesting was a survival adaptation that evolved in purple martins. It continues to be an important part of their nesting biology today.
Natural Nesting Sites
Historically, purple martins nested in tree cavities made by woodpeckers and natural hollows in dead trees. Some martins still use natural sites today, but the numbers are declining.
Tree Cavities
Tree cavities excavated by woodpeckers, especially larger species like pileated woodpeckers, provide crucial natural nesting sites for purple martins. Martins compete with other cavity nesters like starlings, tree swallows, squirrels, and screech-owls for these sites. Tree cavities offer deep, protected spaces ideal for martin nests. However, dead trees are often removed from forests for safety reasons before martins can use them.
Cactuses
In the desert southwest United States, purple martins nest in cavities in saguaro cacti. Woodpeckers excavate these holes, which the martins then appropriate. Saguaro cavities are a rare and declining habitat for martins as cacti are slow growing and take decades to form suitable nesting holes.
Cliffs
Purple martins can occasionally nest in cracks and crevices in cliff faces. This may provide protection from some predators, but exposes the birds to weather and requires them to build a sturdier nest structure. Cliffs are not a preferred or common nest site.
Decaying Logs
In a few parts of their range, purple martins have been found nesting in downward-slanting hollows in decaying logs on the ground. These logs are in advanced stages of decomposition. Such nests are vulnerable to flooding and predators. Very few martins nest in this way.
Decline of Natural Sites
Natural nesting cavities for purple martins are disappearing across North America. Several factors are driving this:
- Habitat loss – Development, logging, and land clearing removes old trees with cavities.
- Competition – Other cavity nesting birds vie for the remaining natural sites.
- Predation – Squirrels, raccoons, and snakes prey on martin eggs and nestlings.
- Forestry practices – Removing dead and dying trees eliminates potential nesting cavities.
The decline in natural cavities contributed to the purple martin’s adaptation to using artificial nest boxes and gourds provided by humans.
| Habitat | Availability | Prevalence of Use by Martins |
|---|---|---|
| Tree cavities | Declining due to human activities | Uncommon currently |
| Cactus cavities | Rare habitat | Locally common where habitat exists |
| Cliffs | Limited locations | Rare |
| Decaying logs | Vulnerable sites | Very rare |
Human Structures for Nesting
As natural cavities became harder to find, purple martins adapted to nesting in human-made structures, especially nest boxes. This shift was vital to sustaining martin populations near people.
Nest Boxes and Gourds
Specially constructed nest boxes and gourds provide substitute cavities that martins have embraced. Placement near human habitation offers martins protection from some predators. Proper design is important so martins are sheltered but not trapped if predators attack. Routinely cleaning and maintaining nest boxes helps martins successfully raise young.
Bridges and Buildings
Purple martins sometimes nest under bridges, in weep holes in brick buildings, and occasionally in vent pipes, abandoned autos, and light fixtures. Such sites mimic natural cavities but can leave martins vulnerable if humans disturb the nests.
Native American Structures
Some Native American groups historically hung gourd houses to attract martins and protect local crops from insects the birds consumed. This practice created a model for our modern martin housing.
How Cavity Availability Affects Distribution
The availability of suitable nesting cavities strongly influences purple martin distribution and abundance. Martins congregate where prime nesting sites exist.
Westward Expansion
In past centuries, purple martins were mostly an eastern North American species. Westward expansion of European settlers brought deforestation that reduced natural cavities. But settlers also erected nest boxes, allowing martins to greatly expand their range westward.
Martins Near People
Today, most purple martins nest in rural areas and suburbs where people put up nest boxes. Urban areas with few houses or high-rise buildings offer limited sites. Deep wilderness with few old trees for woodpeckers to excavate has little martin habitat. Proximity to humans with nest boxes is key.
East vs. West Numbers
The highest martin breeding populations remain in eastern North America where cavity availability remains highest, though much reduced historically. In the mountainous west, suitable cavities are naturally scarce. Martins are far less abundant, nested mainly in human structures.
Conclusion
Purple martins nested naturally in tree cavities and cactus holes before European settlement. Habitat loss caused a decline in these sites. Martins adapted by shifting to nesting in boxes, gourds, and building crevices provided by humans. This allowed them to expand their range into western North America. Today, properly designed and placed nest structures are critical for sustaining martin populations near people across North America. Management and education continue to promote martin conservation.
