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Location

The top reason people fail to attract martins is that martin housing is not placed correctly, or the housing is placed in an inappropriate habitat. A martin house should be placed at least 30 feet from human housing and at least 40 feet from any trees that are higher than the house. Generally, the farther housing is placed from trees, the better. Purple martin houses should be pole-mounted at a height of 10-20 feet. Be sure not to allow any bushes, shrubs, or vines from growing against the pole. Also, do not attach any wires or strings to the house. Think open! Try to place the house in the center of the most open spot available in your yard.

Timing

It is best to put up martin housing about four weeks after the first purple martins have returned for the season. Martins typically return as follows:
Feb 1: Florida, Texas, Louisiana, Alabama, Georgia, Mississippi, South Carolina
Mar 1: N. Carolina, Tennessee, Kentucky, Virginia, Arkansas, Missouri, Oklahoma, California, New Mexico, Arizona
Apr 1: Pennsylvania, Ohio, Indiana, Illinois, Michigan, Wisconsin, New York, Eastern States, Oregon
May 1: Maine, Canada
The first martins that return, often called "scouts", seldom switch to new housing. Purple martins exhibit a high level of site fidelity, and typically return to the same housing year after year. Houses that are put up too early are often inhabited by competing birds such as sparrows or starlings. No matter where you live, be sure to keep your houses up through August. Martins may arrive as late as the end of June anywhere in the United States, and in July and August, the current year's young will be looking for next year's breeding sites.

Competition

If any other bird is allowed to claim unoccupied housing, any martins that may come around will likely not stay. Often house sparrows or European starlings will settle in unoccupied housing and defend their new homes. If this happens, it will be necessary to lower the house and clean out the new nests. Starling-resistant entrance holes may be used to keep starlings out. If other birds attempt to take over empty martin housing, temporarily block entrances with door stops or paper cups and then put up appropriate, single-unit housing elsewhere in the yard. Once these birds have accepted the new housing, reopen the martin housing. Remember that housing should always be closed up or stored inside for the winter to prevent wasps, squirrels, or other birds from claiming the house before the martins return.

Housing

Houses that are white or light in color seem to attract martins best. White houses reflect heat and tend to keep the nestlings cooler.
Minimum compartment size is 6" x 6" x 6". Larger compartments are better and offer greater protection from predators and elements. Entrance holes should be between 1 3/4" and 2 1/4" and should be between 1" and 1 1/2" above the floor of the compartment. Door plugs for these holes can be helpful when closing a house for the winter.
Because successful purple martin management often requires access to martin housing, it is best to purchase a house that may be raised and lowered vertically. Access may be required for competitor nest removal or periodic nest checks. Nest checks will not cause martins to abandon their nests and can help in identifying predation. Telescopic poles, lanyard systems, and winches can prove helpful.

Replacing Active Housing

Purple martins return to the same housing year after year. If the housing they are used to is gone or significantly altered, they may choose to abandon the site. If you wish to add or replace housing, it is best to put the new housing up close by for a season before removing the older housing. This gives the martins an opportunity to get used to the new housing and increases the chances for occupancy in the following year. Landlords with several houses can replace housing between seasons without risk of colony loss.

Predation

The most common reason purple martins abandon there nesting sites is predation. Predators can include owls, snakes, raccoons, hawks, crows, or even squirrels. A pole predator guard should always be used. Note that larger compartments can add extra protection from aerial predators such as owls and hawks.

References:

• The Purple Martin Conservation Association