Improving A Trio Castle:
How to Enlarge the Compartments and Add Owl Guards

Ken Kostka
Purple Martin Conservation Association

Most commercially-manufactured houses have 6" x 6" x 6" cavities. Plans for homemade houses usually call for the same small cavity dimensions. This is unfortunate because martins lay larger clutches, fledge more young, and are much safer from predators in larger, deeper compartments. One reason larger compartments are preferred by martins is the greater protection such compartments afford from predators like hawks, crows, raccoons, and owls. Martins will always build their nest at the furthest point from the entrance hole because they instinctively know their nestlings are as far as possible from the reach of these predators. Nestlings are also are less exposed to driving rains and winds, further enhancing their chances of surviving to fledge. Ideally, compartments should be at least 10-12" deep. Height and width are not as critical, 6" x 6" being adequate.

Enlarging Trio Compartments

Despite this knowledge, most commercial martin house manufacturers continue to make houses with 6-inch-deep cavities, and many plans for homemade houses continue to call for equally shallow compartments. In such housing, it's an easy matter for an owl or raccoon to reach in and extract a meal of martin nestlings. Fortunately, most houses can be modified by drilling holes in the partitions with a wood or metal hole saw. Figure 1 shows a Trio Castle being modified, in position, on the pole, by drilling a hole through the rear corner of a wall partition using a drill with a 2" hole saw. The basic idea is to convert two cavities into one by making a hole in the partition between them and plugging the entrance hole of one of the compartments. Open entrance holes should be spaced as far apart as possible to prevent male porch domination. Keep this in mind when enlarging the compartments and deciding which compartments to plug. In metal houses, be sure to file any sharp edges created by drilling or cutting.

Fig. 1. Enlarging a Trio Castle compartment, in position, by drilling a 2" hole through an interior wall partition.

Metal or plastic commercial houses are easiest to modify while they're being assembled, or after disassembling, but can be converted at any time. The Trio Castle can be disassembled by removing the long rods which run vertically through the house (assuming the nuts haven't rusted) and unbending a few tabs. It's then an easy matter to cut doors in every other partition with a pair of tin snips, or to drill holes on a drill press. Any size internal doorway from 2" to 4," square or round, is adequate. Warning: starlings will attempt to nest in houses that are modified to have enlarged compartments. (One advantage of shallow compartments is that they are usually not attractive to starlings). Be prepared to deal with the starling threat by trapping, shooting, nest removal or the use of starling-resistant entrance holes (SREH's).

Homemade Owl Guards

Even with deep compartments, the Trio Castle should be equipped with a homemade owl guard, because while an owl might not be able to reach the nestlings, the commotion created when it lands and attempts to extract the martins often causes the adults to flush from the cavity and be captured. An owl guard prevents the owl from landing on the porches and creating this commotion.

Fig. 2. Metal fencing cut to function as an owl guard for a Trio Castle. "Please note: We now recommend that the 2"x 4" hardware cloth guard wire be oriented so that the openings are 2" TALL and 4" WIDE." Fig. 3. Homemade owl guard attached to a Trio Castle with binder clips.

An effective owl guard can be constructed from 12 or 14 gauge hardware cloth (2" x 4" rectangular holes) available at most hardware, or lawn & garden stores. A 7-foot-long x 3-foot-wide piece is needed. It can easily be bent to fit snugly around the house (see figures 2, 3, 4). You may wish to cut it in half, making removal for nest checks easier. One half will cover three sides of the (six-sided) Castle. The rectangular mesh should be aligned so that it is 4" tall and 2" wide. Use enough 1&1/4" binder clips (an office supply item) to attach the guard to the porch rails securely on all sides (see figure 4). The binder clips fit nicely over Trio guard rails and secure the owl guard tightly, but are easy to remove for nest checks and monitoring. Don't install the guard on a house that hasn't yet attracted martins because investigating martins might consider it an obstacle. In a house that has martins nesting for the first time, wait until eggs are laid, or the young are a few days old before attaching the guards. In succeeding years, the owl guard may be kept on throughout the season since martins will already have adapted to it. Don't wait for disaster to strike. Add owl guards this season!

Fig. 4. A close-up of one of the 1&1/4" binder clips (an office supply item) that securely attaches the homemade owl guard to the porch rails of the Trio Castle martin house. At least 6 clips are needed and allow for quick removal for frequent nest checks.

Copyright 1999 by Purple Martin Conservation Association. All Rights Reserved.

Our members benefit from 4 issues annually, packed full of helpful and fascinating information like the article above. You can become a member and support the work of the PMCA by making a tax-deductable donation.