A Two-Chamber Rocket Box

Mark Kiser

Building upon the success of Dan Dourson’s and John MacGregor’s rocket box [TBHR, Spring and Fall 1997], BCI staff designed and tested the first two-chamber rocket box this year. Within three months of installation on Quadra Island, British Columbia, a maternity colony of Yuma myotis moved in. This was also the first rocket box to be installed on a metal pole, which adds strength and longer life. In addition to extra roosting space, the second chamber provides greater temperature diversity, allowing bats to select the most suitable temperatures. We thank the Grant B. Culley, Jr. Foundation for funding this project.

Exterior and bottom views of the new design are provided in Figures 1 and 2, and detailed plans are available upon request. Access ports in the inner shell, near the bottom of each side, enable bats to move between chambers. By extending the length of the inner shell three to six inches below the outer shell, additional landing surface area is created, permitting direct access to either chamber.

Rocket boxes are designed to slide over a wooden 4 x 4-inch post, which serves as both landing area and roosting surface. As an untreated post must be used (treated posts may contain chemicals harmful to bats), service life is shortened because the section of post below ground will eventually decay. By combining a four-foot section of a wooden 4 x 4-inch post (for the landing and roosting surface) with a metal pole, the life span of the bat house can be extended. We suggest using a steel pole with an outside diameter of 1 5/8 to 2 inches.

Starting at one end of the post, cut a 24-inch-long notch down the center with a circular saw. The width and depth of the notch will depend upon the size of the metal pole chosen. Use a jig saw or wood chisel to complete the cut. Secure the metal pole to the wooden post with 3 1/2 -inch screws or bolts. Because the actual finished size of 4 x 4-inch posts varies (typically they are 3 1/2 by 3 1/2 inches), measure the width of the post before cutting the pieces for the inner shell and adjust your measurements accordingly. Build the inner shell first, then measure and cut the pieces for the outer shell. Three-fourths-inch spacer blocks help maintain proper crevice widths.

Eager to try the new design, Honorary Research Associate Kent Borcherding of Hazel Green, Wisconsin constructed his house from BCI’s plans this summer. The house was occupied by a colony of 25 little brown bats within several months. Borcherding will test more of these in Wisconsin and Illinois next year, and we plan to test more in side-by-side comparisons with single-chamber rocket boxes.