From: Ken Kostka, Purple Martin Preservation Alliance, Natrona
Here’s a way to modify a T-14 so that you can quickly attach/replace any type of
entrance hole adapter during the nesting season. Cut 4 in. wide x 3 in. tall
squares with a saber saw. Cut these holes so that the entrance holes will be the
proper height above the floor and porch. Take into consideration whether you are
using nest trays. You may need to reposition the porches. The advantage of this
setup is this: if you’re attempting to attract martins in an area of martin
scarcity, and want to offer a few round holes, you can easily switch to an SREH
if a pair of martins chooses to nest in a round-holed cavity. I usually wait
until there are young that are 7-10 days old. This strategy can be used on other
type of houses as well, both wooden and aluminum.
If you are replacing an old decrepit house containing a small number of nesting pairs, and want to start with mostly round entrance adapters to avoid making too drastic a change; you can then switch to SREH after they have eggs or young. (Note: Normally, we recommend erecting the new house in close proximity to the old and waiting at least one season, until a few pair breed in the new house before removing the old. However, this is not always possible.)
Be aware that starlings love the deep (11 inch) cavities in T-14’s, so I also make the round-holed cavities more shallow (and less attractive to starlings) by inserting a false back wall into the compartments with the round-holed adapters. I do this by wedging a board into the nest tray, about 6 inches back (and securing with 2 small screws). I also remove the artificial mud dam (piece of wood). When I replace the round hole with an SREH, I remove the artificial back wall and push the nest to the back of the nest tray. Eventually, I would end up with all SREH and deep cavities. If I were erecting the house at an already thriving colony site, or in an area of martin abundance where attracting martins was fairly easy , I would not bother to offer any round holes; I’d offer SREH only from the start. By the way, martins do sometimes choose an SREH even when round entrance holes are available.