A road-killed Great Horned Owl used to demonstrate the 8-12 inch reach these birds have. Note how far beyond the back wall of the 6" x 6" Trio aluminum LST-1 martin single this small male owl can reach in search of a meal. These nocturnal predators take a large number of martins each summer from colony sites throughout North America. Since all owls are federally-protected, martin landlords need to make sure all of their housing has been owl-proofed with guards. Photo by James R. Hill, III. Please note: Federal and state salvage permits are needed to possess dead birds of prey.

Purple Martins in houses with 6 x 6 x 6" cavities are at great risk of predation by Great Horned Owls (GHO's), a very common night-time predator of Purple Martins. GHO's are very common in rural, suburban, and urban areas. Wooden or aluminum martin houses with shallow (6-8" deep) cavities should be modified to make them extra deep (long). This can be accomplished by drilling a 2"connecting hole through the wall (1" from the floor) between two compartments, then capping one compartment. Martins will always build their nest in the "inner compartment" furthest from the entrance hole, because they instinctively know their nest/nestlings will be further from the reach of predators.

But even with deep compartments, owls can scare martins out by landing on the porch and making a lot of commotion, so active houses should also have wire hardware cloth guards to keep owls from landing on the porches and attempting to reach in. For a "how-to" article on this topic, go to http://www.purplemartin.org/update/Mod.html.

Increasing the size of the cavities makes the compartments more attractive to starlings (as well as martins), so you may wish to use starling resistant entrance holes (SREH’s) which also exclude Screech owls. See http://purplemartin.org/forumarchives/archive/SOPEH.htm. Active gourds should also be equipped with gourd guards to prevent them from being raided by owls. Go to http://purplemartin.org/forumarchives/archive/owlguards.htm for info on owl guards for gourds.

You should wait until just after the martins have hatched young before installing owl guards. Be prepared to attach them quickly by rehearsing their attachment prior to the martin season. Installing guards before housing is colonized might deter your chances of attracting martins because the investigating martins might perceive the guards as obstacles. However, once martins have learned to negotiate the guards, they can be left on permanently. Install the guards early on a nice day, a few days after the young have hatched. It may take an hour or so for the parents to re-enter the housing after the guards are attached. They will be suspicious, but their instinct to feed their young is very strong, and they will re-enter eventually. We have done this at the PMCA research site and it worked fine.

-Ken Kostka, Purple Martin Conservation Association