Reprinted from: Purple Martin Update 9(4): 1
Purple Martin Conservation Association
All active Purple Martin housing should be equipped with barriers to prevent aerial predators, such as owls, hawks, crows, and gulls, from gaining access to nesting cavities. Owls raid gourds by hovering in front of the entrance hole and grabbing it with one foot, then shaking it to scare martins out. Alarmed, the martins usually attempt to escape through the partially-blocked entrance hole and are captured with the owls free foot as they emerge. Hawks and owls can also reach inside to extract martins and nestlings. Repeated raids on martin nests by aerial predators can cause nest failure and, eventually, total site abandonment.
The key to preventing owls and hawks from raiding Purple Martin gourds is to prevent these predatory birds from hovering in front of and grabbing the entrance holes. This can be accomplished by extending one or more curved metal rods or other stiff material out over the entrance hole in an umbrella-like fashion. 1/4" steel dowel rod works well and can be attached to gourd rack arms with hose clamps. The rods should extend down far enough to bring the ends of the rods even with the bottoms of the entrance holes when viewing the gourd head-on, at eye level. The gap between the end of the rod and the entrance hole should be 6-8", enough room for a martin to fly up under the barrier, but too narrow a gap for a large hawk or owl. Smaller predators, like kestrels and Screech Owls, would not necessarily be stopped by these rods. Finally, the rods should be no further than 3-5" apart at the point where they curve down over the center of the entrance hole, when viewing the gourd head-on, at eye level.
The method of securing the rods to the gourd arm will vary depending on the design of the gourd rack. Rods can be secured to the PMCAs Deluxe Gourd Rack arms with hose clamps. After cutting 28-30" sections of 1/4" steel dowel rod, use a vice to bend the rods so that when one end is secured to the arm, the other end curves out over the entrance hole. Do not attach the guards directly to the body of the gourd since a predator landing on or grabbing the guard/rods would shake or upset the gourd.
People trying to attract martins for the first time may wait until their birds have laid eggs before attaching the guards. Have tools and guards ready so that you can attach them as quickly as possible. To be on the safe side, attach guard(s) to just one active gourd, then back off and observe how the parents react. Dont be alarmed if they are hesitant to enter at first. Their overpowering urge to incubate the eggs will conquer their suspicions. If, after an hour or so, the parents are unable to successfully negotiate the guards, lower the rack and bend the rod slightly upward to give the martins more room to fly up under them; then, re-raise the rack and try again. I used this method to attach the rods to one of the PMCAs gourd racks this summer after we discovered it was being raided nightly by an owl. The martins didnt appear to mind, and even seemed to enjoy using the rods as perches. However, I only extended a single rod out over the entrance hole and this did not seem to stop the owl raids entirely. The owl may have gone to one side of the rod; therefore, a guard may need to have two or three rods extending over each gourd. The PMCA will be testing different designs next season, and hopes its members will do likewise and share the results.
Crescent starling-resistant entrance holes (SREHs) might also help to deter owl predation by presenting the owl with a smaller, narrower entrance hole in which to insert its powerful talons. If a hovering owl were to grab a crescent SREH with one foot, there would be less room for it to insert its other foot to clutch martins; the SREH has about 20% less entrance area than a 2" round hole. Of course, a crescent SREH will also exclude Screech Owls, an occasional predator of martins that can enter a 2" round hole.
When martins nested in tree cavities, it must have been common for dead branches to break in storms, sometimes partially blocking their flight path to the entrance hole. The erection of these guards simply mimics that scenario as far as the martins are concerned. Once the martins have learned how to negotiate these barriers, they will not perceive them as a deterrent in future years, and you can leave them up at the beginning of the season. You might feel badly for making it a bit tougher for the martins to enter their gourds at first, but you can sleep soundly at night with the knowledge that large owls will find it very difficult to raid your gourds.
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