The Great Horned Owls is a known predator of Purple Martins. It is easy to determine if an owl has visited a colony site. Because of its large talons and weight, a great Horned Owl will greatly damage a Martin house. When an Owl lands on a Martin house, the house floor and/or roof will be dented and bent, and entrance doors ripped open. In more costal regions of the Martin's nesting territory, owl guards have also proved effective at guarding Martin nest cavities against the Fish Crow. An owl guard protects your birds against Owl or Fish crow attacks, yet it does not disturb Purple Martins. Allows access to compartments for nest checks and clean out. recommended installation after Martins have nested