Several devices have been designed to prevent predation in nest boxes by mammals and birds. Although snakes are one of the most common predators in cavity–nesters, they have always been difficult to deter. Here we tested a method originally designed to avoid predation by tree–climbing mammals. To prevent snakes from climbing trees and predating on nest boxes, we wrapped a transparent acetate sheet of 80 cm high around tree trunks below a sample of 40 nest boxes used by tits. The acetate sheets were secured with duct tape. The remaining nest boxes (N = 74) in the study area were left as controls. The predation rate in the experimental nest boxes was 20 % and 2 % in control boxes. This method can be useful to increase bird breeding success, improving both the effectiveness of resources to obtain scientific data and the breeding success of endangered species.