Predator guards for nest boxes minimize predation of eggs, nestlings, and adult birds at the boxes. Predation is a major cause of mortality among cavity nesting birds. We monitor nest boxes on four Noble Research Institute farms. Our primary nest box predator is the black rat snake.
We also experience occasional predation from the Great Plains rat snake, domestic cat, raccoon, and probably other animals. Sometimes new nest boxes receive relatively little predation during the first year. When nest boxes are present continuously in the same area, predation generally increases dramatically by the third year.
We use metal cone predator guards successfully with eastern bluebird and wood duck nest boxes. The figure with this article illustrates a metal cone predator guard. The bottom of the metal cone should be at least 3 feet above the ground or water. I prefer to place a cone higher than 3 feet. When I use hose clamps to fasten a metal cone guard to a pipe or round post, I fasten a clamp to the pipe where I want the guard; next I slide the guard over the pipe and jam it against the clamp; then I fasten a clamp on top of the guard over the cut strips of metal; and finally I bend the metal strips over the top clamp.
The metal cone guard significantly reduces predation, but does not eliminate it. Some large rat snakes manage to circumvent it. When a snake overcomes a metal cone guard, we sometimes add a nylon mesh cone below the metal cone to deter or trap the snake. This addition seems to help. I plan to try some metal cone guards with a larger diameter, but I do not know whether they will be practical.
We have tried several other predator guards with eastern bluebird nest boxes, including nylon mesh cones by themselves, greased poles, pepper extract treated poles, and electrified wires. Greased poles and pepper extract treated poles provide relatively little deterrent to black rat snakes in our situation so we quit using these techniques.
Nylon mesh cones by themselves significantly reduce predation, but trap snakes by entangling them in the mesh. The nylon mesh cones should be checked frequently to remove snakes, or birds may abandon nests and snakes may die. Rat snakes are desirable parts of the ecosystem, so we prefer to avoid killing them.
Placing nest boxes on posts of electric fences or wrapping electrified wires around posts or pipes below nest boxes have successfully deterred predation in the relatively few nest boxes that we protect in this manner. This method does require sources of electricity at every nest box. Most existing nest boxes don't have handy sources of electricity.
For more information about nest box management, feel free to contact one of our wildlife and fisheries specialists.