by Holly Grant, NestWatch Project Assistant
One of the best ways to prevent predators from accessing your nest boxes is to mount your box on a pole. This placement can make it easier to add predator guards, such as baffles, which have been shown to prevent climbing animals from accessing nests. Climbing nest predators in North America range from mice, to squirrels and raccoons, to cats and even chipmunks, but perhaps none draws as much public ire as snakes.
While adding baffles to your nest box poles is one of the greatest defenses against snakes, sometimes mounting boxes on a pole is simply not feasible. How do you protect a tree-mounted box, or those mounted on other wider structures, such as utility poles? New research points to a helpful new predator guard design.
In Defense of Snakes
Snakes are an often underappreciated group of animals. There are more than 3,000 species in the world; in North America, we have around 130 species. These reptiles mainly eat small mammals, birds, eggs, frogs, insects and other arthropods, so it comes as no surprise that snakes are a common nest predator for birds. For this reason alone, many people dislike them, but snakes are an important part of the ecosystem and can even be helpful to birds. How? While we may not want them depredating our nest boxes, having snakes nearby can act as a natural control and reduce the abundance of other nest predators, such as mice, chipmunks, small mammals, and even other snakes.
Having snakes nearby can be generally helpful, but it’s still a good idea to protect your nest boxes from predation. Some nest monitors have tried methods which trap the snake, but this can be potentially harmful to you (e.g., if you must handle the snake to release it) or the snake (e.g., if you do not find it in time). Snakes are also often legally protected wildlife (like birds) and trapping them should only be done in accordance with your local regulations.
A new guard for trees
A recent study published in Animal Biodiversity and Conservation discusses a simple new predator guard designed to prevent snakes from reaching tree-mounted boxes. Researchers in Spain tested the efficacy of an acetate sheet (transparent sheet of flexible plastic), that was 80 cm (31.5 in) tall and 1 mm (0.04 in) thick, by wrapping the sheeting around trees beneath mounted nest boxes. The researchers first cleared branches and bushes in a 1-m radius around each tree, and then waited until the host species (Great or Blue Tits) had begun laying eggs. They then added this acetate sheet to the trunk and affixed the sheets to the bark with duct tape. Forty randomly selected nest boxes were protected by this acetate guard, while the remaining 74 occupied boxes in their study were left unguarded as controls. The researchers used nest cameras to verify predator identities.
It’s a wrap
They observed a clear difference: the plastic sheet guards were significantly effective at preventing snakes from depredating the birds’ nests. The authors found that 20% of the control boxes were depredated (15 incidents), while only 2% of boxes protected by the plastic sheets were depredated (1 incident). In the one case where there was a successful predation event on a protected box, the authors suspect that the snake may have used a nearby tree to jump to the nest box, avoiding the plastic altogether. The authors were able to record a snake exhibiting this behavior elsewhere in the study area, to confirm that it was indeed a possible explanation.
How to Do It:
- Acetate sheets can help prevent snakes from accessing tree-mounted nest boxes. Use a sheet that is about 32 inches tall, and a width that will allow the sheet to wrap entirely around the trunk. Use duct tape along the top to secure the guard to the tree.
- If you have a tree-mounted box that is close to other trees or branches which a predator could use as a jumping-off point, add another acetate predator guard on those nearby trees. Be mindful of how the upper branches come into contact with other trees and vegetation as well.
- This guard can be used in combination with a Noel guard or entrance hole extender block to further restrict the entrance from birds, mammals, and other would-be predators.
The researchers also noted that snakes have long-term spatial memory, and so nest predation by snakes may increase over time in any given area once boxes have been discovered. Understanding the behavior of predators and how they fit into the ecosystem can help everyone live more harmoniously. We encourage NestWatchers to try this new method out, especially if you are looking to find a new way to prevent snakes from reaching your tree-mounted boxes, or those mounted on utility poles. Despite the improved nesting success documented with predator guards, it’s good to keep in mind that no predator guard is ever 100% effective, and it’s best to have realistic expectations that you cannot prevent all predators from getting at nests.
- Navalpotro, H., D. Mazzoni, and J. C. Senar. 2021. A plastic device fixed around trees can deter snakes from predating bird nest boxes. Animal Biodiversity and Conservation 44(1):103–108. https://doi.org/10.32800/abc.2021.44.0103