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How to Protect Tree-Mounted Nest Boxes from Snakes

Photo © Holly Grant

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. 

A Wily Predator

A Wily Predator

This nest box's predator guard was circumvented by a Western Ratsnake.

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. 

A New Snake Guard

A New Snake Guard

New research shows adding plastic sheeting (acetate) around a tree trunk can help prevent snakes from reaching nest boxes. Click to enlarge photo.

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.


Reference:

  • 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

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31 responses to “How to Protect Tree-Mounted Nest Boxes from Snakes”

  1. Chickadee Pam says:

    Thanks for this tip. We once watched a bull snake slither into a western bluebird box mounted on a ponderosa pine while the parents flapped about futiley

  2. Maureen says:

    This is great. I am wondering about how exactly the duct tape should be applied. I wish there had been more detail in regard to that. I wouldn’t want to harm my trees by causing the bark to get ill. I am wondering too if this wouldn’t work for preventing caterpillars from climbing trees?

    • DLS says:

      My guess is that it’s not applied directly to the tree, but the acetate wrapped as a tube around the tree and duct taped at the acetate seamline

    • Scott Kane says:

      In the 1950s, maybe even early sixties, trees in Ohio were similarly covered in burlap. Can’t remember; suggest purpose was to protect against caterpillars, possibly spores.
      This was from the Ford Foundation? A Time to Choose? You could probably find a record at Wooster; it was Ohio board of Forestry work.

    • Holly Grant, Project Assistant says:

      Hi Maureen, In the study, the plastic was wrapped around the tree trunk and taped along the top edge to the trunk. It didn’t mention whether the long edge was taped as well. It’s possible that there could be some moisture buildup under the tape, which can case rot/insect infestation (ants). There’s a product called tree wrap which has ventilation holes to prevent this, but it isn’t generally adhesive. Temporary tape might be ok, but it could peel young bark or trees with thin bark. It may be a good idea to apply the predator guard only while the box has an active nest, to give the tree room to breathe and grow.

  3. Barbara Tuset says:

    Thanks for the idea for preserving nest boxes, or natural tree cavities for that matter, from snakes. I am not on favor of using plastic of any kind so I’m looking for an alternative, how about recycling something already in our possession? My first thought is the heavy non-recyclable plastic bags from large dry dog food in place of the acetate sheeting. I think if the bags are washed then used with the slippery inner side facing outward it could be just as effective and give another life to an otherwise very wasteful throwaway material.

    • Holly Grant, Project Assistant says:

      Hi Barbara, This study only looked at acetate sheeting so we can’t speak to the effectiveness of other materials, but I like your idea of recycling this way! Whichever material you use instead, it would be important to have a smooth surface on the outside and one that is taut – any wrinkles or bunching would provide a way for snakes to gain purchase on the material. Acetate sheeting is firmer than most plastic bags which gives it this advantage. Alternatively, you can mount your boxes on poles relatively inexpensively. Here’s some information on how to do so.

  4. Glenn Stout says:

    Around our area of mid Michigan I’ve never had snakes in any of my boxes. I have 3dozen bluebird boxes around 140 acres of fields. All used to be filled with tree swallows, bluebirds, and some barn swallows. I used the love watching them fly around the tractor as I was cutting hay, feeding on the insects flushed out by the hay bind. The year after the authorities sprayed for the Zika mesquito the birds never returned. I am lucky to see one pair of each species. I suspect with the bugs gone, the birds are gone without proper feed.

  5. Linda Myers says:

    I’m wondering the same. Duct tape probably adheres ok to smooth-barked trees and utility poles, but a chunky-barked tree might be a challenge. It certainly won’t be attractive, though duct tape does come in a range of colors besides sheet metal gray. Butternut Brown, perhaps? Also, how does duct tape hold up in wet weather?

  6. Cheryl says:

    This might also prevent small critters like squirrels and chipmunks from climbing a tree they need to climb, e.g., which has their nest in it. Has that possibility been addressed?

    • Holly Grant, Project Assistant says:

      Hi Cheryl, Good question! This design only looked at prevention of predation on nest boxes. That said, squirrels and chipmunks can typically access much thinner and higher branches than snakes, and therefore can travel among treetops a bit easier. Regardless, it’s a good idea to install this guard only when there is an active nest, and to take it down once the young fledge. Alternatively, you can stick to mounting nest boxes on poles. Check out this FAQ for more info on installing a thin nest box pole.

  7. Greta says:

    In hot climates, like California, did they look at whether the plastic harmed the bark, and the tree? It seems considerable heat could build up underneath the plastic.
    Also, this guard should be removed when birds aren’t nesting, to prevent girdling the tree.

    • Holly Grant, Project Assistant says:

      Hi Greta, we agree, this guard should only be applied when the birds have an active nest, in order to prevent harm to the tree. The study was done in Spain, but they did not look at the effects of the plastic on the tree itself. Short term use should help reduce potential side effects.

  8. Rich says:

    Very interesting. Would help if you specified the type of sheeting and where it can be purchased. Also, will this also deter raccoons?

    • Holly Grant, Project Assistant says:

      Hi Rich, This study used 1mm thick acetate sheeting (flexible plastic). You can find flexible acetate sheets from many online retailers, like Amazon, and some hardware stores.

  9. Marty says:

    I have a nest box question.
    How do you attach a nest box to a tree without putting nails, screws, bolts, etc. into the tree itself?

    • Holly Grant, Project Assistant says:

      Hi Marty, Some monitors opt to use an adjustable metal band that can be secured around the trunk (belt-style), which the box is attached to. In this case, it would be important to consider loosening the band as the tree grows to prevent damage to the trunk.

  10. Frank says:

    What is the thickness of the acetate sheet, and where would one buy it?

    • Holly Grant, Project Assistant says:

      Hi Frank, the acetate sheets in this study were about 1mm thick. You can find flexible acetate sheets from many online retailers, like Amazon, and some hardware stores.

  11. Nancy says:

    So there is something living in my bird house on a pole. I will not see it for days and then it will stare at me for hours. I can’t get close enough to see what it is. And I never see it coming or going, just the head. This has been going on for weeks.I work remotely and he is just out the window.

    • cam says:

      Could it be a flying squirrel? They are nocturnal and do occasionally inhabit bird houses. We had one in our birdhouse and so has a friend in the area.

  12. Melanie says:

    I used chicken wire about 18 inches in length wrapped several times around the post/tree and cut many of the sides of it once wrapped so they stuck out like spikes. I attached it with twisties. So far so good and also keeps squirrels away as well !

  13. Jerry Gilnack says:

    Intresting, I have stayed away from tree or even wooden fence posts nrst box mounting just for the easy access preditors have. Even using pipes for poles is no gurarntee. In heavey snake areas I use stove pipe baffels to discorage preditors. This looks looks like it might be a good alternative to look into. Thank you

  14. Terri says:

    where do you buy 32″ sheets

    • Holly Grant, Project Assistant says:

      Hi Terri, You can find flexible acetate sheets from many online retailers, like Amazon, and some hardware stores.

  15. Carolyn says:

    My bluebird boxes are on fence posts with fencing hooked. How does this work for me??

    • Holly Grant, Project Assistant says:

      Hi Carolyn, We’re not aware of any predator guards that would prevent climbing predators from accessing fence-mounted boxes. Cone and stovepipe baffles work best for deterring climbing predators like snakes on thin poles, and the design discussed above works for thicker poles and tree trunks, but fences generally provide lateral access to the box which can be hard to prevent. You can try adding entrance hole-extender blocks or Noel guards to help guard against predators that might reach into the box (but which can’t fit their whole bodies), but if you want to prevent snakes specifically, we recommend re-mounting your box on a pole or tree so that you can apply the appropriate predator guard. Here’s an FAQ with instructions on how to install a pole in your yard relatively inexpensively.

      • Marcia Van Horn says:

        I commend you for what you do to encourage individuals to help cavity nesting birds, but I take issue with your instructional .pdf at your FAQ you link to which states that a predator guard is “optional….” If we lure birds by placing boxes, we, as landlord s who lure, are 100% responsible for blocking climbing predators with a proven method. The cone you utilize does not block the constricting rat snake, nor does it block a raccoon. Most wild bird store owners would instruct you not to use that guard against snakes and raccoons. For five years, on now over 200 poles, I have blocked climbing predators 100% of the time by essentially the same theory used on the tree in your article. I know this without even being there to witness the attempts through use of a lithium line detector on the 5-foot HVAC duct. We must first do no harm. Predator guards are not optional and only proven methods should be used. It may be a hobby for us, it’s life and death for the birds.

        • Holly Grant, Project Assistant says:

          Hi Marcia, We strongly recommend using predator guards on nest boxes, but it’s true that they are optional, because birds will be attracted to nest boxes without predator guards. Every predator situation is different, and some guards work in some areas where they might not in others. No box can be made 100% predator-proof, but the tips on our webpages are certainly great places to start. We’re glad to hear you’ve found a solution that works for your nest boxes.

          • Holly, I’m not sure I understand your statement that birds will be attracted to boxes without guards so guards are therefore optional. (“…they are optional, because birds will be attracted to nest boxes without predator guards.”) Also, certainly some guards may work in some areas where they might not, only because the predator species doesn’t exist there, but that doesn’t mean the guards will work in some areas and not others due to the fact that raccoons or snakes will alter their climbing methods. They will climb the same way no matter the habitat. Saying “no box can be made 100% predator-proof” is not what I have found so far with respect to the two main climbing predators. And, my boxes are located in suburban back yards, golf courses, and farms, i.e. different habitats. The predator guard you show in your video and in your .pdf does not guard against raccoons or snakes even though you call it a “predator guard.” Photos and videos show they don’t stop raccoons and snakes. Finally, to not place a guard on a pole is actually unnatural. A bird nesting in a cavity high in a tree is naturally guarded because it is harder to see and hear the nestlings and the feeding activity. To mimic the safety of that at a lower height in a box, we have to guard the box. Not using one boosts the risk of predation over a natural cavity, because the box is easier to spot by a climbing predator. It’s best practice to try to duplicate the natural living conditions, so not guarding a pole placed a level made easy for us, yet risky for them, in my opinion and through my experience, can harm.

  16. Alisa Dear says:

    We have a barn swallow pair that nests above our front door every spring/summer. A few nights ago our camera caught a rat snake climbing up and taking one of the baby birds. I know of no way to keep a snake from climbing up the side of a door – does anyone else?

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Cornell Lab of Ornithology

Cornell Lab of Ornithology