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What Should You Do About Ants in Birds’ Nests?

Photo © Lisa Larson

by Robyn Bailey, NestWatch project leader

NestWatchers occasionally encounter ant colonies in bird nests and wonder if their presence is a problem for birds. The answer depends on where you are located, which influences the species of ants nearby. Over most of the United States and Canada, common ants like the odorous house ant (Tapinoma sessile) or the acorn ant (Temnothorax curvispinosus) can be found inhabiting birds’ nests. These ants may either colonize birds’ nests on their own, or the birds themselves may incorporate some ant colonies into their nests by picking up twigs or sticks which harbor ant colonies inside (Gibson et al. 2019). Most native ants are not directly harmful to eggs or nestlings, and may even be beneficial (Brown et al. 2015). 

Ants In The Nest

Ants In The Nest

Look closely and you can see that these Eastern Bluebird nestlings have ants crawling on them.

One study from Illinois examined 134 nests from 10 open-nesting bird species (i.e., those not nesting in cavities) and found that the presence of ants or ant colonies had no detectable effect on nest success across species (Gibson et al. 2019). The ants apparently did not suppress the numbers of other parasitic insects (e.g., mites, ticks, fly larvae) as hypothesized. They coexisted as roommates, taking advantage of the shelter and thermal environment.

However, Gibson et al. (2019) conducted their study in Illinois where aggressive ants were not found in the examined nests. European fire ant (Myrmica rubra), red imported fire ants (Solenopsis invicta), and Argentine ants (Linepithema humile) are non-native to North America, and their presence can lead to adverse outcomes such as nestling mortality, slowed nestling growth rates, and erratic incubation behaviors (Suarez et al. 2005, DeFisher and Bonter 2013). Some native species of ants will also prey on young nestlings.

Swarmed

Swarmed

European fire ants (Myrmica rubra) swarm a newly-hatched Herring Gull chick. Although such attacks can be fatal, researchers have found that about half may survive (DeFisher and Bonter 2013).

What can you do?

It’s important to remember that ants are not usually a major source of nest failure in North America, but they can be a problem in some situations. It is also more difficult to manage nests that aren’t in nest boxes, so we will limit our tips to nests in boxes. Below are some suggestions that NestWatchers have tried over the years, including some that weren’t favored.

Options for ants that seem to be coexisting:

  • Do nothing. The ants will eventually leave when they are done and you can clean the box later.
  • Once an active bird’s nest is finished, prop open the box with a stick and when the sun shines in, the ants will pick up their larvae and leave.

Options for predatory ants:

  • Avoid mounting nest boxes on trees when possible. Mount nest boxes on poles or posts with a baffle and create a physical barrier by caulking the gap between the pole and the baffle so the ants can’t get through. Just before birds start laying eggs, spray a few inches of the post/pole underneath the baffle with ant spray. The caulk keeps rain from washing it away. 
  • Sprinkle diatomaceous earth (sold at gardening supply stores) underneath the nest box at the beginning of the season. Diatomaceous earth is labeled as “Generally Recognized as Safe,” by the Food & Drug Administration, but you should follow label directions and wear a mask to avoid inhaling the dust (Bunch et al. 2013). 

Less preferred options:

  • Tanglefoot is a natural product that is sticky like tree sap. It can keep ants away, but it is messy and hard to clean. There is also a concern about the product getting onto other animals’ fur or feathers. Applying a key principle from healthcare ethics—“First, do no harm”—we’d suggest avoiding this one.
  • Cinnamon extract can be used as a natural ant repellent in an indoor environment. One nest box monitor and beekeeper mentioned that cinnamon is sometimes used to keep ants away from bee hives. He said he’s never had much luck with it, either for beehives or nest boxes. Given its dubious performance and unclear evidence of safety, it’s probably best to skip this method.
  • Some people have tried a thin strip of petroleum jelly underneath the baffle, but be advised that it will dry out and need to be reapplied every few days. It can also melt in hot weather and run down the pole, making a mess.
Defending The Colony

Defending The Colony

Many ants are not harmful and simply seek shelter in birds' nests. They can be bothersome to people who monitor nest boxes.

Unless you have a known nest of invasive, aggressive ants, we do not advise treating ant nests in your yard. Ants are an important prey species for many animals, including declining birds like the Northern Flicker. Eating poisoned ants could be harmful to native wildlife. Considering all sources of mortality, relatively few nests in birdhouses fail due to predatory ants, and it’s important to remember all of the helpful ecosystem services that ants provide (e.g., seed dispersal, recycling dead organic matter, prey for other animals).


References:

  • Brown C.R., C.E. Page, G.A. Robison, V.A. O’Brien, and W. Booth. 2015. Predation by ants controls swallow bug (Hemiptera: Cimicidae: Oeciacus vicarius) infestations. Journal of Vector Ecology 40(1): 152–157.  https://doi.org/10.1111/jvec.12144
  • Bunch, T.R., C. Bond, K. Buhl, and D. Stone. 2013. Diatomaceous Earth General Fact Sheet. National Pesticide Information Center. Oregon State University Extension Services. Available at http://npic.orst.edu/factsheets/degen.html
  • Gibson, J.C., A.V. Suarez, D. Qazi, T.J. Benson, S.J. Chiavacci, and L. Merrill. 2019. Prevalence and consequences of ants and other arthropods in active nests of Midwestern birds. Canadian Journal of Zoology 97(8): 696–704. https://doi.org/10.1139/cjz-2018-0182

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9 responses to “What Should You Do About Ants in Birds’ Nests?”

  1. Penny Brandau says:

    In northern Ohio we sometimes encounter ants in bluebird nestboxes . Almost without exception the ants seem to be found in nests which had wet decaying nest material or else a cracked egg or dead chick. I have always recommended a nest change for ant infestations to our monitors in order to remove the source of their attraction. We also apply plain Vaseline around the pole below the nest box to prevent the ants from easily returning . Is this advice incorrect?

    • Holly Grant, Project Assistant says:

      Hi Penny, Thanks for asking! The level of contact that people should have with wild birds in the name of conservation can be a contentious issue. Without careful research, it can be tough to decide what is helping wildlife and what might be inadvertently causing them too much stress. We are aware of at least one study that shows, at least for routine infestations of parasites, that replacing nests does not clearly confer a benefit to the host birds. Also, handling birds in general is strongly discouraged because it stresses them and can result in injury. We recommend the more hands-off suggestions above as the best way forward.

  2. Jim Adams says:

    My policy is to leave nature to nature and don’t interfere. That being said, I’ve never had a problem with ants in any of my bird, owl or bat boxes. This is something new to me

  3. Helen Raschick says:

    Don’t dismiss cinnamon. When we had a major infestation of Raspberry Crazy Ants in our camper we used cinnamon oil and ground cinnamon along interior edges and it totally worked!

    • Robyn Bailey says:

      Thanks for sharing this advice about cinnamon. Cinnamon has proven effective in indoor environments in laboratory studies. However its effectiveness outside under natural weather conditions is not well-studied. Given that diatomaceous earth is less expensive and is a common gardening product, we chose to emphasize that; but if you’ve tried cinnamon outdoors and had success, we’d like to hear that feedback as well.

  4. Sharon Emanuelli says:

    In the house and around the bin for yard and kitchen waste that is picked up by City Sanitation, I often use Terra traps that contain Boric Acid. Is this okay? Or does it cause problems?

  5. Theresa says:

    I was an Oceanographer and went over seas for months at a time. One year I returned after the time my purple martins would have fledged. As I went to clean out the purple martin house I noticed skeletons of baby birds. Then I noticed pieces of ant bodies. Fire Ants ate the chicks alive. Obviously the parents tried to kill them, but there were too many. Since then I put out fire ant bait to keep them out of the purple martin house. Two years ago I discovered crazy ants in my yard. I now have the yard around my house to keep the crazy ants from burning down my house. I ask the gentlemen spraying around my house to also spray around the base of the pole of the martin house. This should keep my martins safe. I hope. I live in Long Beach, Mississippi.

    • Robyn Bailey says:

      Were you able to identify the ant fragments as fire ants? Another possibility is that the chicks died of something else and ants (or other insects) scavenged them. It can be difficult to piece together what happened long after a nest has failed.

  6. Marcia Braun says:

    Here in Central Texas we treat the bottom of Eastern Bluebird nest poles for imported fire ants, as they will kill baby birds and force the adults to abandon the nest. We also had an unfortunate experience with Raspberry Crazy Ants. They invaded a Barn Swallow nest and left only skeletons of the young birds. We saw the ants and treated the bottom of the wall, but didin’t realize that they were going inside the wall and coming down from the attic. The nest has been successful since we treated the attic as well.

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Cornell Lab of Ornithology

Cornell Lab of Ornithology