Predators Near the Nest: Bluebirds Weigh the Risks

Photo © JanetandPhil

by Robyn Bailey, NestWatch Project Leader

Oh, Rats!
Oh, Rats!

This rat snake found an Eastern Bluebird nest in Texas. Although named for their propensity to eat rodents, they are agile climbers and will also prey on birds' nests.

To Stay or To Go?

At NestWatch, we often get questions from participants about what could cause nest abandonment. These questions are particularly likely to be asked by those who have spotted a predator in their area shortly before the nest was abandoned. It’s logical to assume that if we see a predator in the area, the breeding pair has almost certainly seen it too, and perhaps has fled to safer ground. After all, predators are the leading cause of nest failure for most species of birds, and birds are therefore responsive to seeing or hearing predators in their nesting area. Birds can respond to nest threats in several ways: mobbing the predator as a means of defending the nest; abandoning the nest and starting fresh in a new area; or staying committed to their nest in the face of the risk. Each decision has consequences and trade-offs that affect the evolutionary fitness of that individual bird. The earlier in the nesting cycle, the less “investment” the bird has made in a particular nest, so choices made during the early nest-building stage are particularly interesting because they reveal the potential “costs” to birds and their ability to weigh pros and cons (e.g., is the predator likely to return?).

The Old Rubber Snake Trick

Recently, researchers designed an experiment to investigate the reactions of Eastern Bluebirds to a common nest predator, the black rat snake (Stanback et al. 2018). They used a rubber snake to simulate a predator on a nest box which contained a half-constructed nest, positioning the head in the entrance hole. The rubber snake was left in place for 15 minutes, and the nest was subsequently monitored for abandonment. The researchers were interested in determining if the bluebirds would abandon their nests after just one visit by a nest predator, or if they would stay invested in them. To control for any impacts of human presence, they presented control nests with a model of a Northern Cardinal, a harmless, non-predatory bird.

It is rather difficult to study real predators because they are wild creatures that don’t behave normally in the presence of people. While spooking birds with a rubber snake may sound mean, using simulated predators is actually a technique that has been used for decades to study predator-prey interactions, the evolution of defensive behaviors, and the benefits of cooperative breeding. Models of red squirrels, house cats, and owls have all been used to simulate predators in other studies.

By limiting the study to nests that were still under construction, the researchers avoided any potential harm to eggs or nestlings, while also allowing the bluebirds to make proactive choices (rather than reactive) about whether to abandon a nest. Nest boxes were paired (two boxes were placed 33 feet apart) so that birds could decide whether to stay, move to a nearby box, or leave the area altogether.

Too Close For Comfort
Too Close For Comfort

While a single visit from a predatory species may not deter nesting birds, you should consider relocating nearby food sources that would attract predators to nests. Frequent visits by a predator would almost certainly discourage nesting.

Surprising Results

The results obtained by Stanback and colleagues were counter-intuitive. The bluebirds were no more likely to abandon their nest after being presented with a rubber snake than they were when presented with a cardinal on their nest box. The bluebirds which were shown the cardinal even mobbed the cardinal in nearly equal numbers to those which were shown the snake. In fact, the majority of Eastern Bluebirds in the study continued to build their nest despite this visit by a nest predator. This suggests that if they actually had eggs or young in their nests, they probably would have continued to attend the nest after a real predator sighting.

What does this mean? Well, it’s always possible that the rubber snake was not realistic enough, and the bluebirds did not consider it a threat, but this would not explain why one-third of them mobbed the snake (or why 12% mobbed the cardinal, for that matter). Perhaps so many Eastern Bluebirds chose to keep their current nest sites because even a half-built nest represents an investment of time and energy that is not easily recouped. Furthermore, nest cavities may be limited, and a good nesting cavity on a good territory may still be better than a marginal nest cavity on a territory of unknown quality (even if such a cavity could be obtained later in the season). In any case, nest predators are usually common in most areas, and it may not make sense to abandon a nest if you’re just as likely to encounter a predator somewhere else.

Racer Versus Wrens
Racer Versus Wrens

Once a predator has caused a nest to fail, the nest site is unlikely to be used again in the near future. We'd recommend relocating such a box and attaching a predator guard.

When To Leave

A similar study was conducted in 2006 by Fisher and Wiebe, in which Northern Flickers were presented with a model of a nest predator (red squirrel) at their nest cavity. Northern Flickers excavate their own nest cavity, so abandoning the nest and excavating a new cavity would be even costlier for this species than it is for Eastern Bluebirds. Like the bluebirds, the flickers did not abandon their nest site after being exposed to a predator mount.

So to answer the common question of whether a predator observed near a nest box caused a nest to be abandoned, the answer is “probably not” unless the visit by the predator was extensive and particularly disruptive. In some cases, birds are known to seek out areas with predators for nesting due to their suppression of other predatory species. Eggs and nestlings may be abandoned for other reasons, such as inadequate food, poor weather, or the death of the parent(s). However, parents are very likely to avoid a nest site that has previously failed due to predators (a reactive response); so if your nest box has stopped attracting birds, you may want to relocate it.


References:

  • Fisher, R. J. and K. L. Wiebe. 2006. Breeding dispersal of Northern Flickers Colaptes auratus in relation to natural nest predation and experimentally increased perception of predation risk. Ibis 148:772-781. DOI: https://doi.org/10.1111/j.1474-919X.2006.00582.x
  • Stanback, M. T., N. A. DiLuzio, A. N. Mercadante, and E. S. Diamant. 2018. Eastern bluebirds (Sialia sialis) do not abandon their chosen nest site in response to a single visit by a nest predator. Wilson Journal of Ornithology 130(2):568-573. DOI: https://doi.org/10.1676/17-027.1

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5 Comments

  1. Carol Bledsoe says:

    Would a snake have suffocated young Bluebirds and left them in the nest?

  2. Terri Sandvik says:

    Since my picture was used in this article, I want to let everyone know that this box is mounted in a location where I am not allowed to use pressure guards.

  3. Terri Sandvik says:

    That autocorrect changed pressure to predater & I failed to notice.

  4. Rick B. says:

    I lost a nest of three young Eastern Bluebirds this year to a snake I found curled up inside the box. I had a conical predator guard attached to the post. So the only thing I can figure happened is the snake climbed out on a tree branch protruding several feet above the box, lowered itself downward, and finally dropped the last few feet onto the roof. From there, it would have been easy to climb inside the box. Snakes are pretty smart when it comes to figuring out ways to reach their prey. Make sure there are no ways for them to bypass the guard from above or from the side.

  5. Holly Faulkner says:

    Hi Carol,
    Snakes will generally access nests to eat the contents, and therefore it’s unlikely that it would have left your bluebirds in the nest. There are a number of things that can happen to a nest, and we have a handy troubleshooting guide to help nest monitors figure out what might have gone wrong here: https://nestwatch.org/learn/all-about-birdhouses/nest-box-landlord-trouble-shooting-guide/.
    -Holly Faulkner, Project Assistant

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