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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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36 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

  6. Heidi from Maine says:

    Pretty much the only predator I have are the House Sparrows!!! I’m ok for their first clutch, but after that it’s a challenge. I will take all but 2 Nesting boxes down…last year I wrapped the corners with shiny tinfoil. The Bluebirds don’t seem to care…I just have to say this, which I’ve witnessed….House Sparrows are smart and will get pissed off and try to find ways to get into the Nesting box. Be aware and if it gets too stressful for the Bluebirds to nest take the Nesting box down….the outcome probably won’t be a good one.

  7. Miranda says:

    I just found a birds nest in my shed a few days ago, I came out to check on it today and the baby birds and the nest were on the ground. I carefully put the nest and babies back. I’m unsure of what knocked it over, we have chipmunks and squirrels in there frenquently but we also have an outside cat. I’m hoping it wasn’t her… anyway, my question is would the parents come back if they hadn’t noticed the accident yet? The babies are a few days old. Could I feed them? I see the mother come back with moths and worms so I know their diet.

    • Holly Faulkner, Project Assistant says:

      Hi Miranda, Only veterinarians or federally licensed wildlife rehabilitators can legally handle and treat wild birds, and the nestlings’ chances of success are very low when raised by humans. Our best recommendation is to leave the nest be. If you don’t see the parents return within a few hours, then you should contact your local wildlife rehabilitator for further advice. We also recommend keeping cats indoors, or in enclosed outdoors spaces such as “catios,” as they are a common predator of birds and their nests. You can learn more about how to guard against predators on our Dealing with Predators webpage.

  8. MMM,risky proposition for the poor birds.

  9. Lisa Bauer says:

    Hi! Just this week, I’ve had a rat snake and a mouse visit my bluebird house. I got rid of the snake while the birds weren’t around. However, The mouse was inside the nest and it did not look like there were eggs yet. The birds were flying around the box trying to get him out but he wasnt leaving so I intervened by opening the side flap. It didn’t seem to phase them because they’ve been coming back the last 2 days, but they will not enter the birdhouse. Are they signaling they’re not sure about nesting here again? Also, how well do noel guards work and is there a certain point I should attach to birdhouse?

    • Holly Faulkner, Project Assistant says:

      Hi Lisa, Noel Guards should be installed before the breeding season begins, or at least before the box is being actively used. There’s no way to tell whether the birds will re-nest in the same box, but it’s a good sign that they’re still on or near the box. Often, however, predator attacks will cause the adults to abandon the nest attempt and start again elsewhere. Our best advice is to give them plenty of space and wait it out to see if they will begin to nest again – it’s possible they may be sneaking into the box when you are not looking. Be sure to only check the box every 3-4 days, and keep your visit to 1 minute or less. Email us if you have any questions at nestwatch@cornell.edu.

  10. Cory Delvecchio says:

    I have a robin’s nest in the backyard and last night I noticed the birds were dive-bombing the nest so I went out to see what was going on and there was what I believe is a bull snake in the nest. I grab the hose and squirted a little water up towards it just to get the snake out which worked. I saw a baby sitting on the side of the nest so I left it alone went back today to check on the nest and it doesn’t seem like the baby or the mom or dad red Robin are around. what a robin move their babies or just abandon the nest altogether because it was disrupted and so traumatic for all of us?

    • Holly Faulkner, Project Assistant says:

      Hi Cory, Most adult birds are unable to move their young. A predator attack often causes the adults to abandon a nest. It’s hard to tell exactly what happened here, but our guess is that the snake likely came back overnight and the parents abandoned the nest. Predator guards are the best way to prevent snakes from accessing nests, such as metal cone baffles, but they are generally designed for nest boxes rather than open cup nests. You can learn more about them on our Dealing with Predators page. Please let us know if you have any other questions by emailing nestwatch@cornell.edu.

  11. Cheryl says:

    I’m sooo sad! I had house finches build a nest in an artificial forsythia wreath on my front screen door. Of the first 4 eggs, one hatched but alas something got the baby. To my surprise, either the same momma finch came back and laid 4 more eggs. Last nightsomething came and attacked the nest. All eggs splattered all on my door and porch. It must have been a heck of a fight because there are feathers every where. If i were to guess, finch feathers. Maybe they got momma too!
    I put out coyote urine to keep the neighborhood cat away. So, I wonder what go them!

  12. Christy Horton says:

    DO NOT have outdoor cats and bird feeders or nests! That is like a trap for poor birds. Put a bell on a cat collar so they can’t sneak up on them, but talk to the neighbor about it. It can kill a mama and then babies starve and die.

  13. CT says:

    I had a bluebird making its nest in the birdhouse that last year was occupied by wrens. One day while I was having lunch I saw the bluebird on the roof of the birdhouse making some type of distress call, all of a sudden I saw a red tail hawk fly over. The bluebird flew away and is still AWOL. It’s been about 2 weeks, guess it won’t come back. Should I empty the birdhouse in hope of getting a new pair?

    • Holly Faulkner, Project Assistant says:

      Hi CT, You do not need to empty the nest box, but you can if you are absolutely sure there is no nesting activity going on inside the box. Bluebirds will readily build nests on top of other nests, so we recommend leaving it as-is to be on the safe side. This bird may indeed have been scared away by the predator, but it’s also true that birds are seldom seen in the first few weeks of nesting – they only return to a box once per day to lay an egg, and then will be inside incubating constantly, except for one or two trips out to forage per day. If you check the nest box and don’t see anything in the way of eggs or young, then it’s more likely that this bird has moved on to nest elsewhere.

  14. CT says:

    Got some good news looks like the bluebird is back. I went yesterday to the bird house, gently tapped the back of it, and the bluebird flew out, sorry to have scared it, but I think it is back…

  15. Mary Green says:

    Can a bluebird move her babies from a nest if threatened by a predator?
    Absent family after 3 days.

  16. CT says:

    The pair of bluebirds is definitely back and casting for the young chicks that are very hungry with several trips to the birdhouse every hour of daylight.

  17. Karen Harevey says:

    Can I put a bluebird feeding box near a tree with an active cardinal nest?

    • Holly Faulkner, Project Assistant says:

      Hi Karen, I’m not sure what you mean by a feeding box. If you mean a nest box, it’s best to put bluebird boxes out in the open, facing open meadows or fields. Placing a box near trees or shrubs may encourage other species to nest in the box, such as House Wrens or chickadees. That being said, birds generally don’t mind other species nesting nearby, it matters more where other nests of the same species are located. For example, Eastern Bluebirds will not nest within 300 feet of another Eastern Bluebird. Let us know if you have further questions at NestWatch@cornell.edu.

  18. Jessica says:

    I had a chickadee nest in one of my bluebird houses. Soon house sparrows showed up and tried to smash the eggs but I got them out in time. I moved the house and put a different house on that pole and the sparrows used it. After the chickadee eggs hatched, a sparrow attacked again and killed two babies while knocking four out of the nest. I was able to save four. Then two were gone about the time the would fledge. Two days later, the nest was empty. Would they fledge two first and then return to the smaller two two days later?

  19. Nancy says:

    I have a bluebird house with a predator guard set up that currently has 1 11-day old baby in it. This morning I noticed Mr and Mrs Bluebird making calls of distress and harrying a branch that they typically perch on before entering the house to bring food to the baby. I grabbed some binoculars and realized it was a snake. I removed the snake from the branch with a long stick and it slithered off into some underbrush, but after researching and realizing it’s a rat snake I’m worried it will come back. Can rat snakes climb around a predator guard? Is there anything else I can do to protect my birds?

    • Holly Faulkner, Project Assistant says:

      Hi Nancy, snakes are generally stopped by predator guards, such as cone baffles and stovepipe baffles, on the pole of a box. However, if your box is mounted on a tree, or the snake is trying to access the box from above, there’s little that can be done. Read this page about dealing with predators, including snakes. You can try adding sheet metal to a tree trunk above and below the box to prevent access by the snake, or trim the branches near the box. If you decide to do these things, it’s best to reduce disturbance to the active nest as much as possible so try to work quickly and carefully. Then, when this nestling fledges, we recommend installing the box on a pole instead, if it is not on one already. If you have more questions, please email us at nestwatch@cornell.edu. The comment sections are not monitored regularly.

  20. Donna Thoreson says:

    We have a Bluebird nest in our Bluebird house. Everything was going well, had 5 eggs as of yesterday. Tonight I checked the box and one egg was missing. It was on the ground and looked like something got to it. Very sad! I have seen a sparrow in the backyard, but not very often..
    More concerning is that we have not seen the Male Bluebird in a couple days. This is our first nesting box and I’m not sure what is happening. I haven read anything about a Male abandoning the Female. I need help!!!

    • Holly Faulkner, Project Assistant says:

      Hi Donna, When you find eggs or nestlings displaced or dead, we recommend checking into our Nest Box Troubleshooting Guide which shows common culprits and solutions. Predators can often drive away adults from a nest; House Sparrows especially have been known to attack adult bluebirds as well as their eggs and young. That being said, this theory can usually be tested by watching the box for the addition of the sparrow’s nesting material. Alternatively, adults can run into predators in their own time off the nest; males do not spend much time at the nest themselves while the female is laying eggs or incubating, but he is often nearby. Finally, birds can also be sneaky – we’ve had several repost of nests where the adult was ‘missing’ but then the clutch hatched and fledged as normal, indicating that the birds were doing their job, just staying out of sight of the monitor. If you have more questions, please contact us at nestwatch@cornell.edu.

  21. Micky says:

    We have a bluebird box where they laid 4 eggs. Have not seen either parent in 2 days. What should I do about the eggs or is it too late to save them?

    • Holly Faulkner, Project Assistant says:

      Hi Micky, whenever you think you find an abandoned nest, we recommend waiting 4 weeks from the day you last saw the adults. This time accounts for a delay in incubation , which sometimes happens, and also a normal incubation period. If after this period the eggs are unchanged, the nest is likely abandoned and you can clean the box out if you wish. Please note it is against federal law to disturb a bird’s nest and its contents while it’s still “active” – so this time period helps ensure that the nest truly is “inactive” before taking action.

  22. Lisa Gay says:

    First timer with nesting bluebirds in a box that I cannot readily open…clearly there are babies since we see parents feeding and removing fecal sacs. We have a cone baffle on it and have cut away branches and vines. Is it too late to put on a Noel guard or any other shield around the hole? We are Seeing parents having to fend off catbirds and Sparrows. Wondering if there is anything we can do to make it more difficult for their predators or is it too late with this brood? Thank you.

    • Holly Faulkner, Project Assistant says:

      Hi Lisa, When a nest is active (i.e. containing eggs/young), it’s best to keep your disturbance as low as possible. Our Code of Conduct suggests that your nests visits should last 60 seconds or less, and that those visit should only occur every 3-4 days at the most – which is not conducive to installing a predator guard (not to mention, the box itself might be shaken/moved about). The adult bluebirds are going to be quite defensive of their nest – catbirds pose no danger, and the only sparrow species that does are House Sparrows. Unfortunately for now, the best thing you can do is leave the nest as it is. However, to help reduce competition with possible House Sparrows, we suggest putting up an additional box. This may discourage the House Sparrow from competing with the bluebirds – once a House Sparrow nests in the second box, you are welcome to engage in one of the legal management techniques listed on this page. Egg Oiling is often a good choice, as it encourages House Sparrows to sit on eggs that will never hatch for a longer period of time than normal, which lessens the amount of time they will have to harass other species. You can learn about Egg oiling in more detail on Page 16 of our latest NestWatch Digest.

  23. Jan R Noone says:

    Wrens have driven out blue birds from 2 nest boxes after they’ve built nest some cases they’ve laid eggs and even the wren guards they’ve gotten past should I just be happy with one fledgling and take the nests down. There is one wren box the wrens I see are coming from out of the bushes

    • Holly Faulkner, Project Assistant says:

      Hi Jan, to reduce the chances of wrens overtaking boxes intended for other species, the best thing to do is either reduce competition by adding more boxes, or moving th nest boxes (after the breeding season is over and no eggs or live young are inside anymore) away from wren habitat. Wrens like shrubby habitat, especially at the edges of forests or woodlands. Bluebirds and swallows prefer more open habitat such as in the middle of a meadow or open field.

  24. Barbara says:

    I had a housefinch nest next to my front door, wedged into a wallhanging next to the door light. I blocked the walkway, did not use my front door, did not turn on the light at night. 5 babies hatched within the last 2 days. Yesterday, i heard no chirping, and there was no “to and fro” activity. Checked on the nest around 5 pm. Four babies gone, nest looks intact, one baby deep down in nest, looked like it was suffering. This morning, I assumed I’d be burying the nest and the baby. The baby was still hanging on. Amazing. I took it to a wildlife rehabilitator. Now I’m second guessing myself. Would the momma have returned to care for this one left, if the other 4 were predated? Also, should I have left the light on at night? Maybe that would have kept predators away?

    • Holly Faulkner, Project Assistant says:

      Hi Barbara, The best thing to do in a situation like this is to contact a wildlife rehabilitator immediately before taking any action – they can advise you best how to proceed. If a predator attacked a nest, then there’s less of a chance that the adults will return to the nest, but it’s still possible. Certified rehabilitators will be able to give you the best advice depending on the situation. As for the porch light – I’m not aware of any research that suggests leaving a light on would keep predators away – unfortunately it can be quite difficult to prevent predators from accessing open cup nests. If you have any further questions, please feel free to contact us at nestwatch@cornell.edu.

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Cornell Lab of Ornithology

Cornell Lab of Ornithology