If you’re a bluebird enthusiast, or love all native cavity nesters, you’ve probably heard advice about adding 6# monofilament line to your nest boxes. Unfortunately, in our experience after observing multiple nest boxes, monofilament is only moderately good at deterring house sparrows from harassing the nest; however, there is still some reasons to use it.
Take a look, as we show a few examples of different nest boxes where monofilament was installed, and old world, invasive sparrows (in this case, Eurasian Tree Sparrows) still perch.
When to add monofilament line to deter house sparrows:
The only way for monofilament line to deter house sparrows is to add it to a nest box that has never been used by house sparrows. And the new nest box cannot be put in the same place as an old one that was occupied by house sparrows.
Even then, it is likely curious house sparrows will land on the birdhouse and investigate. Our observations have allowed us to also discover them even entering the nest box. However, they did not begin to build a nest. And after a week, house sparrows and Eurasian tree sparrows gave up on the nest box.
Unfortunately, what we also discovered when adding monofilament to a nest box before birds had discovered it, is that no bird made unbonded boxes their homes.
What that means is if you’re trying to attract bluebirds or chickadees or other cavity nesters to a nest box, you might want to wait to add monofilament line until you see significant daily activity by the same pair at your birdhouse.
Waiting, however, means you run the risk of house sparrows and Eurasian tree sparrows (if you live near St. Louis, MO) to the nest box.
Another important point to mention is that we do not guarantee the effectiveness of monofilament, even if no sparrow has bonded to the box! The observations you’ll see in this article further show the perseverance of sparrows.
I heard monofilament works, but only if a sparrow hasn’t bonded with a nest box. Is that part true?
Whether a sparrow has bonded to the nest box or not, we have found that even after adding monofilament line, sparrows were still interested and investigated the nest box.
Bonding to the nest box doesn’t just mean they’re bonded to a very particular box. They are also bonded to the location. This means that if you take a box down for a week, even two weeks and then put it back up, bonded sparrows will build a nest in it right away. Or if you replace the box with a brand new one, and put it in the same location, invasive (not native) sparrows will build in the box. And if they have built in a box in past years, they will be more interested in that box again.
In this first photo, monofilament line was added to a bluebird nest box that had been taken down for cleaning, modifications and reinstallation. Previously, there was no monofilament on this box. The nest box was down for at least two weeks before being put back on the pole with monofilament line running vertically down the front of the nest box. Four days after installation, a sparrow nest was discovered in the box.
The video below shows a Eurasian tree sparrow trying to access a nest box. It is confirmed that the sparrow was not bonded to this box, but had discovered it before monofilament line was added. However, we have discovered that when monofilament is added to a brand-new box and placed in a brand new location, they will check out the nest box, but eventually give up.
Monofilament line runs down the front of this nest box. And line is added to the top. Because chickadees have built a nest in this nest box, a 1 ⅛ inch hole reducer protects them from house sparrow invasion. These hole reducers are not small enough to prevent Eurasian tree sparrows from entering. Therefore, we added some overlap when installing this hole reducer, making the access ⅛ of an inch smaller. This allowed chickadee entrance but not Eurasian tree sparrow entrance.
During this day of observation (video below), a Eurasian tree sparrow landed on the top of the box. Though not filmed, we had visual observation of the sparrow perching on the side of the box and looking into the hole.
If monofilament line kind of deters sparrows, why do some bluebird landlords still use it?
One of the most important keys to whether monofilament line will work to deter sparrows is whether invasive sparrows have discover that nest box, whether they have nested in that nest box before, and whether they’re bonded to that nest box or location or not.
This is our observation from various nest boxes. After speaking to many bluebird landlords, there is debate about the effectiveness of using monofilament line as a sparrow deterrent. For some, there has been great success. For others, it has been a heartbreaking letdown.
Despite our findings, we recommend still adding monofilament to your nest box for a few reasons.
- Obsessively protecting cavity nesters just feels right. And it might stress you out not to take this step. There is that feeling of “If I had only done this, then maybe ____ would have worked better.” So to spare spiraling thoughts, it doesn’t take much work to add the monofilament to your nest box. Just remember to keep it 6# or else you might deter other native birds.
- Some bluebird landlords and trail monitors have had success with this approach. It is possible it might work for you. However, do this with caution. Be able to keep a close eye on your nest boxes to determine whether it’s working for you or not. Sparrows are quick nest builders. If you have them in your area and you’ve set up boxes with monofilament, check your boxes at least four days after install, and continue monitoring closely thereafter. Nest cams are another great way to keep tabs on your nest boxes.
- If you already have a native cavity nesting pair interested in your box, adding monofilament might be a helpful short-term solution until you can get a Van Ert trap. In our experience, monofilament will ward off unbonded sparrows for about 2 days. This might buy you enough time to get things set up.
If you are awaiting a sparrow trap and need something to buy you a little bit of time, we still do recommend getting 6# monofilament line to deter the sparrows. We have a tutorial about how to install the monofilament. It’s very simple to do.
Would thicker monofilament line be better at deterring sparrows from nest boxes?
The 6 pound line does seem pretty thin. So would thicker monofilament line work better to deter sparrows from nest boxes? We actually experimented with 30 pound monofilament. And yes, it kept sparrows off – for a week. But it also deterred the good birds too.
We have not yet experimented with something in the middle – 10 or 15 pounds. If we do, we’ll update this post or release another one.
Before experimenting with a thicker line, we observed bluebird pairs, chickadee pairs and eurasian tree sparrow pairs interested in the boxes. After that observation, 30 pound line was installed to the top and front of the boxes. The line was on the box for one week, and no bird investigated the nests further.
For now, we recommend sticking with 6 pound monofilament.
If monofilament line is not a long-term solution, what can I do about sparrows getting into my nest boxes?
So if monofilament isn’t a viable long-term solution against old world sparrows like house sparrows, what is? The only way to truly protect your nest boxes from invasive sparrows is by trapping and eliminating them. The thought of dispatching a sparrow can be a tough answer to accept. The idea of eliminating any bird sounds cruel.
If you’re new to landlording native cavity nesting birds, you might not be able to bring yourself to do this. At least, not in the first year of landlording. For many, it’s not until they witness the horror of what house sparrows do to their tenant birds that they get the courage to trap and dispatch sparrows.
House sparrows are known for killing nestlings and adult birds in their nests.
While this information might feel as if it’s crushing your hope, what you’ll discover in landlording our native birds is that the harder you fight, the more you get invested. And the more invested you are, the better chance these birds have of thriving and increasing their numbers. So keep up the good work of researching, learning and fighting for your birds.
