Keeping Squirrels out of Owl Nest Boxes

I've been lucky this year and had no problems with squirrels in my screech owl nest box. Significantly, I think I know why.

Upper-left: Screech owl nest box in gray.
Middle: Fox squirrel nest box in green.

Though I've seen fox squirrels taking an interest in my squirrel nest box at various times, for lack of built-in video cameras or other instrumentation, I remained unclear as to what, if anything, was going on in there. However, lately, I've seen a good sized female relaxing on the horizontal board that serves the double-purpose of being a stand-off from the tree limb that keeps the nest box hanging vertically from a non-vertical limb, and as a walkway from the tree trunk to the entry hole in the bottom of the box. Around these parts, it's the right time of year for a fox squirrel to have her winter pups (well fed, as I expect my bird feeder ensures, a fox squirrel can produce three litters a year). Taking the afternoon air on that horizontal board, Mme. Squirrel could easily listen to her pups’ activity in the nest, while keeping an eye on everything going on around her, and from a position of complete safety, so it’s a good arrangement for her.

It’s gratifying to see the squirrel box serving its intended purpose, but I’ve also come to a reasonable degree of confidence that it’s serving an additional role: keeping other squirrels out of the screech owl nest box. The way it does this is very simple: fox squirrel territoriality. While they may tolerate each other to varying degrees in foraging encounters, they do not seem willing to allow other fox squirrels to setup a nest in the tree that holds their own nest. (Perhaps in really enormous trees there’s room for “my side; your side”-style compromise, but around here, even when a tree contains multiple squirrel nests, they all seem to be property of the same squirrel or matriarchal family group.) So, having an established squirrel nest more-or-less next door to the screech owl nest box ensures that it has a squirrel preventing all other squirrels from moving in, and you could not ask for a better, or more devoted, defense than squirrel self-interest.

So, if you've been having a problem with squirrels in your screech owl nest box, perhaps building and installing a squirrel nest box (similar dimensions to a screech owl box, but with an entry hole in a rear corner of the floor, and, perhaps, multiple internal levels) nearby is worth trying. And please report your results back here.

The Best Screech Owl Photo I Never Showed You

Readers with long memories may recall my mentioning that I'd setup an experiment during the last week of my 2012 screech owl nesting. The experiment was a home-brew motion sensitive trigger for my digital SLR camera, with trigger and SLR mounted such that they could catch the adults as they flew back and forth from the nest box. (You can see the sensor in the photos; it's the white plastic thing with the exposed pale wood behind it, mounted under the owlet rail. The clamp on the bottom edge of the nest box is providing mechanical isolation for the cable running to/from the motion sensor. Yeah, it was a kludge, but you have to get real-world experience before you can know how to finalize a design.) The experiment worked, albeit with a host of first-time-in-the-field problems that meant the experiment lasted only part of one night. Nonetheless, it was worth nearly being killed by a folding ladder with a very poor sense of timing about when it should fold, the frustration of having to cancel the experiment after a matter of hours, etc., because it produced the photo below.

Delivering a june bug to the nest box.
May 23, 2012, 10:47 PM CDT.

This photo shows the delivery of a june bug, probably Rhizotrogus majalis. And I believe it answers my question about the more-or-less round, white food items that I've seen, via the internal infrared camera, delivered year after year, but which I’ve never been able to identify. The beetles roll-up, to the extent that they can, into a defensive shape, and their shells reflect near-infrared light well, so they appear white in such illumination. Combine that with the low-resolution of standard-definition closed circuit video cameras, and there’s not much to go on in terms of making a species identification. However, combine this high-res, color external shot with the internal shots that followed, and the mystery is solved, at long last.

The motion-trigger experiment produced some other photos of interest, too. I’d intended to post all of them sequentially with commentary, but since I keep failing to do that, I’ve started with my favorite, and will take it from there.

Mme. Owl Spends the Day in the Next Box

Mme. Owl, in a move that I hope has nothing to do with impending eggs (it’d be about 10 weeks too early for eggs around here), spent the entire day of January 2nd, 2013, in the nest box, starting at 6:28 AM CST and ending at 6:01 PM CST. The movie below shows all of the portions that triggered automatic recording. (In future, I will allow movement in more of the nest box to trigger recording.)

January 2nd, 2013, 6:28 AM to 6:01 PM CST
706 MB MPEG-4 movie, of 47:47 duration.

If impending eggs aren’t the source of this daytime stay, it may be that one of the several construction sites up the block disturbed or destroyed her normal daytime roost, forcing her to seek alternate, but safe and familiar, accommodations.

The perch in the nest box continues to be a well-liked feature, as you can see, though I wonder if an adult screech owl would prefer that it was another half inch, or thereabouts, further from the far wall, to allow ample room for tail feathers, etc. With the floor of the box only measuring 8" x 10" (which is larger than the 8" x 8" usually recommended), I wanted to try the perch experiment, but keep it out of the way of normal owl business as much as I guessed was possible. However, the years have proved it is popular enough with adults and owlets alike that allotting it more space might be appropriate. And it's not merely a perch for the adults, or a climbing target for the owlets; both adults and owlets will hide beneath it when they feel threatened. It might provide very little protection, but it seems clear that any protection is immediately recognized as better than none.