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.

May 3, 2012: A Most Unusual Food Delivery

What makes this food delivery unusual? First it happened during the day. Second, it was a whole mouse (wondered where my bold bird feeder mice went to). Third, Mme. Owl was spending daytime outside the nest due to the heat, but showed-up, called to her mate, and eventually he arrived with this mouse, still in daylight, a time when screech owls especially do not want to be seen. So this is one of the oddest food deliveries I can remember seeing.

Fortunately, the S2071 automatically recorded these events. Unfortunately, it was set to start recording six seconds before motion was detected in the entryway, and to stop sixty seconds afterward, so you don’t so much get a slice of owl life from this movie, as slices. When there's been a meaningful gap between consecutive recordings, you will notice breaks in the video (some very obvious, some less so). Those correspond to periods when there was no motion in the entryway to trigger recording. Nonetheless, take all the fragments for a 56 minute period, even though they capture only 18 minutes of it, run them together into a single movie, and the result is still of interest (IMHO). The sound was a problem however: my nightmare hum was present at the time all of this occurred. I've filtered it out, but the side-effects of doing so are over-emphasized high frequencies, and the addition, at times, of what sounds like an echo, which is definitely not a sonic characteristic of the nest box.

May 3, 2012, 7:24 to 8:21 PM CDT
266.7 MB MPEG-4 movie, of 17:56 duration.

As this movie begins, the eldest owlet was 9 days, 4 hours and 20 minutes old, ±16 minutes, while the youngest owlet was exactly 6 days, 3 hours and 31 minutes old. No idea how old the mouse was, or even its species.

December 29/30 Mme. Owl Visits Again

Mme. Owl visits the nest box once again, complete with egg depression work. Not sure what more to say, except that, if all of these visits start to look the same, the frequency of them is well worth observing and recording. Also, at some point, we should witness the male owl call to his mate from the nest site, as part of, what I believe to be, the start of the mating ritual (“See: I found you a good place to nest. Can we have sex now?”).

December 30, 2012, 6:09 AM CST
19.7 MB MPEG-4 movie.

December 28/29 Nest Site Preparation

Mme. Owl stopped by the nest box at 6:04 AM CST to do a little nest site prep. That consists, as usual, of kicking around the bedding material to make a depression for the eggs she’ll eventually lay. Another means to the same end, not seen here, is for the female to push the bedding material around using her breast like the blade of a bulldozer. That behavior, however, may be reserved for looser material than that currently present in the nest box.

December 29, 2012, 6:04 AM CST
40.6 MB MPEG-4 movie.

The material in the nest box, BTW, is what was left after last season's breeding. I haven't made any effort to replace it. One reason is laziness; egg laying shouldn't begin in these parts until March, and the last time I went looking to buy wood shavings (as used in hamster cages, for instance) all I could find was something resembling cat litter. The other reason is that I need to move the nest box to a living tree before nesting commences (a project I admit to putting off due to the fact that it's going to be a major pain in the ass), and leaving the old bedding material, rich with familiar owlet smells, in the box should help reassure the adult owls that this is the same nest they've trusted for who-knows-how-long, just miraculously relocated. I have no evidence that that will be considered significant by the owls, but it seems like a reasonable possibility with no downside.

December 27/28 Mouse Withdrawal

Whichever of the owls (I suspect Mme. Owl) deposited the mouse in the nest box last night (December 26/27) was back tonight to make the withdrawal. Think of it as getting dinner from the leftovers in the fridge.

December 28, 2012, 3:14 AM CST
29.6 MB MPEG-4 movie.