Wednesday, February 21, 2007

FAQ: Screech Owls vs. Squirrels, and Squirrel Eviction

In answer to some questions from readers:

Can a screech owl occupying a nest box prevent a squirrel from taking-over the box? I've often wondered about this, but in seven years of continuous observation, I have never had the opportunity (or, perhaps, misfortune) to witness such an encounter. However, here are some relevant facts: Even the smallest adult fox squirrel, Sciurus niger, weighs more than three times as much as the largest screech owl (adult fox squirrels range from 600-1,300 grams, while, according to Gehlbach, the mean weight of nesting female eastern screech owls, Megascops asio, is 168-183 grams), so that's not a fight the owl is going to want. If memory serves, the The Book of Owls states that eastern screech owls will prey on creatures that weigh as much as they do, where most other owls won't tangle with anything that's more than half their weight. Sallie, the raptor rehabilitator, assures me that screech owls would only tackle prey as large as themselves if they were desperate, but, either way, we're talking about an owl with the metaphorical heart of a lion. Add to that the biological imperative to protect her nest, and I think we can assume that most nesting females would put up some fight against an invading fox squirrel. To continue speculating, I think we can also assume that this would not be enough to deter a determined fox squirrel (in the worst case, it'd be a lot like a 150 lb. person taking on an 1,100 lb. grizzly bear armed only with two fistfuls of knives), but it might well put off one that is merely conducting a casual investigation.

Will the fox squirrels learn to avoid the screech owl nest box after repeated evictions? Yes. In my experience, a fox squirrel must be evicted three times before it will give-up on a nest box, but after that it will avoid the box for about a year. Possibly telling us something about fox squirrel memory, I find that a fox squirrel will return to a nest box from which it has been evicted after a bit less than a week. So the complete eviction cycle requires three evictions over a period of about two weeks.

If the owls keep finding the squirrels in the nest box in between evictions, will it cause the owls to choose another nest site? In the nine years since I put up my first screech owl nest box, I believe I've had to evict squirrels every single year, but the local screech owls have nested in my box every one of those years. This may mean that screech owls are tolerant of these occasional intrusions, or that there are no other suitable nesting cavities in my neighborhood (therefore they're too desperate to be choosy), or both. In any case, as far as I know, it's not a major problem.

What about species of squirrels other than fox squirrels? Beats me. Fox squirrels are the only squirrels I've observed in my area. However, I'd guess that the situation would be much the same even with the smaller gray squirrels, Sciurus carolinensis.


  1. Anonymous10:55 PM CST

    I've been watching an ongoing battle between a pair of Eastern Screech Owls and a gray squirrel. When I put up the box, a squirrel moved in almost immediately. About 2 weeks later I observed a (male I think) screech owl under my deck on a ledge in the floor joists. He sat there all day but did not return the next day (or later). However, 1 or 2 days later, I noticed a female screech owl in the nest previously occupied by the squirrel. The male would come get her at around dusk and they would leave together, returning at near dawn. Then I put up a second owl house - larger than the first - designed for a barred owl. 1 or 2 days later, the owl went into the original box around midnight. The next day the squirrel repeatedly came to the box & sat on top for a few seconds, then left. I have a security camera aimed at the box, but I never saw the owl leave. Around 6:30 pm the squirrel came down the tree and went directly into the box, then came out 2 minutes later and left. After another 2 minutes it came back & went inside again. Around 6:50 the male arrived as usual and landed at the door, climbing about 3/4 of the way inside. He immediately pulled out & flew off. The squirrel poked it's head out the door for a minute, then went back in for the night. I'm not sure what happened but the next day the female owl had taken up residence in the barred owl house about 60 feet away.

  2. I've had a screech owl box up for 4 years. The first 3 years the gray squirrels took over. The first year I saw an owl in there right away but when I cleaned out the box in the fall I found gray squirrel nesting material laid on top of 3 owl the owls didn't or couldn't protect their nest. The third year I covered the hole in the box in the fall with a piece of wood I screwed on...planning on removing it during owl nesting season when all the squirrels hopefully had already built and occupied their winter homes. BUT the squirrels managed to rip the board off and use it anyway. Last year, in late January, my dog caught and killed the female squirrel nesting in the owl box. The babies were no match for the owls and they took over right away and managed to raise and fledge their young. This year the squirrels are in there again. It is now January. I plan on opening the side of the nest box, cleaning it out and leaving it open for several weeks. I'll then add some wood chips and shut it up and hope the owls find it and use it like last year. I'll let you know what happens.

  3. I love this question that states, "Can a screech owl occupying a nest box prevent a squirrel from taking-over the box?" What do you guys think? I love these specific animals. They are so cute and unique.