I have never read anywhere that I recall that the oval holes are a deterent to HOSP. A HOSP can easily get in a 1 1/4 inch hole, I have watched them go in and claim a box that had a 1 1/4 inch hole. So I wouldn't think the 1 and 3/8 would deter them. The only thing I recall reading is that some believe the PVC round nestbox aren't as accepted by the HOSP, I don't have one so I cannot attest to this but I do recall reading it. Personally I like the Gilwood nestbox with the half moon opening with the bar running through it. The Blues and the TRES seem to love them and it is so easy for trapping the HOSP in. Just my opinion of course. My dh made a bunch for me last year. I think the sialis site has the link to the blueprints for building one yourself.

Donna

Here is a link that might be useful: Gilwood nestbox

Keith, I cannot answer your question, but on Bet's web site, she compiles a ton of info from other folks. You might find some helpful info by going to the http://www.sialis.org/ website, and type "oval" in the search box.
Hope that helps.
BlueBars

Here is a link that might be useful: http://www.sialis.org/sitemap.htm

Donna and BlueBars:
I agree with you on the 1 3/8 inch holes don't seem to help deter the house sparrows, maybe they help deter larger birds. But for some reason people started building blue bird house with the 1 3/8 inch holes, me being one of them. My gut feelings is it is to help deter larger birds other then house sparrows.
I did find this below while continuing to search sialis site.

Minimum diameter for House Sparrow entry

according to this web site I probably could reduce the hole another 1/8" to 1 1/4" on the oval holes. I wouldn't want to get any smaller then the bluebirds might look for other nesting boxes in the neighborhood for a box with bigger holes.
I might try making some with the gilwood hole or a slot at the top of box. Looking at trying to make and sell a few houses for sale, don't want to be telling someone something that has been proven false or isn't really true. Documentation helps on that.

Keith

Hi Keith
stop dont make any more oval holes cow birds will enter and lay there eggs
in the bluebirds nest. Stay with the standard 1/1/2 hole A friend gave me
some Peterson nest box's with oval holes was a big mistake they were filled
with cow bird eggs! And bluebirds dont like to enter a hole that is extra thick
make sure there are no extra blocks for added protection until you see eggs
then ok to add extra wood to hole for protection.
Ray

Ray
I've made several bluebird houses over the years and the extra 3/4" piece of wood added for the hole hasn't kept the bluebirds I've seen from nesting. From my observation it doesn't seem to deter bluebirds, or lets say if they don't like them they sure fooled me.

As far as Cowbirds go that might of been a fluke, From just looking at pictures of Cowbirds they look the same size as a bluebird. So any nesting box that a bluebird would or could use a Cowbird could use.
Keith

Keith, Ray has been doing this for 20 years and has 60 Bluebird boxes. I think there may be something in what he has to say. He has at least 35 pairs of Blues every year and many times more than that.
Cathy

I totally agree with Ray too about the cowbirds, and they are just a bit bigger than the Blues so I don't think they normally can get in a normal round 1 1/2 hole, we have tons of cowbirds in this area.

Ray, I was wondering do you know if cowbirds can get in the Gilwood nestbox? We have several of them all used by TRES last year and didn't have any cowbird eggs but TRES are pretty good about protecting their nests with one always in it at all times. I was hoping to get a Blue using the Gilwood but wondered about the cowbirds.

Donna

If you want bluebirds they prefer the oval hole. See https://georgiawildlife.com/out-my-backdoor-whats-best-bluebird-box-design#:~:text=They%20found%20that%2C%20when%20given,8%2Dinch%20drill%20bit.)

ditto the GA wildlife link, it has the dope on several designs. Meanwhile, has anyone seen metalic guards made for oval holes? My squirrels love to eat away those openings!