[by Chuck Almdale]

I’ve always assumed that bird houses – typically built by humans for avian occupation – had round entrances. After all, bird bodies are fundamentally round in cross-section. At least, I assume they are although I haven’t sawed any crosswise to verify this assumption. And, in support of this assumption, every stinkin’ bird house I’ve ever seen in my entire life had a round hole for a door. Not one single triangular, square or rectangular door, except for the occasional cuckoo clock. So I’m not alone in my assumption. And additionally, birds go through those round doors much like a golf ball rolling down a tilted cylinder. Roundness everywhere.

So the first time I saw a bird house with a long horizontal slot, almost as long as the house was wide, for a door, at O’Melveny Park (Granada Hills, north San Fernando Valley a bit west of the #405), I and the fellow birder standing next to me wondered the same thing — “What’s that about?”

I’m still wondering that.

Birdhouses are typically designed for particular species or for a range of similar-sized cavity-nesting birds (e.g wren-sized, bluebird-sized, owl-sized, wood duck-sized), and the size of the hole is often very exactingly-sized to a 1/4th or 1/8th-inch, letting the target resident bird slip through but preventing larger predators or home-wreckers from getting in. I recall that decades ago I read that Western Bluebirds should get a slightly smaller hole than Eastern Bluebirds, a recommendation that has apparently been reversed as the North American Bluebird Society, to which all bluebirds with any social status belong, now recommends the same sized hole 1 1/2 inch to 1 9/16 inch in diameter for both species.

Or 1 1/8″ to 1 3/16″-high horizontal slot entrance. Yes, you read that right. A slot.

Well, I’m still mystified and wondering.

The most noticeable thing about the bird houses with slot doors at O’Melveny was that European Starlings, one of our most accomplished cavity home stealers and hole-pokers of someone else’s eggs, could shoot right through the slot-entrance without hardly slowing down. They only had to fold their wings a little. I wonder if they found themselves impaled into the rear wall on their very sharp-pointed bills, quivering from the impact like darts in a British pub.

Ok, I may be exaggerating a little there, but not that much. The main point is if the door is supposed to keep out predators and other unwanted guests, the slot wasn’t doing its job. Wasn’t anyone watching?

The Study

According to Cornell Lab of Ornithology, the folks that bring you eBird and Merlin, someone has been watching. No, not just hungry homeless starlings, but real living human researchers, aided by a small army of citizen scientists, or was that an army of small citizen scientists? No matter.

Over a period of four years and 280 Mountain Bluebird nests from 101 boxes, researchers from British Columbia tracked potential data every which way: male and female age, direction the nest box faced, distance to the nearest tree, distance to the nearest Mountain Bluebird and Tree Swallow nests, elevation, year, percentage of tree cover, mean temperature during the nestling period, total rainfall during the nestling period, feeding rate, and a few other potentially-interacting variables that might explain fledging success. The nest boxes did not have predator guards, and were similar in all aspects other than entrance hole style.

The Results Are In

• Bluebirds chose boxes in proportion to their availability (slot entrances were less common). There was no difference in selection based on age or body size. While selection does not always equal preference (birds may not be able to obtain their first choice), there was no strong evidence that bluebirds preferred either box type.
• Birds nesting in round-hole boxes produced significantly more eggs.
• Birds nesting in round-hole boxes had significantly better fledging success.
• Hatching success and first egg date were unaffected by box type.
• Complete nest failure was more common in slot boxes, potentially due to more exposure to the elements.
• For hole boxes, temperature and feeding rate affected fledging success. For slotted boxes, no one factor seemed to predict success.

The Conclusion

The authors recommend the use of nest boxes with round entrance holes over those with slotted entrances. Repeat: round.

Well…I could have told you that and would have had you asked, just as any Starling could tell you that but never would. But it’s always good to do the study, gather the data, and support (or amend) your intuition, aka your hypothesis in its null form.

So…O’Melveny park people, are you listening? Out with the slots, in with the holes. Unless, of course, your intention is to supply wonderful housing accommodations to European Starlings, and not to Western Bluebirds.

As we went to O’Melveny to check out all the birds, nesting or otherwise, here’s the eBird report for your perusal and delectation. We also had fun with the Merlin app on my companion’s phone, pointing out the songs and laughing at the mis-identifications. Canada Goose indeed!

O’Melveny Park, Los Angeles, California, US
Jun 14, 2025 8:20 AM – 10:50 AM
Protocol: Traveling2.0 mile(s)
Checklist Comments:     Temps 68-72°F. No wind, sunny. Not a single warbler.
33 species (+1 other taxa)
California Quail  1     Heard only
Rock Pigeon (Feral Pigeon)  20
swift sp.  20     Too high for details; probably White-throated as they nest in nearby cliffs.
Black-chinned Hummingbird  1 Adult male
Anna’s Hummingbird  3
Allen’s Hummingbird  10
Cooper’s Hawk  1 On electric pole near entrance
Red-shouldered Hawk  1
Acorn Woodpecker  6
Nuttall’s Woodpecker  4
Western Flycatcher  2
Black Phoebe  4
Ash-throated Flycatcher  2
Cassin’s Kingbird  1
Western Kingbird  1
California Scrub-Jay  6
Common Raven  10
Oak Titmouse  1
Violet-green Swallow  6
Barn Swallow  8
Bushtit  4
Wrentit  2
White-breasted Nuthatch  2
Northern House Wren  8     Heard only
Bewick’s Wren  3     2 heard only
European Starling  8
California Thrasher  1     Heard only
Western Bluebird  6
House Finch  16
Lesser Goldfinch  5
Dark-eyed Junco  5     2 heard only
California Towhee  8
Spotted Towhee  3
Black-headed Grosbeak  4     2 heard only
View this checklist online at https://ebird.org/checklist/S250326450