your sense of place

Round Versus Slot Entrances: New Study Says “Go Round”

Photo © Sarah Leroux

by Robyn Bailey, NestWatch Project Leader

A better box?

Which one is right for me?

Which one is right for me?

It may sound silly, but birds make decisions all the time. Where to nest, what to eat, with whom to mate...understanding their perspective helps us make better conservation choices.

Bluebird enthusiasts have been known to debate the merits of round versus slot-shaped entrances for nest boxes for years. And while the situation has not quite reached Hatfield-McCoy level feuding, the debate does get heated for some. A wide slot for an entrance is said to attract bluebirds while being something of a deterrent to House Sparrows, a non-native bird which competes for nest boxes. The slot entrance is also easier to make, as it does not require drilling a hole. Detractors point out that it could expose the nest to more predators.

Citizen Science To The Rescue

Unfortunately, research on nest box design has not kept pace with the popularization of different styles. There are so many varieties of nest boxes that evaluating them all is a daunting task. But recently, researchers from British Columbia, Canada, decided to investigate differences between round and slotted entrances in a study of Mountain Bluebirds (Leroux et al. 2018). They were looking for ways in which weather and parental feeding rates influenced nest success, but what they found was that this depended largely upon which nest box design was used.

House Hunters: British Columbia

House Hunters: British Columbia

A female Mountain Bluebird perches above her nest box in British Columbia. It is one of many boxes monitored by a group of citizen scientists.

Their study system includes a large bluebird trail monitored by a group of citizen scientists from the Kamloops Naturalist Club. The study’s authors designed a series of analyses which included variables accounting for 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. They were able to use 280 nests from 101 boxes over 4 years; nest boxes did not have predator guards, and were similar in all aspects other than entrance hole style.

Results Roundup:

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

A Community Collaboration

Home On The Range

Home On The Range

A male and female Mountain Bluebird survey their territory.

At NestWatch, we believe there will always be friendly debate over what makes the best nest box, but one thing is never in question: the importance of citizen science! If not for a group of citizen science nest monitors, this study wouldn’t have been possible. Principle investigator Dr. Matthew Reudink agrees, saying, “This trail goes back over 30 years, and we’ve been working with the naturalist club since 2011. The most important take-away here is the importance of these long-term data sets and how critical they are for scientists and academics to partner with citizen science groups. Not only that, but it’s a win-win when we can get students involved in working directly with citizen scientists and analyzing these data sets.”


  • Leroux, S. L., A. E. McKellar, N. J. Flood, M. J. Paetkau, J. M. Bailey, and M. W. Reudink. 2018. The influence of weather and parental provisioning on fledging success depends on nest box type in a cavity-nesting passerine, the Mountain Bluebird (Sialia currucoides). Wilson Journal of Ornithology 130(3):708-715.

Filed under:

15 responses to “Round Versus Slot Entrances: New Study Says “Go Round””

  1. Al Serfas says:

    I have nearly 500 bird houses up in the Merritt area. Most of the houses have the circular entrance, but I also built a few with the slot entrance. When 2 or more houses with different entrances where close to each other, the bluebirds tended to choose the circular entrance house.

  2. Nathan Robertson says:

    I love these kinds of studies! I would gladly contribute monetarily to their execution. It would be great if similar work were done with eastern bluebirds.

  3. Michele says:

    I have several of both boxes on my property and I have more success with round entrance and the most popular box on my property are the two XBOX that you order online I can’t say enough about them that’s all I’m using for now on!

  4. SDBluebirds says:

    Slot-boxes allow a bird cornered in the box (by a House Sparrow) to escape and survive. Slot-boxes also provide good air circulation; and do not require an entrance hole drill for construction. However, the wider opening does make it easier for scrub-jays and crows to poke their heads in and snatch nestlings.

  5. Kim says:

    We live on 5 acres in VA and have seen great success pairing Bluebird and Tree Swallow boxes 15-20 feet apart. They have become very tolerant of each other, and may feel some safety in numbers. While all of the birds seem to prefer the round hole boxes, we switched to all slot entry due to the high death rate from House Sparrow attacks. The House Sparrows do not seem to be as interested in the slot type boxes. Brood size was 4 birds per box last year. We also had some success by tying reflective mylar tape to the poles of the round hole boxes after the brood hatched and that seemed to keep the House Sparrows away. If not for the House Sparrows, we would be using the round hole boxes as there are so many more options on the market.

    • Debbie says:

      I put up a well made slot entrance house this year hoping to deter house sparrows. And I watched it as closely as I could to shoo away interested sparrows. The house sparrows moved in and laid an egg by day three. 😕

  6. Eric says:

    About to build some houses to put up on the fringes of common areas of our neighborhood since we have seen quite a few bluebirds around. Since we are in Texas I wonder if the opposite results would be found here since our weather extremes are high temps rather than cold snaps… the slot houses seem like they would ventilate far better than the hole houses.

  7. Chuckporras says:

    I have 2 box nest ,small round hole types how close to each other can I hang them?

    • Holly Faulkner, Project Assistant says:

      Hi, It all depends on the species. For example, if you’re looking to attract Eastern Bluebirds, boxes need to be placed at least 300 feet apart. Other species can tolerate closer distances, and some prefer even farther distances. Sometimes, pairing two nest boxes can encourage two different species to nest near each other (i.e. an Eastern Bluebird may need 300 ft between another Eastern Bluebird, but it won’t care so much about other specie nesting closer). Check out our Right Bird, Right House tool to view several species of cavity-nesting birds, and then if you click on the page for each species, you’ll see the spacing requirements, and the size of the nest box that they prefer. Eastern Bluebirds need a box that’s 5.5 x 5.5 x 9 inches. I hope this helps!

      • Dan says:

        Has any one use a oval hole I heard that sparrow can’t get into the house then and swallows can.

        • NELLY says:

          Had guy build me a oval hole box, dimensions I took from DUNCRAFT bird houses. Set up for my tree swallows. That same day saw bird inside , was hoping to see swallows. Nope another Sparrow. Oval slit I now believe should be under a inch in height. Mine was 1- 1/8th. Too big. Sparrow fit right in no problem.

  8. Norman Hansen says:

    I LIVE IN MERRITT BRITISH COLUMBIA CANADA. I decided to build some Mountain Bluebird nest boxes and researched the plans.
    The hole size recommended by several sites states 1 9/16 inch. But when I checked out a bluebird trail I measured the hole diameters and found out of 31 boxes measured only one was at 1 9/16 inches; all other 30 boxes are under by 1/16 to 3/16 inches and most by 3/16 inches. So: the big question is what does the scientific research say about the proper hole size? The brass plate on the boxes is “Vancouver Avian Research Center”.

    • Holly Grant, Project Assistant says:

      Hi Norman, Mountain Bluebirds need an entrance hole that is 1 9/16 inch in diameter. However, they also will not nest closer than 300 feet from another Mountain Bluebird. If the boxes are closer than 300 feet from one another, then those boxes may be intended for other species, such as chickadees, wrens, or swallows. Swallows in particular can use boxes with entrance holes that are 1 3/16 inches in diameter.

  9. Clark Blackwell says:

    Is there recent research as to breeding-pair preference and fledgling success rate specifically for EASTERN Bluebirds? I’m in north-central Georgia (US). The county Parks & Rec department seems to have closely adhered to proper (300-ft apart) spacing when installing dozens of pole-mounted bluebird boxes around the open-area perimeters of all public parks. These boxes were installed at least one year ago. Entrance types vary as to slot, round, oval and predator-exclusion-plate-added, most being 1-9/16ths-inch round. In this area, we primarily see Eastern but will occasionally spot a Mountain.

Leave a Reply

Your email address will not be published. Required fields are marked *

Cornell Lab of Ornithology

Cornell Lab of Ornithology