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.”


Reference:

  • 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. https://doi.org/10.1676/17-084.1

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2 Comments

  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.

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