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What kind of nest box should I use for a bluebird trail?
In their natural environment, bluebirds occupy abandoned woodpecker holes. Therefore, boxes that simulate these holes work best. Shallow nest boxes with floors that are only 4-5” below the bottom of the oval hole and with an interior platform of 4 “ x 4” or 4 “ x 5”, fledge the most bluebirds. For the BRAW data reported in 2006, the only boxes that fledged more than the statewide average of 3.5 bluebirds/box were, in order of productivity, K-boxes, NABS/NABS-Style & Petersons (Table 6). Moreover, these three boxes had among the lowest swallow occupancy of all boxes. You can’t go wrong by placing these three boxes on your routes if you want to increase production.
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Those using nest boxes which attract fewer birds than the state average should first be sure boxes are placed according to the principles in this guide and then consider replacing them with a higher producing nest box, if still producing fewer bluebirds than the state average.
It should further be noted that each of these box types has an “oversized” opening, or oval in both the NABS-Style and Peterson boxes. Oval holes have been shown to produce more bluebirds than smaller, round holes (www.braw.org /Braw Articles/Close). Although difficult to prove, some investigators think that an oval hole allows adults to “tip-feed” their young without having to completely enter the hole as is the case with a circular opening. This behavior theoretically saves time for the adult.
It should be pointed out that 60.5% of all nest boxes being reported to BRAW are of the three leading types listed. As others see the importance of providing a better style of box for increasing bluebird production, we should see the average number of bluebirds fledged/box increase steadily.
Meanwhile, it is possible to modify your boxes and still increase production, even though you do not replace them with the more productive boxes discussed. When we first built our Peterson boxes for the Audubon Trail, we used a poor model type and the nesting platform was 8” below the bottom of the oval entrance hole. This construction flaw forced bluebirds to build a nest that was nearly double the normal volume.
The second year, we inserted a platform into the box that was only 5” below the level of the oval opening. As Table 3 shows, we increased the bluebird fledgling rate by 60% from 2002 to 2003 with only this major modification of our trail (few boxes were moved). So, for those with deeper boxes, such as Hill Lake & Herman Olsen models, you can improve their productivity by simply inserting a new nest platform to made them shallower. Better still, shift to the “Big 3”, more productive boxes.
Table 6. Eastern Bluebird and Tree Swallow productivity in different box types during 2006. Boxes are listed in decreasing order of bluebirds fledged per box. Only box types with at least 10 presented state-wide are named; the remainder are included under “Other”.
| Box Style | No. Boxes Presented |
Bluebirds Fledged Per Box |
*Range, Bluebirds Fledged/Box |
% Box Occupancy By Bluedbirds |
Swallows Fledged Per Box |
|---|---|---|---|---|---|
| K-Box | 527 | 5.4 | 0.8-6.0 | 61 | 0.5 |
| NABS-Style | 1,233 | 4.4 | 0.4-10.0 | 51 | 0.7 |
| Peterson | 1,881 | 3.6 | 0.3-9.4 | 67 | 1.2 |
| Other | 945 | 3.1 | 0.0-15.0 | 55 | 0.9 |
| Tree Branch | 70 | 2.7 | 1.0-3.6 | 68 | 1.1 |
| Simple | 433 | 2.4 | 0.0-10.0 | 60 | 1.3 |
| Hill Lake | 331 | 2.2 | 0.0-8.0 | 51 | 1.4 |
| Gilbertson | 116 | 2.2 | 0.5-8.4 | 61 | 1.5 |
| Her. Olsen | 389 | 1.9 | 0.0-12.0 | 56 | 1.9 |
| Troyer Slot | 96 | 1.5 | 0.4-6.5 | 48 | 3.3 |
* Values are the minimum and maximum statewide for individual nest-box trails (Table produced by Dr. Peter Dunn, Dept. of Biology, UW-Milwaukee).
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