Western Bluebird prior to release after being rehabbed by Vicki Andersen 12/02.  He was rescued after hitting a glass (view) fence.  Notice the elegant length of his primaries compared to the tail.

 

 

Floor Space Visuals

In addition to the visual photos, an attempt has been made to list the box designs which correlate to the floor size.  Various nestbox designs and information can be viewed at:  http://audubon-omaha.org/bbbox/boxlinks.htm  

4" x 4" Floor 

Boxes with  4"x4" floors are inadequate for western bluebirds.  Adult birds cannot keep small crowded boxes clean and fully feathered nestlings will sometimes be found stuck in filth and covered with feces from siblings who made it to the top of a crowded box.  Plus, a full clutch of feathered nestlings (5-6 chicks) in a crowded box cannot separate from one another to keep cool during hot summer days.   Small nestboxes are also shunned by monitors of mountain bluebirds.  

Boxes with 4x4 floors (or less!):

    Original NABS (4 x 4)   

 

    Vince Bauldry Box (4 x 4)

 

    Lenker Box (4x4)

 

    Kentucky Slot (4 x 4)

 

    Troyer Slot (slant front)  (3.5x4)

 

    Rita Efta Slot Box (4 x 4)

 

    Van Ert Slot (4x4)

 

    Mel Bolt (4 x 4.25 or smaller 3.75 x 4.5)

 

    Huber Flip-Flop (3.75 x 4.5)

    Fox Box (4x4)

    Gilwood (3.5 x 4.25) 

    Gilbertson round PVC tubes (4") see last photos

    Navratil hanging round PVC tube (4")   

The small floor of the Peterson is not included because the height of nest to the sloped front determines the ultimate surface area.  

 

4" x 5.5" Floor

The adult western bluebird seems cramped in the narrow 4" width of this box.  The photo shows the beak placed toward the side of the entry hole.  

Boxes with 4 "x 5.5" floor size:

NABS  (4x5.5)

 

Griffin Box (4x5.5)

 

Springer Chalet (4x5.5)

 

Hill Lake (4x5.5)

 

 

5" x 5" Floor

    NABS (for mountain bluebirds)

    Tuttle Box  (5x5) 

 

    Duncan Box (5x5)

 

    Herman Olson (5x5)

 

    Johnson Slot Box (5.25 x 5.25)

Photo shows a western bluebird in comparison to a 5x5 inch floor.  Wings of an adult can partially open and it would appear that about four to five bluebird nestlings would be quite comfortable on a box floor of this size. 

6" x 5" Floor

By comparison, I am now building oversized boxes with approximately 6" x 5" floors.  Notice that even this 6" width does not allow the wings to extend.  

But the space appears to fit the physical needs of western bluebirds and five or six full-grown nestlings would have some space to move about prior to the fledge.  This floor size is working very well for western bluebirds on my hot-weather trail.  

Betty Lovejoy of Yorba Linda, Calif., has had clutches of seven western bluebirds fledge from her box of this size for three straight years—the large floor area looks a lot different when it is filled with seven active chicks. 

In the photo, four of the seven chicks can be seen at the front edge—two males rump-to-rump at the right, the female at the left and a head peeking out near the floor at the center (three others are hidden toward the back of the box).  There are fecal deposits on the straw but, because of the extra space in the box, it is able to dry into a powdery texture which does not affect the nestlings' feathers.  Around the date of fledging, a layer of slime sometimes occurs even on the large floors but it is much thinner (less danger to nestlings) with the larger surface area.  These robust western bluebird nestlings still had more days of growing before they flew from the nestbox.  

If I did not have to hoist boxes into trees, I would be building larger floors of 6"x6" (see below)

Bob Wilson of Colorado designed an excellent box with 6x6 inch floors using flat double-walled PVC fencing materials:  http://audubon-omaha.org/bbbox/nestbox/bwbox.htm  

4" Circles (PVC Tubes)

Gilbertson (4" tube)

Navratil (4" tube)

 

Here is a western bluebird against a 4" circle—which is the floor size of a Gilbertson PVC tube.  

This floor size is too small for western bluebirds.  Plus, adults have difficulty keeping nests clean in such cramped quarters. 

In crowded boxes, fecal deposits do not have surface air space to dry into a powdery texture—it stays wet under the tight nestling cluster.  See photo of the feathered nestling which was found stuck to a filthy nest in a Gilberton tube.   Note that it has fecal deposits on TOP of its feathers—obviously from its siblings and caused by too small a floor space.

Some monitors resort to the Gilbertson tube where they have house sparrow problems.  It is my understanding they pair a standard wooden box with this tiny tube box with the hopes that the sparrows are satisfied with the standard box and will leave the bluebirds unharmed in the PVC tube.  This seems to work for a year or two on our western bluebird trails in urban settings but house sparrows eventually learn to take both the standard wooden box and the PVC tube. 

Below is a post by one of today's foremost bluebird authorities, Keith Kridler, in response to a question posted on Bluebird-L, a forum for bluebird discussions.  It begins with discussions specific to PVC tubes but contains a superb range of nestbox considerations:

From: "Keith & Sandy Kridler" <kridler@1starnet.com>
To: "BLUEBIRD-L" <BLUEBIRD-L@cornell.edu>
Subject: Re:PVC Boxes
Date: Fri, 3 May 2002 07:56:28 -0500

Keith Kridler Mt. Pleasant, Texas
Sparrow resistance of the PVC nestboxes is a combination of factors and varies with how long the boxes have been used in an area and how many are available and how well they have been monitored over a long period of years. Originally PVC boxes were designed and used by bluebirders who removed ALL nesting House Sparrows so sparrows tended to learn to nest elsewhere. You have to remember that there were less than 325 "Bluebirders" operating "trails" in the 1970's. There were millions of wood nestboxes provided across the country then and today mostly unmonitored!   

A house sparrow seldom is given all of the choices in a single location and they often choose nest sites with entrances only sheet metal thick and some far smaller in cavity size than 4" diameter pipe. What House Sparrows prefer and what they normally do doesn't matter very much when you only have one pair of bluebirds nesting in a PVC box out of hundreds of boxes in the area and sparrows enter and kill the young like they did to Pauline down in Austin Texas....She has lost more bluebirds, killed by sparrows, in PVC boxes than in wooden boxes!  

The original PVC nestboxes were all made from Schedule 40 pipe that has 1/4" thick walls or 4 times the thickness of the Gilbertson boxes made from S&D (Sewer and Drain) 1/16" thick walls. The original PVC bluebird boxes were made from 6" & 5" and 4" diameter round white pipe because white was one of the colors least preferred by both Starlings and House Sparrows when they were given a choice of nestbox colors both interior and exterior. Most of these PVC boxes were VERY deep compared to the PVC boxes used today by most people.  NABS was recommending 10" or more depth from hole to floor and adding 1&1/2" thick wood entrance blocks to these boxes and they were still considered House Sparrow resistant. NABS early tests showed bluebirds fledging from PVC boxes even 13" deep with minimal problems reported even from those with Tree Swallows using the early boxes but these boxes had larger floor areas possibly allowing the swallows to "fly" to the entrance hole.  

The heat tests on these thicker walled PVC boxes conducted by Robert Patterson, NABS first president showed that these PVC boxes heated up SLOWER than 3/4" thick natural wood color pine nestboxes or OPPOSITE of the heat tests of the S&D PVC boxes tested by other people years later but they also tended to cool down quicker the same as the thinner walled pipe. These also reached lower overall temperatures than all of the 3/4" thick wood box styles tested. The Cornell heat study with the data loggers should help give an idea of nestbox orientation and nestbox style for the best possible installation according to heat gain.  

Later fledging from deeper boxes: I would like to see more research done on this!  OK robin's leave the nest before they can fly and in urban settings they believe that up to 60% of the young can perish within a week. Bluebirds on the other hand can normally fly 50 to 100 feet and tend to have higher survival rates than robins. I have seen some young more developed that could fly hundreds of feet and others who were capable of landing on a power line wire for the first landing while others hop out of the box and crash land within a dozen feet. I "BELIEVE" that we could dramatically increase survival of the young birds IF they were to stay in a nestbox an extra day or two or three.

Depth of box and slickness of insides of the box tend to have little effect on keeping the young bluebirds inside the nestbox because they tend to be able to hop to the entrance hole well before they are ready to fly. Survival rates would have to be determined with radio controlled devices from different box styles (various floor sizes) and fledge dates to determine if one box style were better at fledging stronger young birds better able to keep up with the parents and more importantly able to avoid predators. I believe that larger floor area will allow more exercise and just because all 7 baby bird fledge from a 4" pipe box does not mean they are ready to start and fly a marathon when that predator gets them in their sights hours after they fledge!  KK 

[End of quote by Keith Kridler, Texas]

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