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Floor Space Visuals  

The ultimate goal of nestboxes is to produce strong flight-ready fledglings that are able to survive after they leave the nestbox.  


Each bluebird chick should have sufficient space for proper growth and exercise inside a nestbox.  I'd recommend at least four square inches per chick and, thus, bluebird box choices limited to those offering a minimum of 24 square inches of floor space to accommodate clutches up to six chicks. There should be ample space for adults to enter, feed chicks and remove fecal sacs.  Notice the poor feather condition of this adult Tree Swallow in the photo below.

Before you build or purchase a box for bluebirds, consider the floor space visuals on this web page showing a Western Bluebird next to various floor sizes.  Its size is comparable to all three bluebird species. 

Sufficient space inside the box is also essential to ease heat stress.  Young chicks will attempt to separate from one another in hot weather.  See:  Heat Stressed Purple Martins  Quick-scroll through the video to about the 6-minute mark.   Floor space (thankfully) was sufficient for the Purple Martin chicks to separate.


Boxes with approx. 4" x 4" Floors

I would NOT recommend building or buying any of the following box designs since at least one of the floor dimensions is less than 5".  They are too small or too narrow for bluebirds. Notice that the tail and wing tip areas of an adult bluebird would become worn and damaged in small nestboxes.  If you are considering small boxes (HOSP-resistant boxes)  please see Keys to Success .

Floor sizes of the Texas Bluebird Boxes are not included because they give the builder a choice of sizes.  Here is their link: Texas Box Plans 



House Sparrow Resistant:

BEFORE YOU DECIDE TO BUILD OR BUY a House Sparrow Resistant box, read the following discussion (Bluebird-L):  Gilbertson Box Discussion  The discussion has application to any small House Sparrow Resistant nestbox.

Whether bluebirds will nest in small boxes and fledge chicks is quite different from discussions of the space per bird that is required for optimum growth and survivability after the fledge.  The only poster who consistently tries to bring back the discussion to after-fledge survivability is Keith Kridler.   Pay close attention to the information and background he offers. 

Photo shows a Gilwood (3.5" x 4.25" floor space) with a peanut butter jar inside to give perspective to the tiny size of the box.


Various other nestbox designs and information can be viewed at:  Nestbox Links  and at  Nestbox Pros and Cons




4" x 5.5" Floor

I do not recommend any bluebird box that has less than 5" on any side.  The adult bluebird is cramped in this box size regardless of which side the entry hole might be placed.

NABS  (4x5.5)

Griffin Box (4x5.5)

Hill Lake (4x5.5)

Plain (4x5.5)

Springer Chalet (4x5.5)

X-Box (4x5.5)  (entry hole on long side)


Small boxes are often referred to a "House Sparrow Resistant".  If House Sparrows shun a nestbox, there is a reason.



    A Note on Tree Swallows:

Here's a great link for Tree Swallows which states, "Many published box plans are intended for bluebirds which although larger than swallows average fewer nestlings per brood.  Bluebird designs often have interior dimensions that are far too small for swallow broods of up to seven young.  You may meet bluebird hobbyists who say these smaller bluebird boxes are just fine for swallows.  They are NOT!  It is true that Tree Swallows will accept small boxes due to  a shortage of nest cavities, but their nestlings may not all thrive or fledge successfully.  Boxes with small internal volumes can put nestlings at risk of death from hyperthermia (overheating) during hot spells.  Smaller nestlings may get trampled by their siblings, have their feathers soiled with feces, and be unable to reach food brought by parents.  Swallow nestlings also require space to exercise their wings so they can fly strongly when they fledge.  Small boxes can also be detrimental to adult swallows. To avoid potential problems be certain your floors are at least 5" x 5.  Narrow, cramped designs like the Peterson and Gilwood Bluebird Boxes are totally unacceptable in a Tree Swallow project." 

Boxes with small floors are inadequate for Bluebirds because of the same reasons listed for Tree Swallows. 


5" x 5" Floor


Wings of an adult bluebird 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. 


Many folks think they have to build or purchase a small-sized box (4" PVC tube, or a small slot box) because they have been told it is House Sparrow Resistant.  The better option would be a larger slot box, Spookers on a 5x5 standard box, 2-holed boxes (see Home page)

Also see:  Depth of Box  and Designing Your Own Box (slots, etc.)


6" x 5" Floor or Larger

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.  

 2-Holed Mansions (6 x 5)

 Long Point (Tree Swallow Project) (6 x 5.5)

Large Slot Box Plans (7 x 5.5)


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

Bob Wilson of Colorado designed an excellent box with 6x6 inch floors using flat double-walled PVC fencing materials:  Bob Wilson's 6x6 PVC Plans  


The 6" x 5" floor appears to fit the physical needs of Western Bluebirds.  There is space to move about and exercise wings even with clutch sizes of six or seven chicks.   

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. These robust western bluebird nestlings still had more days of growing before they flew from the nestbox. 

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. 


4" Circles (PVC Tubes)   NOT RECOMMENDED

Gilbertson (4" tube)

Navratil (4" tube)

Hutchings (4" tube)


The photo to the right is a Western Bluebird.  The circle underneath it approximates the size of a Gilbertson PVC tube. 

Obviously, that floor size is too small for western bluebirds.  Yet, it is my understanding that Steve Gilbertson sold about 3,000 of these nesting tubes per year for over a decade.  Cheap to make, cheap to ship.  As of 2012, he's no longer making them but he leaves behind a legacy of incredibly small boxes  that are promoted and sold by the North American Bluebird Society (NABS).

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 Gilbertson tube.   Note that it has fecal deposits on TOP of its feathers—obviously from its siblings standing on top of it.  The strongest chicks in a small box will stand on top of the weaker siblings.  As they do so, weaker siblings will get stuck in wet fecal deposits under them (their own feces) in addition to feces of siblings standing on top of them.  Even if they continue to be fed, their feathers are in poor condition.  They cannot clean or preen themselves nor exercise their wings prior to their fledge.  I witnessed this in 4x4 boxes on my own trail before retrofitting the trail with larger boxes.

Some monitors resort to the Gilbertson tube where they have house sparrow problems.   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 nesting or two on our western bluebird trails in urban settings but house sparrows eventually take both the standard wooden box and the PVC tube and you have a bigger problem after pairing.

I have been lobbying against small boxes ever since I saw the results of small 4x4 floored boxes on my own trail.  The 4" tubes are even smaller.

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" <>
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]

You may contact me, Linda Violett






As of June 15, 2012