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 2-Holed Mansion Plans  . . . 

Or you can build a Slot Box Version


The unique feature of the large 2-holed box design is the second hole which allows birds a chance to escape danger and take a battle with House Sparrows outside the nestbox.

If you look closely at the photo, you will see I tried adding plexiglas/skylight roofs based on the myth that extra light deters House Sparrows.  Skylights do NOT deter HOSP.

Another advantage of this large box is that it provides an 8.5 inch drop (bottom of holes to the floor) for protection from avian predators See: Box Depth   page.

NOTE:  Give your birds plenty of space inside deeper boxes to partially open their wings to flutter up to the holes.  See: Floor Size.   Birds have difficulty flying upward from deep narrow boxes.


Wonderfully detailed plans on how to build the 2-holer (and other box plans) at Fred Stille's web pages:  Nestbox Builder and  2-Holed Mansion Plans/Layout


Basic Dimensions of 2-Holed Mansion:


  • 1 Back =  6" wide x 13" long

  • 2 Sides = 8" wide x 13" long

  • Door face = 6" wide x 13" long

  • Floor = approx 6" x 5.5"

  • Roof = approx 10" x 12"

  • Hole Guard (find scrap of hardwood trim)

  • Screw floor to back and recess it 1/4" from the bottom edge of back to create a drip edge.
  • Screw sides to back and then the floor (tilt floor slightly downward toward front of box so that *if* water gets into box, it will drain out the front gaps.
  • Drill two holes on front, keeping about 1.5" solid wood above holes (I use 1&9/16" holes, some prefer 1.5" holes)  See Photo below and info on adding kerfs (toeholds) to inside front panel.
  • Fasten the door to the sides toward the top with pivot screws.  Or for a drop-down door, put a hinge at the bottom.  Builder's choice.  Not critical element to 2-holers.
  • Add the roof.  Position the roof with more overhang toward the front.
  • You can add a hardwood hole guard (use a scrap of wood to fit over the front holes, draw the circles on the scrap, drill the holes, screw the hole guard over the front.  Or you can add metal rings to the outside of the holes such as cheap electrical washers from Home Depot (sold by the bag).


Any nestbox style can be used to make a 2-holer just as long as you buy lumber that is a couple of inches wider and longer than lumber specified in the box style of your choice.  Design a Box

Be creative, but please don't add or subtract an inch or two or make some modification to an already existing box and end up burdening the birding community with an unending string of box "names" to sort through.

My own trail has some 2-holers built with backs, fronts & sides all from 8" wide lumber because that was what was available at the time.  The original prototype was smaller.  Some folks are even expanding the Peterson box so 2 holes can be added. 


Cut interior toeholds on box face (kerfs), or squirt on silicone caulk "ladders", or screw on a length of gutter guard to the inside door front so birds can catch their claws on the rough surface for easy exits.  One of my favorite toehold materials is a 6" long by 4" wide piece of rough tread from old bicycle tires which is fastened on with screws.

How the door opens and choice of hardware is per monitor preference.  If you close the door with a square hook at bottom; you need to cut an opening slit in the bottom middle of the door about 1.5 inches.  And then use two pivot screws a couple of inches down from the top and screw through the side into the door so the door swings upward.  

When the box is finished, you should have about 8.5 inches from the bottom of the holes to the top of the floor (safer from avian predators such as crows, jays).  Again, those measurements are approximate.  There is plenty of builder's wiggle room built into these large boxes.

You can add hardwood face guards for added protection from avian predators and woodpeckers.  If you cut the guard wider than the door (and your box style and dimensions allow) it will cover some of the side seam gaps, as well.  If you prefer extra ventilation through the door seams, or an easy assembly, cut the guard to fit flush to the door.




A Note on Hole Sizes

Hole sizes for Eastern Bluebirds are usually recommended at 1.5" diameters.  Hole sizes for Western and Mountain Bluebirds are usually recommended at 1&9/16".   There will be size variance amongst individual bluebirds regardless of species.  On my own trail, some of the Western population cannot or will not enter 1.5" holes.  Monitors who consistently use smaller (1.5") hole sizes will effectively exclude the largest bluebirds from their nestboxes.  Monitors who offer slightly larger holes (about 1&9/16 inch holes) will be able to accommodate the larger bluebird segment of the local population, regardless of bluebird species.  My recommendation is that bluebird monitors offer the largest hole size possible that will still exclude Starlings.

Any hole size to exclude House Sparrows will also exclude bluebirds


Hole Size to Exclude House Sparrows is 1&1/8"

Hole Size to Deter most House Sparrows on my trail is 1&1/4"

Whether the holes have added thickness (guards) makes a difference as to the ease of entry, as well.


Metal Hole Guards:   Woodpecker-proof holes can be made by inserting PVC rings (slices) into box face holes and then covered on the outside with inexpensive metal "electrical washers" from Home Depot (sold by the bag).  The PVC pipe will be slightly larger than 1.5 inch diameter.  Builders should cut slices from the PVC to correspond with lumber thickness of the holes and clip out a small segment of the PVC ring so it can be squeezed (reduced a bit) to fit inside the builder's choice of hole size.  You will need to practice on a wood scrap before drilling the final holes on your box face.

Roof:  Position the roof so that there is about 2" overhang at the back and about 3" to 4" overhang toward the front.  If you use post-mounts, by all means add a larger roof.  In southern California, we hoist hanging boxes into trees and have to consider the overall weight of the box.

Finish:  Smear clear silicone caulk all over the outside of the box and let it dry for several days. The clear silicone provides a continuous waterproof "skin" for the boxes that doesn't crack during hot/cold weather changes.  Many monitors do not add any protective finish to their boxes.  However, unfinished boxes age and crack more quickly.  Flip through the pages of the "Bluebird Monitor's Guide" and note the poor condition of the active boxes shown on pgs. 91, 94, 100 (first edition).


The above portion is HOW to build the box.

And the box goes hand-in-hand with normal bluebirding techniques (correct spacing between boxes, remove HOSP nests and don't feed HOSP).  See: Keys to Success

Below are details along with commentaries as to WHY these features and sizes benefit nesting bluebirds.  It also gives a glimpse as to why these web pages had to be created at the grassroots level to get the word out that bluebirds can outcompete House Sparrows without trapping and without resorting to substandard (House Sparrow Resistant) nestboxes.

House Sparrows will use these boxes in the absence of Bluebird defense.  Links to no-trap, no-gadget test site logs are provided on the Home  page to show what can be expected as a site transitions from HOSP problem 1-holers to problem-free Defensible 2-holers.

My own trail had to be rebuilt in quick succession to overcome problems of small 1-holed boxes.  This web site is to help others avoid time-consuming dead-ends pertaining to House Sparrow control and compromises pertaining to box design.


Large 2-holed boxes have not been approved by the North American Bluebird Society 

Monitors who want a NABS-approved and/or promoted House Sparrow Resistant box can buy those directly from NABS at: NABS Nestboxes.   Just be aware that you will be getting a bluebird box with a built-in disadvantage.  The disadvantage (usually too small or too shallow) is purposefully built into the box design so that House Sparrows will avoid (resist) the box.

Photo shows a Western Bluebird compared to the floor space of the (NABS-promoted) Gilbertson PVC tube nestbox. 

The photo should be self-explanatory.  But since thousands of "House Sparrow Resistant" boxes are sold every year, I'll spell it out: THE FLOOR SPACE IS TOO SMALL.


Small boxes tend to damage feathers of adult bluebirds trying to maneuver inside them.  And chicks do not have enough space for proper growth and wing exercise.  You may very well get bluebirds and/or Tree Swallows to nest in small boxes, but the ultimate goal of providing nestboxes is to produce flight-ready native birds that are able to survive once they leave the nest box.

House Sparrow Resistant Boxes:  Box designers trying to solve House Sparrow/bluebird competition have focused their attention on building something substandard (usually too small) into the nestbox so that HOSP won't nest in them.  The following link shows a bluebird compared to floor space of popular box styles : Floor Sizes    Or House Sparrow Resistant boxes will be built too shallow.  Boxes that are too shallow are prone to missing eggs and chicks from avian predation.  Even standard 6.5" hole-to-floor drops were not safe from avian predation on my trail. 

NABS keeps trying to come up with a better House Sparrow resistant box.  A few years ago, a committee of seven ended up with a "new" design that was basically a regular NABS-size box (4"x5.5") and switched the hole placement to the long 5.5" side and made a few roof changes. The big difference is that they made it shallow (5" hole-to-floor).  Only Texas (Keith Kridler) had the good sense to stand firm on not approving the shallow 5" drop but was outvoted because the group wanted something "House Sparrow Resistant" built into it.  Since HOSP Resistant boxes have built-in design shortfalls, there is a never-ending series of  "House Sparrow Resistant" boxes with a different combination of built-in design shortfalls.

Defensible Boxes:  With "Defensible" boxes, the success of the box depends ENTIRELY on bluebirds being able and willing to defend the box and survive a battle.  That can best be done if Bluebirds are given an avenue of escape (box with second hole or large slot opening).   Interactive behaviors between House Sparrows and bluebirds had to be taken into consideration as well as their responses to site conditions.  See  Keys to SuccessIn order to have Defensible boxes, feeders and extra boxes will have to be removed in addition to providing a defensible nestbox.  If that is done on my trail, bluebirds consistently outcompete House Sparrows without the need to trap, without adding spookers and without having to resort to boxes that even House Sparrows will avoid.

Two very different approaches to solve the House Sparrow problem.

Why aren't more monitors using Defensible boxes?

Comments by high-profile sources (almost always associated with NABS) are highlighted with this color shading and illustrate why monitors have to use their own good judgment when buying/building boxes to solve problems.   

Keith Kridler, (past NABS Board Member), stated on Bluebird-L,  Feb. 20, 2002 " My thinking on bluebirds getting killed in the box is that the bluebird has NO intention of 'escaping.'"   

Keith's thinking is incorrect. 

Here's what actually happens:  Bird attack/defense/escape behaviors  Video cams show that birds will attempt to defend their nestbox and chicks but will try to escape out of a nestbox during a failing defense and their lives are in danger.   Slot boxes also give an opportunity of escape but slots are usually built as HOSP-Resistant (tiny) boxes.  If you like the idea of a slot box, try building a larger version of it.



Photo by Andrew Plaza, Pennsylvania (1998)

Hole Placement:   Having both holes (openings) on the same side allows monitors to turn holes away from wind and rain. Unfeathered nestlings may perish during a cold snap if they get too cold or wet.  


Keith Kridler, March 23, 2006 " To me it makes more sense for a two holed nestbox to have entrances on different sides of the box. Cats will sometimes sit on the roof of the nestbox and it is harder for a four legged predator to guard entrance exit holes on opposite sides or front and back of a nestbox. If a raccoon or cat is up to their arm pits reaching into a nestbox then very possibly another entrance hole a couple of inches away from the first hole is also going to be covered or might actually be used by the other paw of the animal. "

Nestboxes with cats and raccoons sitting on roofs and reaching into them are doomed no matter which side is used for the extra hole. 

The underlying conflict is probably something along the lines that you can't put two side-by-side holes on a small mass-produced box that is cheap to build, cheap to ship.


    ~ ~   ~    ~   ~ ~~ Sun Wars ~~  ~   ~   ~    ~


If you are using post-mounted boxes in sun, paint your boxes a light color and/or add heat shields (shade panels added to the box with air space between shield and box).

See video showing temp comparisons:   "Heat Tests (Color & Shields)


Emergency Sun Shield:

You can make a quick Darth Vader style helmet made from a plastic plant container with the front section cut out.  There is a hole in the middle bottom of the container.  Bungee cords were hooked at the back lip of the box and brought up the outside back and down through the container hole at the top.  Then another bungee cord was looped around to the first cord and secured under the roof (see under roof in photo). This quick, simple and secure sunshield that can be installed in a matter of minutes during a heatwave. 



Floor Space During Hot Weather:

These large boxes buffer the affects of hot weather extremes because they have two entry holes, a taller interior space (attic room) with chicks at a further distance from a hot roof.  The larger floor space also allows chicks to move away from hot walls and to separate from one another.   See: Floor Sizes

Birds do not have perspiration glands to keep cool—feathered nestlings need to hold their wings away from their bodies to cool down in hot weather and they cannot do that in small crowded boxes.  Small boxes are being touted as "sparrow resistant" without considering how many desirable birds are perishing from overcrowding and heat exhaustion.   If you don't like box choices on the market (I didn't) then here's a link to get you started: Designing your own box


Wrens & Trees:

Some monitors feel they must place nestboxes in full sun away from shady trees because of the potential of attracting House Wren problems.  On my own trail, wrens may strike (rare event) but bluebirds seem to be more diligent about preventing it from happening twice.  Wrens have never destroyed two bluebird clutches in the same box during the same season on my trail.

Wrens are a protected native species and it is against the law to remove a wren NEST.  However, male wrens put sticks in a multitude of boxes (not a nest) so the female wren can choose a box and build her nestcup.  My recommendation is to remove dummy nest sticks (legal) before the female builds a nestcup.



Plans for 2-Holed Mansions do not specify side or back vents.  If you want to add them, fine.  I don't.  The two entry-sized openings on the front panel of these large and extra-tall boxes provides attic space and ventilation.  The large floor (5x6) allows nestlings to move away from hot walls to separate from each other and lift wings up away from their bodies to flutter/fan themselves.  Why not add an extra opening (escape hole) instead of a series of small vent holes?  Extra hot/cold protection is provided if monitors decide to hang their boxes under tree canopies as we do here in Southern California (semi-desert conditions).  See how to Attach Hangers.

You may contact me with comments or questions:   Linda Violett

HOT BOX  Perception:     Evelyn Cooper (NABS Hotline Advisor) March 23, 2006 says, "Two holes in the door do not produce the cross ventilation that holes on each side of the box produces. That is what the holes (vents) on each side at the top are for."
and on May 17, 2006, she wrote " Linda, you should modify your recommendations to state that vent slots need to be added to your plan for people with extreme heat problems."

Monitors who use standard 1-holed small boxes WILL need ventilation slots or holes.  Cross ventilation holes on different sides of the box will take in wind and water.  Monitors who choose to put openings on different sides of boxes have to go around and close openings during inclement weather or risk hypothermia to nesting birds.  



Drafty Box Perception (2-Holer)


Well Ventilated (Gilwood Box)

Keith Kridler writes in the "Bluebird Monitors Guide that "an extra hole would just make a box drafty and chilly."  (page 105)

HOWEVER,  Keith writes a favorable review (page 103) of the NABS-promoted Gilwood box hole. The mouse hole opening of the Gilwood  is equivalent to the combined open area of two standard holes.  

Keith says (page 103) , "Thanks to the large opening at the very top of the box, hot air rises and escapes rather than being trapped inside. . . . If the Gilwood box is used in the South or if the upside-down mouse hole is replaced by a circular or oval hole, additional ventilation might be needed."

At the time, the 2-holers and the Gilwood had comparable size entry openings (about 4 sq" openings on the front panel) and no other vent openings.  Only the small Gilwood box (that NABS sells) was given a favorable review on ventilation.  


OK, back to the House Sparrow issues,

Keys to Success contain only three basic rules for successful nestbox placement:

  • Space boxes at least 300 feet apart per minimum bluebird territory standards
  • Stop feeding House Sparrows
  • Don't let House Sparrows breed (remove their nests/eggs)

If those three bulleted items are followed on my trail in addition to the 2-holer, bluebirds can outcompete House Sparrows and they consistently do so after a short learning curve.

Another benefit of having "Defensible" boxes is that the longer those boxes are in place, the more confident (and effective) bluebirds become in defending them.

Whereas, Spookers and Magic Halos tend to lose their effectiveness to deter House Sparrows as time passes.  And, thus, monitors have to put them on when bluebird eggs are laid and remove them after the fledge.


I have provided 2-holed boxes to several individuals over the years with the understanding that the 2-holers would be placed at active House Sparrow/bluebird contested sites and per Keys to Success for a 3-year observation test.   Monitors at test sites are instructed to follow the Keys and to not interfere with battle outcomes (no trapping and no add-on gadgets or swapping box styles) for three years.  To date, only TWO monitors have done so:

    Shari Kastner of Wisconsin Test Logs  . . . good results as far as HOSP/bluebirds, but the wren problem persisted during the first few years.  Test is complete but Shari continues to provide updates.  In 2012, Tree Swallows fledged first and then Eastern Bluebirds took the 2-holer and fledged chicks.

    Christine Boran of Virginia Test Logs . . .  3-year test was completed in 2012.  Three bluebird clutches fledged in 2012.

    Special thanks to both of those no-trap testers.   Other monitors are using 2-holers but the above two monitors are the only ones who have set up test boxes at sites with intense Bluebird/HOSP competition according to the Keys to Success, without trapping and with weekly ongoing logs of what actually occurs during the 3-year transition.

As a fairly new bluebirder struggling with House Sparrow problems in 1998, I tried using 2-holed boxes after reading a Letter to the Editor by Andrew Plaza.  The 2-holers worked.   I shared the good news with the birding community and sent boxes to some of the more experienced birders.  You can view those initial posts from 1999 at:  Sharing the Good News

Unfortunately, experienced bluebirders (especially NABS folks) want to put the second (escape hole) on different sides of the box, or place boxes too close to one another or they build a smaller version of the box.  Other NABS folks thought the box was supposed to repel House Sparrows and when it didn't, they started trapping.

Keith Kridler (co-author of Cornell's Bluebird Monitor's Guide and past NABS board stated on July 6, 2002, "At some of the sites I have trapped multiple pairs of sparrows using the two holed boxes after they have laid eggs so I am not seeing any advantage to larger and deeper two holed boxes when compared to house sparrow competition." 

Keith missed the main points:

There is absolutely nothing substandard built into a 2-holed box to discourage use by House Sparrows.  Success of 2-holers depends entirely on bluebirds being able to defend the box after the monitor removes extra boxes and feeders.  A HOSP nesting inside a 2-holed box is NOT a competition failure, especially if it is early in the season (March/April) before bluebirds are ready to nest. 

(Photo shows one of Keith's 2-holed set-ups.)


NABS nestbox placement guidelines NABS Getting Started state:

In many cases, bluebirds have been observed nesting closer than the distances recommended. However, it is better to start a bluebird trail with boxes placed too far apart than too close together.

  • Western Bluebirds - approximately 100 yards

  • Eastern Bluebirds - 125 to 150 yards

  • Mountain Bluebirds - 200 to 300 yards

Maynard Sumner of Flint, Michigan (NABS Committee member) stated on Bluebird-L in 2005:   "I had one Bluebird nest in the two-hole box in 2003 and one in 2004. All the other boxes were filled up so they had to use the two-hole box. Most of the time they will not go near the two-hole box."   

Note:  "All the other boxes" should have been removed prior to placing a 2-holed box at a House Sparrow observation site.


So far ONLY grassroots monitors have been correctly placing 2-holers built to spec and per standard spacing guidelines.  And those monitors are having good results.

This successful 2-holer was set up by Linda Hammond of Virginia. Two EABL chicks are at the holes about ready to fledge.

Trapping was done at this location and, thus, could not be included as a test site.




When you have a moment to relax, here is an absolutely wonderful time-lapse, slow-motion video of Bluebirds feeding chicks in a nestbox and it is set to music   Connie's bluebirds  

You may contact me, Linda Violett





As of June 15, 2012