Construction  . . . 

BASIC 2-HOLED MANSION

You may contact me, Linda Violett,  (lviolett(at)earthlink(dot)net) 

I monitor between 70 and 100 boxes each year and this box design was the answer to prayers for many problems encountered on complex trails in congested urban southern California.  The good results have often been shared with other monitors on "Bluebird-L" a public forum for discussing Bluebirds and other cavity nesters.   

Comments by "experts" (usually associated with NABS*) on Bluebird-L (a public forum for discussing how to help cavity nesters) are highlighted in shaded boxes and illustrate why monitors have to use their own good judgement when building boxes to solve problems.    *NABS, North American Bluebird Society

Adult Survival:  Attacks can be made on any nestbox at any time.  Adult birds in 2-holed mansions, have an avenue of escape whether the box is attacked by cats, snakes, raccoons, vandals, flying squirrels or large avian predators.  If you give the adults a chance to escape, it gives you time to better protect the box and adults will usually survive to renest.  

Escape Hole:  The vast majority of adult Bluebirds can escape danger if 2-holed mansions are provided for them.  In my opinion, most of the sad stories of adult nesting birds found dead in standard 1-holed nestboxes could be eliminated if monitors would build 2-holed mansions.   Some monitors trap and kill House Sparrows as a partial solution. Boxes are attacked by many predators, not just House Sparrows and predators (including House Sparrows) can attack without warning.  

Keith Kridler, (past NABS Board Member) an authority on Bluebirds, 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.'"   

Yet,  Susan Bulger and I reopened a House Sparrow infested trail in La Mirada, California to test 2-holed mansions.  Dozens of active 2-holed Mansions were attacked by House Sparrows.   Some chicks and eggs were destroyed but not even one adult Bluebird has been killed by House Sparrows in 2-holed Mansions at that test park.  See Home Page test sites for more details.  Adult Bluebirds were found killed by House Sparrows in 1-holers at the above site before 2-holers were installed but not afterwards.

Evelyn Cooper (Past NABS "Hotline" Expert) President of Louisiana Bayou Bluebird Society) says the disadvantage of 2-holers is that baby birds cannot fly out of the second (escape) hole during an attack and a statement to that effect should be made each time 2-holers are mentioned.   

Evelyn prefers using standard 1-holed boxes where babies AND adults are at risk during a nestbox attack. 

Basic 2-Holed Mansion Dimensions:  The interior box floor is approximately 6 inches wide, and 5 inches front to back.  The interior height measurement is about 12 inches and the finished hole drop (bottom of holes to floor) will be about 8.5 inches; standard (NABS) boxes have only about 6.5" drops.  Therefore, these deep boxes with 8.5" drops put the nestcup 2" further down in the box away from entrance holes (away from predator reach) than NABS-style boxes.  

I prefer front-opening boxes but any basic rectangular box design can be adapted to these overall measurements.  (The comprehensive "Bluebird Monitor's Guide" provides several basic nestbox layouts.) You will not be able to build this large box out of one simple plank of wood.  If you only want to build a box or two, pick through scraps at a lumber yard.  Double-check to make sure the scraps include your wide back and roof.  

Screw sides to back; make sure top edges are flush (so roof will sit tight).

Floor = measure inside width (approx 6").  Then measure back to front but LESS 1/2" (or the lumber thickness of space your door needs to sit between the sides), it will be approx 5".  Position floor between sides and leave about 1/4" recessed from bottom, screw in.

The box face is about the same length as your sides and back (approx. 13") by about 6" wide and will be recessed between the sides.

Drill two equally spaced holes on the face with about 2 inches of solid wood above the holes.  When finished, you should have about 8.5 inches from the bottom of the holes to the top of the floor.  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.

If you close the door with a square hook at bottom; you need to saw an opening slit in the bottom middle of the door about 1.5 inches.  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.  

I prefer to add hardwood face guards for better protection from starlings, jays, hawks and crows.  If you cut the guard wider than the door, it will cover some of the side seam gaps.

 

Attach the roof:  position the roof so that there is about 1" overhang at the back and about 2"-3" overhang toward the front.

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).

Hole-to-Floor Depth Debate:

Some monitors don't build deeper boxes because they have been advised that bluebirds will waste energy filling in extra depth and build high nests up to the hole.  

Ann Wick (Past NABS Board Member) on Jan. 22, 2002 said "Every Eastern Bluebird pair that has used a deeper box on my trails has built a deep nest."  And Gary Springer (builder/designer of Springer Chalet and NABS Executive Director in 2006) responded "Regarding deeper nest boxes and the distance from the nest box hole down to the bottom of the nestcup upon which the eggs and chicks rest, my experience apparently mirrors Ann Wick's. On my trails, more than 50 percent of the time the Eastern Bluebird builds the nest high enough so she can look out the hole while sitting on the nest. "

If Bluebirds can look out the nestbox hole while sitting on the nest, that nest can easily be raided by crows, jays, grackles (photo below by Jay Hoover of Texas).   

Most Western Bluebirds on my trail make full use of the extra deep boxes and build normal size nests with eggs nestled an inch or two from the floor.    See average nests at: http://home.earthlink.net/~lviolett/claimstraws.html

The extra height (hole-to-floor depth) of the 2-Holed Mansion and face guard help protect nestlings from predators, especially large avian predators such as crows, jays, grackles and hawks which can't be stopped by standard pole guards.

 

I see no need to drill holes in the floor 

FLOOR DRAIN HOLES:   The vast majority of Bluebird monitors add water drainage holes in the floors of their boxes.  Since my box exteriors are completely sealed with a waterproof "skin" of silicone caulk,  they do not have water problems and I do not add drainage holes.  Bruce Burdett says floor holes drain water out of boxes AND create "convection currents" whereby cold air comes up through the bottom holes and warm air exits through the upper holes.  

Bruce Burdett wrote to Bluebird-L (Date: Fri, 28 Mar 2003 07:40:18 -0500) 
In my opinion, these bottom holes are for air circulation as much as for water drainage, and I also provide 2 or 3 holes at the very top of each side-board, just under the roof (eaves). This circulation of air is especially important in the heat of summer, when we don't want hot air to collect (build up) inside the house. As the hot air rises and exits the upper holes, cooler air replaces it via the bottom holes. "Nature abhors a vacuum." (These are called convection currents.)  Bruce Burdett, SW NH 

In my opinion, the above statement on drainage holes creates two problems:

1)  Water Drainage:  It gives the false impression that water in nestboxes is the expected norm and, thus, boxes NEED to have holes in the floor); and

2)  Convection Currents:  At the very time drainage would theoretically be needed (wet/cold/windy spring months) is the very time when birds don't need cold "convection currents" coming through floor holes and up under the nestcup area siphoning off warmth.  Even if upper box vent holes are sealed off during cold months, the largest hole (entry hole) must stay open at all times. In other words, if convection currents coming up from the floor holes keep nests cooler during summer months as Bruce states, then convection currents would also make a drafty box during cold months of spring nestings.  

The logical solution is to completely waterproof boxes (silicone caulk is effective) instead of drilling holes in the floor.

Attention to Details

1)  Make sure the box tilts slightly toward the ground (not as drastic as the Peterson Box face angle, but same principle). 

2) Give both the inside and outside box seams an extra layer of waterproof silicone caulk. 

3) Screw in the box  floors so they are slightly higher in the back; any water that gets inside the box should flow toward the front door and leak out from the gaps at the front side seams at the door face.  Be sure to face your boxes away from wind/water. 

4) Add a length of wood trim to the front edge of the box—spaced so it sticks a little above the roof edge to divert water to the sides of the box and a little below and/or  form a drip edge.  

5) Interior:  Add squiggly lines of silicone caulk "ladders" on interior front, sides and back.

6) For added roof strength, L-brackets are added under the roof at the front side edges.   I hang boxes from tree branches about 15 to 18 feet high  (see photo) to keep boxes out of reach of vandals.  Vandals/thieves will try to jab the box out of branches and the L-brackets help keep the box intact.  Using screws instead of nails will also keep the box intact when rocks are thrown at the box.  

7)  For those in urban area who plan on hanging the box, add a 1" x 4" brace across the outside roof.  Use long (3" to 4") screws through each end of the brace, through the roof and down into the sides of the box.  This precaution will keep the hanger firmly in place even if vandals keep jabbing at the box from below and destroy the roof edges.  As long as the hanger attachment holds secure to the box remnants during the attack, the babies should survive the attack.  This has occurred on my trail multiple times.

 Holes:   Some bluebirds seem to have difficulty entering the hole with guard.  Therefore, I've been rounding out the hole edges and adding a toehold grip.  Bluebirds seem to prefer the guards to be somewhat thicker at the bottom where their claws can get a good grip.  See photo of new hardwood trim shape which should work better. 

  

Hole Placement:   Having both holes on the same side allows monitors to face both holes away from wind/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. "

My response is that if a cat or raccoon is reaching into a box, the nest will be lost regardless of the placement of holes.  Therefore,  holes should be placed for an advantage.  Both holes on the face of the box allow holes to be turned away from wind/rain.  

These boxes are used on my mountain forest trail in Big Bear, Calif. next to a ski resort with freezing spring conditions and are used on my hot southern Calif. urban trail (semi-desert).  Placement of holes and size of the box are important factors in the success of nesting birds in both weather extremes.

Ventilation:    Plans for 2-Holed Mansions do not specify side or back vents.  Anyone can add/drill side vents to any box if they feel it is necessary.  These large boxes are naturally cooler boxes during hot weather because they have two entry holes, a taller interior space (attic room) and the larger floor space allows chicks to separate from one another during hot weather.  The floor size of the mansion is approximately 6"x5" (30 square inches of space).  A standard box has a 4"x4" floor space (about half the floor space of a Mansion).  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.   Obviously, boxes should not be placed out in the open in direct sun if monitors have trees or other shelter options.  Boxes in the open expose them to weather extremes and predator attacks such as hawks. 

Most monitors depend on adding a series of ventilation holes or slots on the sides and backs of their (smaller) boxes in an attempt to cool down hot boxes.  The combined open areas of ventilation holes/slots, large openings of Gilwood boxes, etc. roughly equal or exceed the open space of two entry holes.  Yet, having 2-holes on a box has been singled out by those on Bluebird-L as either having too much or too little ventilation.

 Too HOT?     

HOT BOX  Perception:    From: Evelyn Cooper (Past NABS Hotline Expert) March 23, 2006 "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, " Linda, you should modify your recommendations to state that vent slots need to be added to your plan for people with extreme heat problems. 

Evelyn is fond of critiquing 2-holed boxes without ever having used the design.  Some monitors over-emphasize cross ventilation (side vents/slots) and de-emphasize floor size and box height.   For example,  if you and your family are packed shoulder to shoulder in a tight closet, overhead side vents aren't really going to do much.  But if the floor space is doubled so you can separate from one another and move away from hot sun-baked walls, that will make a big difference.  If extra "attic" space is provided as a buffer from the hot roof, that too, will make a big difference.

And, if extra ventilation is needed on a box, why not add an extra escape hole which serves the dual purpose of added ventilation and added safety for adult birds?

Or Too COLD? 

And, then again, other "experts" argue the 2-holer has too much ventilation:

Bluebird Monitors Guide:  Page 105 on 2-holed boxes: "  an extra hole would just make a box drafty and chilly." 

The large opening of the "Gilwood" box is about the same open area as having two holes.  But the Bluebird Monitors Guide (pg. 103) has good things to say about the large Gilwood hole:  "Ventilation is provided by the entrance hole.  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."

The Bluebird Monitor's Guide is one of the best Bluebird references currently on the market.  The favorable ventilation review of the large Gilwood hole compared to the unfavorable "drafty and chilly" ventilation review of an extra entry hole illustrates why it is important for box builders to filter out author bias when comparing various box features.  

Keep in mind that chicks in 2-holed mansions are  nestled further down into the nestbox away from entry holes during windy wet weather.   But shallower Gilwood box would have cold wet wind blowing directly over the nest in close proximity to eggs/chick/incubating female.

The design combination of 2-Holed Mansion—ample floor space, extra depth (hole-to-floor distance), interior height (attic space) and two side-by-side holes on the face of the box rather than vents—buffers both hot and cold weather extremes.

Floor Space:

With deep boxes, you should be providing sufficient wing space so birds can  flutter/jump to the hole.   To visualize the space bluebirds (or swallows) would need to flutter their wings, please see: http://home.earthlink.net/~lviolett/floorsize.html.   In my opinion, the vast majority of boxes on today's market are too small and narrow.  Small boxes are being touted as "sparrow resistant" without considering how many desirable birds are perishing from side affects such crowding and heat exhaustion.    Well fed nestlings just prior to fledge have body mass larger than their parents (excluding length of feathers) and small boxes could very well cause premature fledges for nestlings trying to escape crowded dirty conditions of a small box.  When visiting at the above "Floor Size" link,  try to visualize five or six fledge-sized nestlings on the various floor sizes.  

House Sparrow Tests:

Keith Kridler (co-author of Cornell's Bluebird Monitor's Guide and one of today's foremost bluebird authorities) built and tried several large, deep 2-holed boxes and  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 did not build his boxes to "2-holed mansion" specs nor did he follow recommended monitoring techniques.  Anyone experimenting with these boxes should build them to spec and follow all of the "Possible Keys to Success" (see link on Home page).  It is recommended that you also review the on-going logs of problem trails acquired for testing these boxes which give detailed monitoring techniques. (See test links on the Home page )

 

For monitors who are redesigning boxes to work around their trail problems, look for an earlier edition (1993) "Bluebird Trails, A Guide to Success" by Dorene Scriven which contains a size comparison of five major bluebird boxes on page 66.  For example, it lists the Hill Lake box with a hole-to-floor drop of eleven inches—which gave me confidence to build larger, deeper boxes.  

BEES

From Bruce Burdett to Bluebird-L entitled  "Honeybees and 2- or 3- Holers"             
                "Another possible drawback for 2-hole boxes has to do with the greater total entrance-size of the 2-holers. I would guess that honeybees would find a 1-hole box less to their liking. A normal modern wooden beehive has an entrance slot the whole length of the landing-board, which is around 20 inches. In the heat of summer, when the bees are working full blast, the traffic at that slot is intense, and every inch of the entrance is in use. A single 1 1/2" hole would not provide them with the entrance size they need, and they would prefer to find a cavity with 2 or even 3 holes . . . ."
  
Bruce Burdett  5/16/06

A swarm's cavity selection has nothing to do with the number of holes on a nestbox.    It has everything to do with a cavity's proximity to a mother colony, water sources, food sources, size of the cavity and moderated temperatures.  In fact, bees will often add a tar-like substance to seal cracks and close off extra openings in their effort toward making their hives more secure.   But, other monitors started adding the bee connection to 2-holed boxes:

May 2007, Sialis.org (Bet Zimmerman on Bees):   

http://www.sialis.org/predatorid.htm  (Bees):  "Avoid large two-holed boxes hanging high in trees? (Multiple holes may also make it more awkward to contain bees while lowering)."

Ironically, the above author, Bet Zimmerman (NABS Director, 2008) has a problem with wasps in her 1-holed boxes.  Yet, she does not recommend avoiding 1-holed boxes as a solution to wasp problem.  

Two-holed nestboxes are an option for monitors who have the ability to base their decisions on facts rather than perception.

Logically, if  more openings on a box is a significant aspect of bees in nestboxes, we would expect similar warnings about other box styles such as the large entrance hole of the Gilwood box http://audubon-omaha.org/bbbox/nestbox/gilwood.htm  and  the Hughes Slot box http://audubon-omaha.org/bbbox/nestbox/hughes.htm   But notice that only "two-holed" boxes (multi-holed) are mentioned as the Bluebird box design to avoid in connection with bees. 

If you build it, will they come?

Maynard Sumner of Flint, Michigan (NABS Committee member) says "no" by stating 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."   

Yet,  Linda Hammond of Virginia had Eastern Bluebirds at her home use a 2-holer shortly after installation.   She also added a "Sparrow Spooker" (see  photo below) on the roof as an extra House Sparrow deterrent.

House Sparrow "Ambush" Theory:

According to NABS-associated "experts" (specifically Maynard Sumner),  House Sparrow pairs will synchronize an attack where both male and female House Sparrow land at the holes and trap adult birds inside the box. This has been dubbed "The House Sparrow Ambush".    Maynard is the only person I know who claims to have witnessed an "Ambush" but the theory is being fanned by Cher who hosts the Bluebird web site: http://www.bluebirdnut.com/ 

April 2007 Cher wrote to Bluebird-L " . . .  I recently heard separate accounts of HOSP in two different areas of the country adapting their "ambush" strategy where two-holed boxes are used - the female blocking one entrance hole while the male uses the other to enter the box and kill the sitting female."

House Sparrows can easily usurp smaller birds whether they are using 1-holed boxes or 2-holed boxes.

Hole Reducers:  Chickadees and other small birds can be protected from House Sparrows by adding a 1 & 1/8"  hole reducer.  And most House Sparrows on my trail won't use boxes with 1 & 1/4" round hole guards made from 3/4" thick wood.  Thin metal hole guards would, of course, be easier to enter (the same concept as a wide vs. thin ring on a finger).  Thin metal hole reducers won't protect small birds as effectively as a thicker wooden hole guard of an equal size.

 

LIFTER BASKETS (for hanging):

For monitors who plan to hang their boxes, an easy-to-make lifter box can be built from a kitty litter container (Susan Bulger design).  The PVC neck will slip into the end of a standard pool-cleaning pole and the yoke pivots around under the box during lifts and takedowns.  Or, a custom lightweight lifter box can be made by forming a wire basket and wood rim (Dick Purvis design).  A 12" ruler leans against the wire basket for visual perspective.

 

Urbanization of California continues to impact the population of Western Bluebirds.  Post-mounted boxes in heavily used green spaces (parks, sports fields) is not an option.  However, hanging boxes about 15 to 20 feet in treetops has enabled Bluebirds to survive in the cities and about 50% of California's nestbox-raised bluebirds now fledge from hanging boxes.

 

Potential Problems:

Since Bluebirds need about 2 acres of forage per nestbox and our urban areas have about 5 homes per acre, one nesting bluebird family is directly impacted by the occupants of about TEN houses.   Sadly, at least two homeowners in that area will be feeding house sparrows and rats.  An irony of birdseed is that it helps the house sparrow and rat populations increase which, thus, harms our native species.  But, since rats are nocturnal, they go largely unnoticed by homeowners.  Rats will also eat eggs, nestlings and birds; and they can raid nests on the ground, in trees and, of course, hanging nestboxes.   Currently, there is no answer for keeping rats out of hanging boxes.  

 

Snakes:

Snakes can access hanging boxes same as post-mounts  For those in black rat snake territory, try putting a snake guard on the tree trunk same as you would a post-mounted box.

 

 

 

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