Construction  . . . 

This is HOW to build a 2-holed Mansion; the  Home page explains why.

 

2-Holed Mansions

You may contact me, Linda Violett

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 unique feature of the box design is the second hole on the box which allows birds a chance to escape danger.  And just as important, it allows Bluebirds to take a battle outside the nestbox where Bluebirds outcompete House Sparrows. See log notes at Golf Course test site.

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.   See more on : Box Depth

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).

Comments by high-profile sources (usually associated with NABS*) are highlighted in shaded boxes and illustrate why monitors have to use their own good judgment 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 make changes.  The adult bluebirds will usually survive to renest.

 

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.'"   

 

Inbox video cams are showing that birds inside a nestbox will try to escape out of a nestbox if their lives are in danger.

 

Metal Hole Guards:   Woodpecker-proof holes can be made by inserting 1.5" diameter PVC rings (slices) into box face for the holes and covering the outside with inexpensive metal "electrical washers" from Home Depot (sold by the bag).

 

I see no need to drill holes in the floor 

FLOOR DRAIN HOLES:   The vast majority of Bluebird monitors cut drainage holes in their box floors. 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 holes in floors will cool down boxes by creating "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 

First and foremost, water should not be getting into nestboxes.  And, a small nestbox would not have convection currents coming up through a nest with eggs/chicks and/or an incubating female. 

The logical solution is to completely waterproof boxes (silicone caulk is effective) instead of drilling holes in the floor.  If you have a top-opening box and water is getting into the box, then yes, you will need to add floor drain holes.

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 kerfs (toeholds) on the inside front under the holes.

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 areas who plan on hanging the box, add a 1" x 4" brace across the outside roof.  Use long screws through each end of the brace, through the roof and down through the sides.   As long as the hanger attachment holds secure to the box remnants during the attack, the babies should survive.

 

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.  

More background:   The only way you can put two holes on a small box is to place holes on different sides.  Small boxes are cheaper to build, cheaper to ship with more profit to the seller/organization.  And, I believe that is why holes on different sides of the boxes are often suggested as an alternate to two face holes.

 

 

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.  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).  The following link shows a Bluebird compared to : 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.   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 overcrowding and heat exhaustion.    Well fed nestlings just prior to fledge have body mass at least as large as their parents.  When visiting at the above "Floor Size" link,  try to visualize five or six fledge-sized nestlings on the various floor sizes.  

Monitors with boxes in full sun should consider painting boxes high-gloss white to reflect heat. 

Sun shields are another option.

 

Wrens & Trees:

Many people do not put boxes near trees because of the potential of attracting House Wren problems.

One 2-holed test site being monitored by Shari Kastner  2008 — 2010  Wisconsin shows there is a possibility that Bluebirds may become more diligent in protecting 2-holers from both House Sparrows AND House Wrens.  I have done no tests in that regard. 

 

Ventilation:   

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. 

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. 

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.  But if that is done, the monitor risks having water blown into the box unless the vents are plugged during rainstorms.

Drafty Box  vs. Well Ventilated

Keith Kridler writes in the Bluebird Monitors Guide that "an extra hole would just make a box drafty and chilly."  In the same chapter, he writes a favorable review of the large "Gilwood" hole (the Gilwood hole size is equivalent to having two holes).   Photo of Gilwood box below.

"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."

And, the NABS-approved Gilwood box has only 5.5 inches from the bottom of the hole to the floor.  The nest itself would take up at least 2" of that depth.   Thus, the Gilwood would actually be more prone to causing hypothermia with a large hole in very close proximity to the nestcup. 

Chicks in 2-holed mansions are nestled much further down into the nestbox where they are protected from wind chill during windy wet weather.  

The design combination of 2-Holed Mansion buffers both hot and cold weather extremes. 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.

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. 

See Keys to Success page, which is basically placing one 2-holer by itself on two acres of forage and removing any House Sparrow nest/eggs each week.   Bluebirds learn they can outcompete House Sparrows by escaping through the hole so the battle can take place OUTSIDE the box where Bluebirds have the advantage.  Simple and elegant solution.  Bluebirds need ample forage (two acres) around the nestbox so they don't have to leave the box undefended to find food.   Any extra Bluebird-sized box within that territory is an invitation for House Sparrow problems. 

For monitors who are redesigning boxes to work around their trail problems, see how to Design a Box

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)."

Logically, if  larger entry space 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 NABS-approved Gilwood box or boxes with large openings such as the Hughes Slot box  http://audubon-omaha.org/bbbox/nestbox/hughes.htm 

 

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."   

Maynard says he has only had a few Bluebird nestings in his 2-holers . . . ever.   But he claims claims to have witnessed a male House Sparrow and the female House Sparrow simultaneously land at both holes of a 2-holer and trapping/killing Bluebirds inside the box.

I've been using these boxes for about ten years on my own trail and I can almost guarantee that anyone using 2-holed boxes will not have adult Bluebirds trapped and killed inside the box by House Sparrows as Maynard Sumner claims happened on his trail.  As more monitors start using these boxes, that will become evident.

 

 

Hole Reducers:  Chickadees and other small birds can be protected from House Sparrow attacks by adding hole reducers. 

 

Photo taken by Linda Hammond of Virginia

 

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.

 

 

 Home