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Box Depth

Standard boxes have a measurement of 6.5" from the bottom of the hole to the inside floor.  I refer to this measurement as the "drop".   On my trail, boxes with 6.5" drops are at risk from jays, crows, starlings and hawks reaching into boxes and pulling out young chicks or eggs.  (photo taken by Jay Hoover of Texas, below shows a Scrub Jay at a box).

Once avian predators discover how to raid one nestbox, the problem quickly escalates as they find other targets. The hole-to-floor (drop) measurement for boxes on my trail was initially increased to 7.5 inches which kept most avian predators from reaching in.  But avian predation didn't end until the drop measurement was eventually increased to 8.5 inches.  That is a full two inches deeper than the standard 6.5 inch drop.

Apparently, 6.5" drops became standard based on box plans that could be cut from one standard fence board.  A convenience to the box builder.


There is no downside to building in a couple extra inches of box depth as long as you don't build a tiny box that restricts bluebirds from fluttering up to the holes.  Any bluebird box should have at least 5" x 5" floor to allow birds "fluttering" space.

See:  Floor Size




Tree Swallows:

Some people state that 6.5 inch drops are standard because Tree Swallows, with their tiny legs, can't get out of deep boxes. 

Here is a link "The Wrong Nestbox" (  authored by Bet Zimmerman via where folks are advised NOT to build deep boxes.  Item #11 states, "The box is too deep. Nestlings (especially Tree Swallows) may be unable to get out of the box when they are ready to fledge."

However, in 2009, I acquired a trail that had nesting Tree Swallows each year.  This provided a great opportunity to see if Tree Swallows could use deeper boxes.  Thus, all 6.5" drop boxes were removed and replaced with a mixture of box depths ranging from 7.5" drops to 9" drops.  Kerfs (inside toeholds) are always added to my boxes so that birds can catch their their toes on a rough surface to help them flutter up to the hole.  

I would NOT recommend deep boxes in conjunction with tiny 4"x4" floor boxes.  Give your birds plenty of space inside the box to partially open their wings to flutter up to the holes.  See: Floor Size.

Tree Swallows used the various box depths with no apparent preference or avoidance based on box depth.  The Tree Swallow nest and eggs (pictured) was built in a 9" drop box.  Both adults and chicks had no problem using the extra deep boxes (see photo of same box as Tree Swallow chicks are at both holes and about ready to fledge. 


Some sources claim that if you provide a deeper box, birds will just build a higher nest.  

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. "

Notice in the previous photo that Tree Swallows built a normal sized nest in a 9" hole-to-floor nestbox.

Birds on my 100-box trail tend to build average nests and most build their nest in a far back corner away from nestbox holes (away from predator reach).

Thus, they take advantage of both the extra depth of the box and the extra floor space for maximum distance from the holes.

Cold Weather, House Sparrows and Deep Boxes:

Keith Kridler, (one of today's foremost authority Bluebirds and co-author of "The Bluebird Monitor's Guide") stated on 4/25/07 "The short answer about the 6" depth of the box is that in the Northeast, cold weather on average kills more bluebirds than all of the predators.  Native House Wrens are probably #2 at disrupting bluebird nests, then sparrows. Back 30 years ago House Sparrows when given a choice would nearly ALWAYS pick a DEEPER and larger floored nestbox so that they could build a  big and bulky nest."

The first sentence in the above statement suggests that deeper boxes are cold.    However, temperature inside a nestbox with an open hole during the winter, regardless of size, will be the same as the outside temperature. 

Keith's statement also suggests that House Sparrows are attracted to deeper boxes with large floors.  I doubt that House Sparrows would choose a deep box "so they could build a big and bulky nest."  But I do think they will choose the safest nesting sites available to them.  For example, House Sparrows will nest in tiny roof crevices high above predators (small areas) but will avoid nesting in shallow nestboxes where their nests would be within easy reach of predators through the entrance hole.  Common sense.  Monitors have to remember that House Sparrows do not need a nestbox, but Bluebirds do need a box.  Bluebirds will often choose unsafe boxes because there is no other better choice and will pay the ultimate price.

Since I was adding an escape hole at the same time boxes were deepened, I was able to provide safe boxes that Bluebirds could defend from House Sparrow competition.  Best of both worlds.

As time passes, Keith is becoming a supporter of deeper boxes and stated on 3/7/09

"I tend to prefer deeper nestboxes. Raccoons arms allow them to reach into and then down quite easily 6" from the entrance holes. We seem to have years when the European Starlings are more of a problem trying to reach into nestboxes. There are several avian predators that have pretty good reach into the nestboxes.
So if money and weight of the nestboxes is not an issue then an 8" drop from the entrance hole to the top of the floor is a starting point. In deeper nestboxes the females can always build up a deeper nest. But once you have lots of shallow nestboxes and IF they become a problem you cannot go back and make them deeper.
The wire Noel guard and Don Hutchings PVC guard can be added to the nestbox fronts to help with shallow boxes and predation after the boxes are installed. Keith Kridler"

Photo below from Bird Guardian

Folks using standard 6" drops will often add outside tunnel-type guards since their boxes weren't built deep enough:


Below are more quotes from NABS-connected experts posting to Bluebird-L, a public forum.   This is why it is so difficult to find deep boxes on the market.  Monitors wanting safer boxes will have to start building their own.  See:   Designing a Box.

6/14/07 from Phil Berry (NABS Board Member)  "The problem with a deep box is that the bird builds a taller nest, thereby negating any advantage man tries to give them. I have woodpecker boxes out and invariably a bluebird takes them over, building the biggest nest you've ever seen.";
Phil Berry

6/14/07  Evelyn Cooper (NABS Hotline)  "I found no advantage in the deeper box as the bird built right on up to the
entry hole, exactly three inches from it. Weekly monitoring means that things can happen before the monitor gets back to lower the nest and lives and eggs can be lost. "

6/14/07 Maynard Sumner (NABS Speakers Bureau), "Evelyn, I am with you on this. Most of the time the birds put the nest right by the hole so the female can look out. If you are putting out more them one kind of box the birds can pick the one they would like to use. " And later that same day Maynard said "I have removed part of the nest and the female will put more into the nest and move the eggs up so she can see out of the hole." 

6/10/09 from Bet Zimmerman (NABS Board Member)  "The biggest nests I have ever seen in my LIFE (volume wise) were in Linda’s boxes – the one bluebird nest was HUGE (really really deep), and two nuthatch nests were also quite voluminous. They certainly required a LOT of nesting material."

Another from Evelyn Cooper (NABS Hotline) on 4/30/07:  "I don’t want any deeper boxes on my trail as females commonly do build higher nests in them. There are times when it might be over a week before I can get back to check them. I have one this year and sure enough she did build the nest too high. I also had a female that built a nest , laid 5 eggs and were incubating in the 7 day period before I checked again. Had it been a high nest in a deeper box, she would have been at high risk."

Keith Kridler, though, is seeing the same thing on his trail using deeper boxes that I see on mine:  He stated, "In limited tests NONE of the bluebirds built their nests up high enough to be able to "look out" of the box and over half built AWAY from the entrance in a back corner of the box. Nesting material in the larger nestboxes has just about matched the AMOUNT of material in my standard 4&3/4" square bottom nestboxes. Even with the deeper boxes 8" deep compared to 6&3/4" deep MOST of the eggs are within 1" of the bottom board."

You may contact me, Linda Violett


As of June 15, 2012