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Violett's Keys to

House Sparrow vs. Bluebird Success  . . . 

Including Behavioral Tendencies and how to tip the advantage to bluebirds


Keys to Success: 



Photo courtesy of  Debbie Highfill of Joshua, Texas, taken during March 2009


Defensible Box Style

Other than the simple instructions listed above, the only other monitor requirement is to recognize that 2-holers are Defensible boxes (not House Sparrow Resistant). There is nothing substandard built into the box to deter House Sparrows. 

Residual HOSP/bluebird behaviors:   If a House Sparrow builds a nest in a 2-holed box, that is not a failure.  It means that Bluebirds have not yet chosen to defend the box.  Simply remove any House Sparrow nests and wait for bluebirds to take control of the situation as was done here:  Woolwine Test 


Why Bluebirds succeed in 2-holers 

And why monitors using 2-holers per "Keys" do not have to trap

The escape hole takes advantage of nesting bird behaviors.  Adult birds will attempt to defend their nestbox (eggs and chicks) but will try to escape if the battle becomes deadly.  Following is a link showing Purple Martin nestlings being protected by their parent during a Starling threat.  When the battle advances to a life-threatening stage for the adult Purple Martin, it escapes out of the large hole:   Bird attack/defense/escape behaviors 

    —   Bluebirds Are Survivors   —

Bluebirds (just like the Purple Martins shown in the video) will try to defend eggs/nestlings but will exit the box (or not enter) if the situation becomes life-threatening during the defense.  Most of the time, the male bluebird will signal to an incubating female if he spots any sign of trouble.  The female will either pop up to look out the hole or exit upon a warning.  She will often join her mate in the defense of the nestbox.

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.'"   

Whether bluebirds will try to escape from a risky situation in the nestbox is fairly easy for monitors to confirm.   Do not confuse a female sitting on eggs during a weekly check as something she would do if the box were under attack from House Sparrows, snakes or raccoons.   If a female is sitting on a nest when you open the door, she will continue to sit as long as she does not feel threatened.  If you gently and quietly close the door, she will usually stay.  But if you move your hand towards her (a riskier situation for her) she will usually leave.  You can test the "escape" theory by small degrees by the way in which you advance your hand toward a sitting female.  A hand approaching from above the hen is more threatening than a hand approaching from below.  The greater the perceived threat, the quicker she will leave.  



At the startup of the no-trap Woolwine Test a few years ago, a convoluted discussion took place on the Bluebird Nut Cafe bluebird forum: A site to watch (2-holed test)

Most of the misinformation surrounding defensible nestboxes can easily be discerned by understanding HOSP/bluebird behaviors and applying common sense.  Any bluebirder who actually witnesses a House Sparrow preparing an initial attack on adult occupants in a nestbox would also have plenty of time to intervene.

See  1984 Paper Describing Bluebirds Killed by House Sparrows.  It gives insight on the methodology and detailed timelines of a House Sparrow attack from its inception until the final death on the following day. 

"On 15 May 1981 from 1230 to 1330 I watched a male House Sparrow repeatedly enter a nesting box containing five 12-day old nestling bluebirds.  On each visit  the House Sparrow remained in the box for 1 to 3 min. The House Sparrow entered and exited flying quickly and silently without stopping to perch on the front of the box.  The adult male and female bluebird residents were foraging from a high wire 30 m behind the nesting box where their view of the nest hole was obscured. I interrupted the sparrow' visits after the first 20 min and found 5 bluebirds nestlings 4 of which had bloody crown wounds; of these 4 one was already dead. Within 5 min of returning to my blind, a male House Sparrow entered the box and the pattern of entrance and exit at 1 to 3 min intervals resumed.  At 1330 four nestlings were dead and the fifth died the next day."

The entire scientific observation article on HOSP attacks is well worth reading.  Concentrate only on observations and timeframes.  Do not assume that the nearby adult male and female bluebirds were unaware that nestlings were being systematically mutilated by the House Sparrow. 

During progressive attack conditions where adult bluebirds are giving up the defense, observant monitors will see adult bluebirds perched near the nestbox but reluctant to re-enter the box.  In those no-win battles, adult bluebirds with chicks in the box will be reluctant to take mealworms into the box to feed chicks of boxes under attack. That signals that the adults are in the process of giving up defending a nestbox, or have already done so.

As stated, House Sparrow attacks are not quick sudden events.  Here is another video which shows House Sparrow Initializing an Attack on Nestlings.  Homeowners saw live video and stopped the attack before nestlings were harmed.






After escaping from a box under attack, will adult bluebirds sail off into the wild blue yonder leaving the kids behind. 

The answer is a qualified "No". 


Assuming you have provided bluebirds with an avenue of escape and enough space where they do not feel they are risking their lives, they will continue to defend eggs and chicks.  If they can stage the battle outside the box where they have the competitive advantage, they will do so.  If the HOSP confrontation requires that bluebirds must enter (or re-enter) a nestbox, bluebirds will do so as long as they can escape from a risky situation.  They can do that fairly easily from 2-holed boxes and, thus, will continue to defend 2-holed boxes from House Sparrows even if the confrontation requires re-entry into the nestbox. 

  Standard guidelines of two acres forage should be available around each nestbox so bluebirds do not have to leave the box undefended in order to find food.  

Monitors who have grasped the concept of bluebirds escaping from nestboxes for continued nestbox defense can advance to the next topic of which type of defensible entry they prefer (slot, over-sized hole, multiple holes).


My recommendation would be for monitors to use box openings of their choice as long as it  provides an escape for bluebirds, still excludes starlings, and any additional openings are placed on just one side of the nestbox.



Abandonments may occur (regardless of box style) when a homeowner places a nestbox next to feeders that attract House Sparrows, starlings and other critters.  Bluebirds become overwhelmed; they cannot continue to forage, incubate and defend the nestbox from non-stop battles.  And that is why removing feeders (one of the Keys to Success) is so important.

No Trapping Necessary

Early test sites on my own trail (such as the YL Golf Course) were conducted over a decade ago to identify what I now call "Keys to Success".   And if those "Keys" are used, monitors will have a smooth transition to problem-free 2-holers without having to depend on trapping.  Trapping can be done if monitors see HOSP nests going into 2-holers during the transition; but it is not necessary.

Monitors using 2-holed boxes and following the Keys should not expect any bluebird losses (adults, chicks or eggs) from HOSP attacks.  As long as monitors set up 2-holers per the "Keys", those nestboxes without trapping are proving to be safer for bluebirds compared to 1-holers with trapping.



Why Bluebirds Sometimes Fail 


And why monitors using 1-holed boxes have to trap

As mentioned, adult bluebirds will try to defend a nestbox under attack but will attempt to save themselves by escaping out of the box if a successful defense is not possible. They are found dead in nestboxes when they cannot get out of the nestbox during a struggle.  In situations with too much activity and repeated challenges next to feeders and paired boxes, there may be a hesitation in exit response time compared to sites with less activity and challenges.

Even in 1-holers, there are comparatively few dead adult bluebirds found in failed boxes compared to the amount of destroyed clutches and mutilated nestlings. Trying to exit a 1-holed box under attack is a lot riskier than exiting out of an extra hole if the situation inside a nestbox becomes deadly.  Under life-threatening conditions, adult bluebirds are reluctant to continue re-entering the nestbox to protect eggs or chicks. 

Vulnerable 1-Holers:  The most vulnerable type of entry for HOSP/bluebird battles is one single round hole.  A single round hole is the easiest hole for trappers to use.  And since most nestbox designs are created so that trappers can use them, the majority of nestboxes have a single round hole.  And in areas with Bluebird/HOSP competition, trapping will be necessary if 1-holed boxes are used.  A loop.



—   Expert Trapping has at least a 10% Risk Factor   —

In the following link to a scientific paper describing House Sparrow attacks 1984 Paper Describing Bluebirds Killed by House Sparrows, it states, "In 18 (90%) of the traumatic bluebird deaths, House Sparrows were at the nesting boxes on my visits to boxes preceding my discovery of dead bluebirds."

Which means that 10% of the time, House Sparrows were not seen at the nestbox by the observers prior to the attack.  And that explains why even the best of trappers will have at least some periodic losses to HOSP.  Trappers don't always see a developing problem in time to trap and even if they do, they are not always successful.


House Sparrow Nest Removal vs. Killing Rampages:

Do not be afraid to remove House Sparrow nests from 2-holed boxes.  In fact, it is important that you do so.



You may have read scary warnings about removing House Sparrow nests similar to the excerpt below:

Bluebird Monitors Guide, (pgs. 80 and 81) states, "Also be aware that when you remove the nest or toss out the eggs, your strategy may backfire. The sparrows may become agitated, and in some cases, the male house sparrow will go marauding. 'If you pull the nest of a house sparrow, usually it will go into the nearest nestbox and kill whatever is inside,' says Alicia Craig of Caramel, Indiana.  Dean Sheldon of Huron County, Ohio, agrees. 'Destroy a sparrow nest, and the male will move on down the line, wreaking death and destruction wherever he goes.'"

And here are other web sites that include the "Rampage" theory: "Managing House Sparrows".">.  Scroll halfway down page.

Steve Kroenke's "House Sparrow Revenge"    Article relating to Purple Martins and often quoted (including in Bluebird advice as the reason pulling HOSP is not recommended AND justification that trapping is the only effective way to control House Sparrows. 


Luckily, with 2-holed boxes, that does NOT happen. You simply remove any HOSP nest/eggs during the transition period until bluebirds gain confidence to take the box from House Sparrows and start their own nesting.  If you have a trail of thirty 2-holed boxes with active bluebird nests and you pull a HOSP nest out of one of the boxes, no problem. You will NOT see the male HOSP moving down the line "wreaking death and destruction wherever he goes". 






House Sparrow Resistant Boxes and/or Pairing: Pairing boxes does NOT work for any extended time in battles between HOSP and bluebirds.  Bluebirds have the tendency to move to an uncontested "extra" box within their territory and House Sparrows have the tendency to harass nesting bluebirds, regardless of the number of boxes available.


One of the North American Bluebird Society (NABS) recommendations for House Sparrow Control is provided below:


"Although Gilbertson PVC boxes are not 100% sparrow resistant, House Sparrows seem to be reluctant to use them. If PVC boxes are paired with a wooden box, sparrows tend to choose the wooden box. If you are trying to attract bluebirds in an area where you suspect sparrow problems, it might be wise to start off with the PVC box."




To the right is an adult bluebird against a circle that represents the floor space of a Gilbertson PVC tube/box

In some instances, pairing a Gilbertson tube with a standard box for House Sparrows will get you through a first nesting before House Sparrows take both boxes.  However, there is a real possibility that HOSP will not quietly nest in the better box of the pair without causing problems at the HOSP-Resistant box. 

Even if HOSP aren't interested in taking the tiny box for themselves, they sometimes prevent (kill) other species trying to use it.  House Sparrows are colonial nesters and have no problem nesting in close proximity to other House Sparrows in paired or clustered arrangements.

Some monitors pair only for the purpose of trapping out HOSP from the standard box.  But that still leaves bluebirds stuck with the Gilbertson or some other tiny "House Sparrow Resistant" box ill suited for bluebirds.  In order for fledglings to survive outside the nestbox, their feathers have to be in good condition and they should have enough space inside the box to exercise their wings prior to leaving the box. 


The withered dead bluebird in this photo was found in a  "House Sparrow Resistant" Gilbertson PVC tube box horribly stuck in "fecal glue".  This is similar to what I found in 4x4 floored boxes on a trail I took over as a new bluebirder.  Notice the thick layer of  fecal droppings under it and also on TOP of its feathers.  Filthy nests such as this attract ants and predators.

House Sparrow Resistant Boxes have something substandard built into the box on purpose in the hopes that House Sparrows will avoid (resist) using them.  Most House Sparrow resistant boxes are built with tiny floor space or they aren't built deep enough.  House Sparrows will recognize the built-in flaws and tend to avoid those boxes.  House Sparrows do not need a cavity or nestbox and can go elsewhere.  Bluebirds do not have that option and will nest in tiny shallow boxes by default.

In one such small box on my early trail, an extremely heat-stressed chick engaged in self-mutilation.  It plucked every feather from its body it could reach and only its head feathers remained.  By the time I opened the box, the rest of the family had left and it was dead (abandoned and starved). 


After my first bluebirding season, the entire trail was retrofitted with larger boxes and that was just the beginning.   The trail had to be rebuilt multiple times over in quick succession to solve a string of problems.  The large  2-holed Box  is the end-result and provides an (almost) problem-free trail.

There is no disadvantage to having two side-by-side holes on the face of a nestbox.  Other species will also benefit from having the second (escape) hole.




Pairing for Tree Swallows:  

 And other species

Joe O'Halloran of the Bluebird Restoration Assoc. of Wisconsin, put together a large study to compare actual results of pairing boxes in Tree Swallow/Bluebird areas.  Tree Swallows increased in the paired setups and Bluebirds decreased.  See:  BRAW Paired Box Study However, note that "paired" boxes in the BRAW study included boxes that were up to100 feet away from one another. 

Tree Swallows protect a relatively small territory around their boxes (about 20 feet).  Anyone wanting dual protection/cooperation from both Tree Swallows and bluebirds at a paired site need to space the two boxes 20 feet or less from each other.  Any other paired setup needs to be at least 300 feet from the first setup to allow for the large bluebird territory.  Additional boxes within the 300-foot bluebird territory will throw the advantage to Tree Swallows (nesting in closer-spaced boxes) at the expense of bluebirds.  Here is another GREAT web page by Chris Gates that covers the subject (including photos) of:  Pairing for Tree Swallows 

At a rather rural location on one of my golf course trails, there is nestbox competition between Western Bluebirds, Tree Swallows and Ash-Throated Flycatchers.  I want to accommodate as many native species as possible but Bluebirds will often usurp Ash-Throated Flycatchers and Tree Swallows.  *IF* there is no history of HOSP problems where I see either Tree Swallow or Ash-Throated Flycatcher nest attempts, I will add another box.  At one location, I ended up hanging three boxes in a tree where all three species were trying to nest a few years ago. This year, bluebirds took one box, ATFL took another and (unfortunately) Tree Swallows failed to make an appearance.  Setups like that are very risky if there is a potential for HOSP problems.  If the location has a history of (or potential for) House Sparrow problems, I avoid multiple box setups. 




Photo courtesy of  Debbie Highfill of Joshua, Texas, taken during March 2009


TIP:  Do not remove 2-Holed boxes during HOSP competition 

And do not put 1-Holers at active competition no-trap test sites

Following is a link where the monitor removed test boxes whenever he saw HOSP nests in them because he was an experienced monitor who was used to having House Sparrows outcompete bluebirds in standard 1-holed boxes.  He also thought the large SIZE of 2-holed boxes (rather than the escape hole) was the important factor.  Purvis Mixed Test   Different box sizes were changed around amongst the problem sites and, apparently, Purvis was not able to recognize consistent 2-holed box successes amongst the chaos of box changeups and 1-holed box failures. 

  • Bluebird nests in 2-holers were not usurped by HOSP  (100% successful in the Purvis Test)
  • Bluebird nests in 1-holers were usurped by HOSP  (at least some 1-holers failed, regardless of box size)

Interesting side note:   Notice at Purvis's test site where this interesting sequence took place: 

  • A "Severe Problem" 1-holer at Amerige Park was replaced with a 2-holer
  • The 2-holer proved successful (two bluebird nests in the 2-holer the first year)
  • Dick removes the successful 2-holer from that site the following year and puts BACK the 1-holer
  • The 1-holer is successful (following the successful year of bluebirds using the 2-holer)

There may be a residual benefit from successful 2-holers carried forward to a transition back to 1-holers.  Similar to what happens when a negative residual is carried forward from problem 1-holers during the transition to 2-holers.



TIP:  When reviewing 2-holed box performance at test sites, don't mix in failures of 1-holers
1-holers do not perform well during HOSP/bluebird battles unless the monitor traps and/or uses Spookers/Halos.  Even then, there are unpleasant surprises with 1-holers.

Standard 1-Holed boxes are vulnerable to House Sparrow attacks.   If any other box styles besides spec 2-holers are used at these no-trap, no-gadget box performance test sites, it is at the monitor's own risk.    So far, only 2-holed boxes have consistently tipped the advantage to bluebirds during active battles with HOSP. 

Bet Zimmerman planned to test 2-holers for three years at a no-trap, no-gadget test site.  See: Bet's Test.  Even though Bet did not set up the test per the "Keys" (spacing), it had the same type of results as the Purvis test site:

  • Bluebird nesting in 2-holer was not usurped by HOSP (100% successful in Bet's test)
  • Bluebird nests in 1-holers were usurped by HOSP  (including deaths in Bet's test)

And (same as Purvis's mix of box styles), the failed 1-holers overshadowed the success of bluebird success in 2-holers. Here is a photo of dead bluebirds from a 1-holer:  Bet's dead bluebirds and her no-trap conclusions  


Common house sparrow control recommendations may reduce, rather than increase, available nestboxes for bluebirds:

  • Remove the Box—Counterproductive, bluebirds need a nestbox, house sparrows do not
  • Move the box to better habitat—Impractical, habitat is getting worse, not better
The difference between what the most and the least learned people know is inexpressibly trivial in relation to that which is unknown." — Albert Einstein

The above quote is a good description of where Bluebirders stand in relation to House Sparrow control in spite of decades of trying. 



You may contact me, Linda Violett
As of June 15, 2012