In September 2006, I set up a experimental two-hole mansion trail with 5 boxes made and donated by Linda Violett of California (see photo, courtesy of Linda Violett). The boxes are hanging from trees on a small town green surrounded by farms and residences. There were 7 existing HOSP-infested, pole-mounted boxes there (see photos) that I stumbled across and obtained permission to manage. Bluebirds have been seen in the area. Note: Two of the boxes went missing - one in 2006, one in 2009 - from climbable trees. I have another two-holed mansion that is pole-mounted on the Chimalis trail. It has not been used yet.
The Experiment: The existing pole mounted boxes (no baffles) were plugged in 2007. In Year 2, someone unplugged the single-holed boxes. No HOSP trapping was conducted, in order to evaluate whether EABLs are able to secure and successfully defend the two-hole mansions despite HOSP competition. I am removing any HOSP nests and eggs on at least a weekly basis.
I'm doing this experiment for two reasons: 1) I believe that about half of serious bluebirders prefer passive alternatives to HOSP control or are unwilling to take any steps to keep HOSp out of boxes, and 2) I want to gather data about two-hole box usage by EABLs. I can't wait to see what happens, although I admit it will be hard to potentially allow some losses to HOSP. I also want to limit the variables so I can assess the two-holers. (I do use ground and inbox traps as needed on my other trails). Squirrels may be an issue with the hanging boxes, as red, grey and flying squirrels are common in this area.
Conclusions SO FAR (as of 03/09):
Here are my opinions, based on two full seasons (2007 and 2008) and one partial season (2008). However, be aware that I only have four of these hanging two holed boxes. I also have 80+ pole mounted boxes.
Increased species diversity (which is something I WANT). In 2008, one of the only TUTI nesting was in a two-holed mansions, along with one of only two WBNU nestings (one in a 2 holer, one in a tree-mounted one holer - both were successful. In 2007, I had only three TUTI's attempts on my trail, of which two were in the hanging boxes (unfortunately both failed due to HOSP and HOWR competition.) The 2008 TUTI attempt in the two-holer failed due to House Wren predation.
One EABL pair able to successfully fledge two young in HOSP-infested territory in 2007. (Nest built after trees leafed out.)
Safer from predation compared to a tree or telephone-pole mounted box. The large overhanging roof, hook and deep box probably offer some protection against raccoons and definitely against almost all cats. (No snakes in my area.) The two-hole mansion is deeper which offers additional protection (although I find users build bigger, higher nests.)
Safer from vandalism or stealing, ONLY if camoflaged (if hung high enough in a tree that is not readily climbed.
Cheaper system (no pole mountings system or predator guards) - a big advantage for a large trail.
Low utilization by Tree Swallows? (None so far in any hanging boxes or the pole-mounted box in Tree Swallow territory)
The two-hole mansion is very roomy inside.
The two hole mansion probably enables adults to escape more readily from a HOSP inbox attack.
HOSP interest seemed to decline after trees leafed out.
Harder to monitor than a pole-mounted box, and takes longer (must have lifter or ladder on hand, takes about 2-3 times longer to monitor, tricky to get the technique down, concern about dropping box and disturbing contents and incubating female.) The two-hole mansion per Linda's specs is very heavy.
Harder to vandalize (as long as they are hung high enough in a tree that is difficult to climb) BUT the only two boxes ever lost on my trail were these. Both are apparently too close to a posh high school. The first one was too low in a climbable apple tree. The second was in a larger, but climbable tree.
Appeal to Flying Squirrels, which may predate nestbox contents. (Personally, I think it is cool to have Flying Squirrels nesting in a box though.) I have never had Flyers in my pole mounted boxes, but in year one had them in 2 out of 4 hanging boxes (however, Linda's boxes are larger than a typical single-holed box.) I have seen Flying Squirrels using pole-mounted boxes in a nearby unmonitored trail on a golf course, where boxes are at forest edge.
Predator problems: Snake access? (no direct experience). Roof rats (not in my area.) Honeybees? (I have only had bumblebees in mouse nests in pole-mounted boxes.)
Low utilization by bluebirds SO FAR. No bluebird attempt until trees had leafed out and HOSP pressure declined in year 1. No bluebird attempts at all in year 2.
Some people don't have good "hanging" trees in suitable habitat.
The two-hole mansion has a large interior, and users tend to build larger, bulkier nests which takes more time and energy.
Unfortunately these boxes are not HOWR-resistant (no boxes I've used so far are!)
BCCH = Black-capped Chickadee
EABL = Eastern Bluebird
HOSP = House Sparrow
NABS = NABs style nestbox
PETE = Peterson style nestbox
SLOT = Slot style entrance on nestbox
TRES = Tree Swallow
WBNU = White-breasted Nuthatch
NOTE: THE 2006 LOG BELOW and drawing SHOW USE OF THE TRAIL BEFORE TWO-HOLE HANGING MANSIONS WERE INSTALLED
Attempt #, Species & Fate 8/10/06
HOSP - blue feathers
HOSP - saw male perching
Pine needles - EABL?
Chewed hole. Cracked.
grass and lots of white poop - HOSP? Top is split.
Mouse nest? Grass and leaves without cup.
HOSP - active, 4 eggs on 8/10
EABL (pine needles, saw them attempting to enter after plugging)
HOSP, poss TRES. seed heads.
Few pieces of grass
8/10/06: cleaned out contents of all boxes. Some split roofs. Looks like boxes had not been emptied in years. See photos. No sign of paper wasps.
9/06: Installed 5 new two-holed mansions in trees. Saw at least one male EABL. See 2007 log of activity.
Daddy dear, tell me please, is the world really round? Tell me where is the bluebird of happiness found?
- Little Child (Mon Enfant), Wes Montgomery, Ralph Towner, Spike Jones
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