Owl Predation on Purple Martin Colonies
(by Steven Kroenke)
The wild hoots of the Barred owls often filled the nights at my old home site in Wacissa, Florida. Oh, how I enjoyed the Barred owls "talking" to each other. Their loud and somewhat scary calls reminded me of nature in the raw and horror movies. And how true that proved to be! Their calls now have a different meaning to me and I no longer wish to hear them anymore. For me, the Barred owl's wild hoots mean death and destruction to any purple martin colonies located in the owl's hunting territory. I can speak with some authority on the subject.
I have studied purple martins for over 30 years and have maintained colonies at three sites: my grandparents' old home, about 18 years, 80 pairs; my previous home in Wacissa, Florida, 8 years, 152 pairs; and at my current home, 12 years, 3 attempts, about 30 pairs.
ALL these colonies have been ravaged by Barred owl predation, resulting in the deaths of hundreds of adult and young martins. My grandparents' colony was destroyed over a several year period and I took all the houses/gourds down. My Wacissa colony was raided numerous times during the last several years of existence and I moved. My current site has been hit on 3 separate occasions, causing all surviving martins to abandon the colony. I'm trying again this year with newly designed gourd setups that hopefully will foil the owls. It seems like all my colonies have been cursed with the Barred owl hex!
How do Barred owls (or Great-horned or Screech) find martin colonies? When they find them, how do the owls raid gourds/houses? What hunting techniques do owls employ to pull martins from gourds/houses? How can martin landlords owl-proof their gourds/houses? I will share my observations and try to provide answers to these questions in the following discussion.
Did you know that owls have some of the best hearing abilities in the bird world? Yes, owls can detect the slightest sounds of a mouse crawling over leaves and the sounds of martins in their nests at night. Martins make a lot "noise" in their nests at night. In the aluminum houses, you can often hear the martins "hitting" against the aluminum room dividers, scratching on the floors, and against the nesting material. In gourds and wooden houses, martin scratch on the gourd bottoms and room floors and move around in the nesting material. The male martins sign their death warrants with all the "singing" they do at night. You can often hear the males gurgling calls at night. Owls have no difficulty in picking up these sounds and will eventually fly to the gourd crossbars or house roof to investigate the potential food source.
The owl will then listen for sounds coming from the gourds or house. If the owl is sitting on a gourd crossbar, he will often "walk like a parrot", then bend over right above the suspended gourd, "hop" off, hover for a few seconds "like a hummingbird" right in front of the gourd entrance hole, reach out with his long legs, grab the bottom portion of the hole or rain canopy, hang there, beat his wings, and then scratch with his free foot at the entrance hole. Adult male martins and non incubating/brooding females always try to escape and will often be grabbed as they try to squeeze through the hole which is partly blocked by the owl's talons. During this struggle, thin shelled gourds maybe broken at the entrance hole when the martin is pulled out, leaving a noticeable gap at the bottom of the hole. Thicker shelled gourds may show scratch marks all over the front of the gourd. Even if the martin escapes, the martin will often abandon the colony.
For houses, the owl simply hops off the roof and hangs onto the wide porches and guard rails. Again, adult male martins and non brooding/incubating females try to escape and they often fly right into the owl's talons which are scratching at the entrance holes. If the owl is after young martins, he will simply insert his long legs into the shallow room and reach around into the nest until he finds a baby martin. Then the owl pulls the baby out. If you are using aluminum houses with flip up door panels, the owl will often pull them open or even off when he drags the martins out. I've had the door panels of the Trio houses completely dislodged and the owl carried them off with the caught martin! The wide porches with guard rails become butchering platforms for hunting owls.
It is important to understand that adult martins in the early part of the season will NOT sit tight at night when a huge Barred or Great-horned owl is hanging on their gourd/house, beating his wings against the gourd/house, and scratching at the nest holes. Instinct tells the martins to get out. Even if the martins escape, martins will quickly abandon the colony if owls are constantly tormenting the martins at night and forcing them to flee for their lives into the darkness. Over a 1 week period in early March one year at my current site, a Barred owl constantly raided 3 gourds poles. All the martins abandon the site even though the owl was not successful in catching a martin every night. I would see the martins return in the evening, go into their gourds, then many would simply leave before total darkness. All the martins had abandon the site within a few days after the first week of Barred owl attacks.
However, martins will not abandon their young, though they will their eggs, because of owl attacks at night. Many of the adult martins will simply quit roosting in the houses at night if owls have been raiding the colony. Of course, the females will try to brood their small young and this places them in grave danger. Other females will no longer brood their young, and the young may die from exposure.
Martins scream loudly when caught and I have been awakened at night to their death cries. It is not a pleasant sound. Sometimes other martins flee from the gourds and houses after one of their colony members is caught and screams. The owl typically carries the martin to a nearby tree and eats it there. Then the owl comes back again and again, spreading death and destruction to any adult martins that haven't already fled the house/gourds or baby martins which can't escape.
What can martin landlords do to owl-proof their gourds/houses? I have studied this problem for many years and have learned quite a bit about owls and their techniques for capturing martins in gourds/houses. Here's what did NOT work for me: bright lights on the colony, scarecrows, oversized house compartments, gourd canopies, and loud hard rock music played at night. I'm not saying this would not work for others; maybe a combination of these ideas would discourage owls, particularly if you start BEFORE any predation takes place. Loud music would certainly prevent the owl from hearing martins in their nests at night.
The real solution lies in preventing the owl from even landing on the house porches or "hovering like a hummingbird" in front of the gourd entrance hole. It is probably easier to owl-proof gourd setups than houses because a swinging gourd is already a challenge for an owl, though the owl will learn quickly how to "meet the challenge".
HOUSES:
For houses, a martin landlord can try a number of possibilities
to foil owls. First, convert any non hexagonal house (6 sided) that has the
shallow, 6" x 6" room dimensions to 6" x 12" if possible.
This conversion for a Trio Musselman house, for instance, will reduce the number
of compartment from 12 to 6, but will give the martins, particularly the babies
better protection. The martins will build their nest as far as possible from
the entrance hole.
Then you need to install some kind of guards to each side of the houses, blocking off the nest holes from view, but leaving enough room for the martins to enter their nests. Attaching dowels (Trio offers such an owl-guard) that leave 2 inch spacings may prevent an owl from reaching as far into the nest as it could without the dowels, but this approach may not save any adults. Owls have very long legs and may still be able to reach through the "dowel bars" and pull out baby martins. The adult martins often try to escape and will usually not sit tight while a huge owl is clinging to the martin house, scratching at the nest holes, flapping its wings against the house, and shaking the house. As the adults fly from the house, the owl will try to grab them. However, the Trio owl guard or a similar design is MUCH BETTER than no guard at all! Photo left: An inexpensive guard made out of 2" x 4" hardware cloth (wire) attached with binder clips (an inexpensive office supply item.)
A possible way to owl-proof this type of house is to install protective sloping aluminum canopies that extend the roof line out about 6 inches beyond its current length. Then bending this extension down so that a narrow 3 inch "slot" is between the end of the extension and the upper porch. Another extension is attached to the upper porch to provide a protective sloping canopy over the nest holes on the lower floor. Martins can fly up under these protective sloping canopies, but large owls cannot, and these predators would have nothing to hold on to. The sloping canopy would not offer any support and the owls could not cling to the porches because the canopy would block the porches. Of course, these canopies would make it more difficult to access the compartments through the flip-up door panels; but that is small price to pay to save your colony from owl predation.
If you are using the hexagonal houses (6 sided), such as the Trio Castle, DuraCraft "Hex" or Heath houses, with multiple floors, there is an easy way to block off nest visibility from owl eyes and talons, yet still provide enough space for martins to go around the "block" and enter their nests. Cut out a 4 to 5 inch wide aluminum (could be thin plywood) strip that is long enough to reach from the bottom porch to the end of the sloping roof. Using bolts or clamps, simply attach these strips vertically to the porches and roof lines of the front of each side, thereby blocking off the nest holes from view. Even with the block in front of the hole, the martins have plenty of space to go around the block, walk behind it along the porch, which is about 3 inches wide, and enter their nests. Owls could not even see the nest holes (and the young within) nor reach around behind the blocks to insert their claws into the nests. I used this approach on my only 24 room Heath house in Wacissa when barred owls were exterminating my colony and not a single nest was raided. This was the only house I had that seemed to be protected from the owls.
Another approach is to completely enclose your houses in fencing, creating a prison or cage around the houses. However, to maximize anti-predator effectiveness, the fencing would need to extend out several inches from each side of the house rather than be flush up against the porches. If it is flush to porches, owls could still hang on the fencing, reach through it, and insert their long legs into the nest holes. Ideally, the fencing, when completely enclosing the house, should extend out about 6 inches from the porches. This way no owl could reach 6 inches through the fencing, then reach another 3 inches (width of the porches) to the hole entrance, and then reach even further into the compartment itself to pull out the martins.
GOURDS:
I am primarily a gourd user as gourds are the very best
and most attractive nesting sites for purple martins. Martins prefer them to
all other housing because martins like the large nest dimensions and nest domination
by possessive males is almost nonexistent. A cluster of 12 gourds will often
achieve full occupancy while a typical 12 room house with rooms side by side
rarely is more than 50% occupied.
Though swinging gourds maybe more challenging to an owl, even huge Barred and Great-horned owls learn how to raid them as I have previously discussed. The key is to prevent the owl from "hovering like a hummingbird" in front of the gourd and grabbing the entrance hole or rain canopy. This is not difficult to prevent and I believe I have several possibilities that would work.
One method involves attaching protective hardware cloth or similar fencing canopies to the crossbars right above each suspended gourd. Think of these as "roofs" over the gourds that extend out about 10 inches beyond the front of the gourd and then curve down, leaving about 10 to 12 inches of space between the gourd hole and the end of the curvature. The canopies curved end would be even with the top of the entrance hole. This setup provides plenty of space for the martins to fly to and from their nests, but would completely block off any owl from hovering in front of the nest hole. The owl could only land on the crossbars and "dream" about having martins for dinner! The owl simply could not hop off, drop down, then come up under the canopy and hover there. There is no room for such a maneuver. These could be separate canopies for each gourd or could be a continuous canopy that provides protection for all gourds on one section of a crossbar. The gourds would have to been facing the same direction for the continuous canopy approach.
Another method involves the using wiring to block off access to the gourds. This method is a little more difficult to explain. I'm trying this method this year. It involves a typical crossbar set up of 8 gourds. The gourds are facing different directions: the four on each end of the crossbars face outward; while the four inside gourds face each other's backside. I have created a "clothes line" pattern of plastic coated wire strung between the each of the 4 ends of the crossbars and another wire that is strung in the middle of each crossbar right above the inside gourds. This pattern would prevent any owl from hopping off the crossbars and hovering in front of the inside gourds. Each inside gourd has 2 wires strung over it, blocking any large winged predator from trying to grab the gourd entrance hole. To protect the 4 outward facing gourds, I have used 2 different techniques. For 2 gourd poles, I have attached aluminum dowels to the crossbar ends, extended them out about 14 inches, then bent them downward to produce a curvature that blocks off the immediate vicinity of the gourd entrance hole. The distance between the curvature and the gourd front is about 10 to 12 inches. Plenty of room for martins fly around the dowel, but, hopefully, a preventive block for a hovering owl. A large owl would have a difficult time trying to hover in front of the gourd without his wings or body hitting the curved dowel.
The second technique involves my other 2 gourd poles and is an extension of the outer wiring that is attached to the crossbar ends and circles the crossbars. I have created a wire baffle or basket that is attached to the ends of each crossbar. This baffle is triangular shaped, extends out about 10 to 12 inches, and curves down near the front of gourds. Think of this as a v-shaped piece of wire, with each end attached to the outer wiring. Another piece of wire is attached to the end of the crossbar and then extends out to the end of the v-shaped wire to add support. This wire also hold up the v-shaped wire. This combination of wires produces a wire "basket" in front of the outside gourds. There is plenty of room for the martins to fly either through the opening in the triangular shaped baffle or up underneath it to reach the gourd nest hole.
I realize it is difficult to explain these owl-guard setups without pictures. A picture is worth a thousand words! I have taken pictures of the setups. I am working with Kent Justus on his purple martin web site called Purple Martin Headquarters. We hope to include a picture of this setup when the web site goes online.
Owl predation is a very serious problem for martins and results in the destruction of many colonies each year. Martin landlords can't understand why their birds are there one day and then gone the next. There is a good chance that an owl raided it the previous night. Small colonies are wiped out over night. The surviving birds just abandon the site. Martins, particularly early in the nesting cycle before any young have hatched, will NOT stay at a colony site where they have to flee for their lives into the darkness every night! All martin gourds/houses should be equipped with owl guards to protect the martins from not only owls, but from Accipiter hawks and crows, too. Now is the time to take action to prevent or reduce the chances for owl predation on your martin colony.
Steve Kroenke - Tallahassee, Florida
