How Owls Raid Gourds
From: Steve Kroenke, Tallahassee, Florida
The bright lights shining on my huge purple martin colony revealed the terrifying secrets of the night. The large, brown sinister shapes sitting on the gourd crossbars were not there to rest; they were there to hunt purple martins. The barred owls had come to feed.
For a long time, the barred owls kept their secret from me and I could not for the life of me understand how a crow-size barred owl could somehow pull martins out of a swinging gourd that had no porch to sit on. How were they doing it? I finally discovered their secret.
But first, how were owls finding my martin colony? It was based on sight and sound, with sound the major factor. Barred owls begin their nocturnal hunts late in the afternoon, so they could no doubt see large numbers of martins gathered around the colony site. Owls can also see martins moving around in their nests in houses with the shallow 6"x 6" rooms and holes only 1 inch above the floor. Martins have no chance in such houses from owl attacks. But it is sound which draws owls like a magnet to purple martin colonies. Martins sign their own death warrants with all the noise they make in their houses/gourds at night. Martins hit against the aluminum dividers inside the Trio/Heath houses. They scratch the nesting material and against the bottoms of gourds. The baby martins are often still calling for food even when it is dark. And the male martins can be heard emitting their gurgling calls. All these sounds mean "possible food" for hunting owls and owls have some of the best hearing abilities in the animal kingdom. Owls can detect the slightest sounds of a mouse scurrying across a forest floor and the sounds of martins in their nests at night.
Now, that we know how owls find martin colonies, how are huge owls, like barred or great horned able to raid swinging gourds? The brights lights turned night into almost day around the my large colony and I could observe the owls' hunting behavior. I learned their secret and watched them hunt my martins on several occasions. Owls like to survey an area for possible prey. They would often land in nearby pine trees first and then eventually fly to the gourd crossbars. Here they would sit and listen for sounds coming from the gourds. Sometimes an owl would "walk like a parrot" on the crossbar as he sized up a gourd for raiding. Then he would bend over, hop off, and "hover like a hummingbird" for a few seconds right in front of the gourd. He would reach out with one of his long legs and grab the bottom of the gourd entrance hole, hang there, sometimes beating his wings against the gourd, and then start scratching with his free foot at the gourd hole. He may also even grab at the entrance hole with his sharp beak. Adult male and nonincubating female purple martins panic and try to escape out the entrance hole. Instinct tells them to get out. The owl's talons are partially blocking the entrance hole so the martin has to squeeze through. The owl then grabs the martin right at the entrance hole with his free foot. During these struggles, thin shell natural gourds may be broken at the entrance hole, leaving a key hole shape. Thicker gourds and plastic gourds may show scratches all over their fronts. Even martin blood with their feathers sticking to the blood may be smeared on front of the gourd. The power of the owl's grasp is so great that I have had gourds completely penetrated by his talons, with several holes near the entrance hole. If martins would sit tight, then they would be safe, as the owl cannot reach in a swinging gourd and extract the martins; he has to grab them as they flee for their lives. Baby martins would most likely be safe in large gourds as long as they did not try escape out the nest hole. [Editorial Comment: I disagree. I'm certain owls could use their 10-12" reach to extract martins from swinging gourds.]
Since I now knew how large owls raided swinging gourds, I reasoned that the
key to preventing owl predation was to preclude
the owls from "hovering like hummingbirds" in front of the gourd entrance
holes. Also, I figured that making it difficult for an owl to even land on the
crossbars in the first place would also help.
I thought of several designs and even incorporated them in my current gourd setups. I posted my a discussion of owl predation and owl baffles about 2 years ago and that article is in the Purple Martin Forum archives, Owl Predation on Purple Martin Colonies. I shared these ideas with other folks, including Kent Justus. There are several ideas that I thought would block the owl's hovering in front of the gourds. This includes using dowels that protrude out in front of the suspended gourd, thereby blocking the immediate front. A more promising design is what I call an owl wire baffle that also extends out in front of the gourd, but provides more coverage while still allowing martins to easily reach their nests. Another option would be to use a continuous flow of hardware cloth or fencing that is attached to the crossbars and then extends out and bends over in front of the gourds. And, stringing wire between the crossbars to create a "spider-web" or "clothesline" effect would protect inside gourds. I'm sure there are a number of options one could consider. The key is to keep large owls from hovering in front of the gourd–that is the bottom line.
I am in the process of modifying all my gourd setups with owl wire baffles and also including a thin wire that will be placed about 4 inches above each crossbar. This wire will make it very difficult for any large owl to "sit comfortably". In fact, he would not be able to hold onto such a thin wire and would be highly unstable. Owls prefer to sit on the gourd crossbars FIRST, listen for martins in the gourds, and then execute their "hummingbird maneuver". As soon as I complete these setups, I will take pictures and hopefully include them on the Purple Martin Forum.
Remember: owls are everywhere and have no difficulty finding purple martin colonies. We have barred owls nesting in a city park, not more than a half mile from downtown Tallahassee. Great horned, barred and screech owls are significant predators of purple martins and can inflict great damage to unprotected martin colonies. Small martin colonies can be wipe out overnight; large colonies take longer to destroy. Martins will readily abandon colony sites where they have to flee for the lives each night because of owl attacks; this is particularly true early in the nesting season before martins have young to feed. Martins will not abandon their young, but they will abandon their eggs if owls constantly bother the martins at night.
Steve
