Vertically Deep Purple Martin Cavities
From: Steve Kroenke, Tallahassee, Florida
Date: 1/12/02
Time: 2:57:10 PM
Comments
Vertically Deep Purple Martin Cavities
How it got started no one knows for sure. But the design concept is now firmly entrenched in purple martin housing. Every bird book says to do it and every commercial martin house from Trio to Lonestar does so. Nearly every homemade martin house probably does it, too, and this commandment has become the gospel: thou shall place the entrance hole around one inch above the compartment bottom. It is the "law of the land" when referring to multi-room martin houses.
This Law Flagrantly Violates The Martin’s Past
Well, just because everyone is doing it doesn’t mean it is the only way to go! This "law of the land" governing entrance hole placement defies thousands of years of biological nesting history for the purple martin species. In geological time using a 24 hour day as a reference point, it could probably be argued that perhaps 23 hours, 59 minutes and 59 seconds of martin nesting behavior in eastern North America was "vertically deep" and just a recent fraction has been "horizontally shallow". Prior to multi-room martin houses and gourds, martins probably nested in vertically deep abandoned woodpecker holes, some of which can be a foot or more in depth, and also in similar natural cavities. Such deep cavities were safer from predators, better insulated, and perhaps gave the female martin a sense of security for her nest. However, these cavities are narrow and tend not to be as "spacious" at the bottom, since woodpeckers sacrifice nest bowl volume for depth. But martins survived for thousands of years in such a nesting environment. And then we humans started messing around and changing things.
Blame It On Their "Short Legs"?
So how in the world did the "one inch hole placement above the compartment bottom standard" arise? I have no idea, but perhaps folks figured martins have short legs and therefore they need a "short" distance from the nest hole to the nest chamber. Total nonsense. Martins may have short legs, but they have strong feet and can use their nails to climb up rustic surfaces. Short legs are better for "crawling, waddling or climbing" in confined areas and perfectly suited for movement inside a rustic, deep nesting chamber. Martins no doubt used their short, but strong feet to climb up and down in those deep but narrow woodpecker cavities. I have watched young martins that have prematurely fledged and landed near the base of a tree climb a good distance right up the tree’s bark. But neither adult nor young martins can do this on slick aluminum, plastic and many painted wooden surfaces of modern day housing. However, martins can "jump fly" from the bottom of a deep non-rustic cavity provided there is enough wingspan room to make the upward move. A very narrow deep non-rustic chamber could become a death trap for a martin, as the martin could not climb out nor "jump fly" out. The long wings of the martin need room for flapping and subsequent upward propulsion.
Maybe We Did It For Our Convenience
So, perhaps building horizontally shallow martin houses was easier for us and permitted more nest compartments per house. Perhaps houses with holes place close to the compartment bottoms are more aesthetically appealing to humans. Our own houses have rather "shallow" doorway entrances, don’t they? We may have designed such martin houses because it was easier and more convenient for us and not necessarily for the martins. Who knows? Though the standard is now in concrete, it can still be circumvented. Well, how do you do that? And why would you want to do it?
Natural Gourds
I’m a hardcore lawbreaker and always "break the one inch law of the land" with natural gourds. I do it with gusto! I have always cut nest holes in natural gourds anywhere from 3 to as much as 8 inches or even more above the lowest point on the bottom, usually averaging 5 or 6 inches. The shape and size of the gourd dictates the placement of the entrance hole, as you don’t want the hole cut on a curve to point upwards too much; this would allow more water inflow from rain. However, this problem can be alleviated with rain canopies. Remember: natural gourds are rustic inside, so martins can cling to the sides more easily if necessary and large diameter gourds provide plenty of wingspan room. Martins have no difficulty exiting vertically deep natural gourds.
Commercial Plastic Gourds
Most of the plastic gourds on the market tend to have entrance holes placed vertically less deep from the bottoms than comparable natural ones. Since plastic gourds are "more curved and sloping" from the entrance hole down to the gourd’s lowest bottom point, this makes it somewhat more difficult to obtain a definitive measurement. However, that said, I took some rough measurements by placing 2 different brands of plastic gourds on a flat surface and then measuring the distance from there to the bottom of entrance holes. The Carroll Industries gourd entrance hole is less than 3 inches from the bottom. The SuperGourd is vertically deeper and is around 4 inches from the bottom of the entrance hole to the lowest point inside the gourd. Though a martin can’t climb out of a slick SuperGourd, there is more than enough wingspan room for him to make the "jump fly" move to reach the entrance hole. I have no data on the Natureline brand of gourds but the entrance holes appear to be less vertically deep than SuperGourds from looking at pictures of them.
Commercial Aluminum Houses
I have used Trio, Heath and Lonestar brands of aluminum houses. All of these have entrance holes one inch from entrance hole to the compartment bottom. If you include any inside nest trays, then the distance decreases to less than an inch. I am not aware of any commercial houses on the market that incorporate vertically deeper compartments.
My Homemade Houses
For many years, I used my homemade houses in at least 3 of my various martin colonies. Most of these houses incorporated the one inch standard, but I did increase this to as much as 3 or 4 inches in a few of my multi-room houses. In addition to multi-room houses, I have used single unit wooden houses suspended on crossbars just like gourds. I did this for several years at my super colony in Wacissa, Florida. These houses were 8" x 8" in diameter and had entrance holes anywhere from 4 to 6 inches above the bottoms. Martins thrived in them and these houses were almost as attractive as natural gourds. The main problem was the weight and a cluster of these was much heavier than a comparable number of natural gourds. This was a big negative for single unit houses.
At my first boyhood colony, martins nested in single unit houses that I had erected in open areas near the main multi-room martin house. These single houses were constructed of exterior plywood, had inside dimensions of around 6" x 6" or larger and were erected for great crested flycatchers or woodpeckers. Some were around 12 inches in vertical depth and martins nested in all of them. There was enough wingspan room for the martins to "fly jump" from the bottoms to the entrance holes.
Which Housing Has Been Most Attractive?
There is no question here when reviewing occupancy levels of all these types of housing in my martin colonies. Natural gourds have ALWAYS had significantly higher occupancy levels than all other types of housing I have used. My natural gourds on average have incorporated vertically deeper nest chambers than any of the commercial or my homemade multi-room houses or plastic gourds. The distance from the entrance holes of my natural gourds to the gourd bottoms have averaged between 5 and 6 inches. Of course, you can’t say the vertical deepness factor is a determining variable as there could be a combination of influences involved. In an earlier posting, "The Darkness, Seclusion and Rustic Factor", which is in the Forum Archives, I had discussed these concepts relative to martin housing attractiveness. I do believe martins, particularly the females, seek out cavities that maximize darkness and seclusion for their nests and young. Such nesting chambers may give the female a sense of security and protection from predators. So, the vertical deepness factor may be an extension of the darkness and seclusion concept and these variables could work together to influence the attractiveness of martin housing. These factors could be key components of the attractiveness of large natural gourds to martins.
Pros And Cons Of Vertically Deep Cavities
There are a number of benefits to using vertically deep cavities to martins. Here are some:
(1) Much safer from predators, particularly from the long legs of barred and great horned owls. When the owl reaches in a vertically deep cavity, his foot for the most part may be grabbing the "empty airspace" of the open area ABOVE the actual nest site. Of course, this depends on the depth of the cavity. It is more physically difficult for him to twist his leg vertically downward. He has a harder time reaching the actual nest and clutching and pulling nesting material that will greatly frighten adult martins. Deep horizontal cavities still allow the long legs of the owl to reach straight into the nest chamber and grab nesting material, disturbing the martins and eventually hooking terrified adults and their young. However, deep vertical compartments would NOT stop adult martins from panicking and trying to fly out the entrance hole, particularly if the owl is shaking the gourd or house. Probably nothing would. But the babies may be more protected. (2) Would significantly reduce the chances of eggs accidentally "rolling out the entrance hole" as the martins exit their nests. This can also occur if martins are fighting inside the nest. Eggs may be dislocated for various reasons from the nest bowl in houses when martins build their nests level with the bottom of shallow entrance holes. (3) May encourage young martins to remain "deeper" in their nest cavity as they wait for their parents to bring food. This may discourage large well feathered out young from prematurely exiting or reduce the chances of them being "pushed out" of nest holes by other babies during feeding time. (4) May give any mate hunting female martin a greater sense of security because of the darkness and seclusion of such a deep cavity thereby encouraging her to accept the male that controls the territory. (5) May be cooler in the summer because hot air rises and would tend to escape out the entrance hole. Woodpecker cavities are excavated where the entrance hole is basically a tight tunnel that gradually enlarges toward the bottom. Such a cavity channels hot air directly from the nest bowl right out the hole. In shallow cavities, the hot air will rise to the top and tends to "stay there" and circulate inside the cavity unless sufficient air vents are provided. (6) May provide a better environment for building more secure nest foundations since the entrance hole is farther away from the nest site bottom. Martins could add more nest material that would not partially block the entrance hole and may reduce the "mud lip" barrier sometimes seen bordering nests in the shallow house compartments. (7) Deep vertical cavities may "activate" stored genetic memories from the martin’s biological past and thereby function as an attractant. This may be just an extension of the security and seclusion factor.
There are possible disadvantages to using vertically deep cavities. These include the following:
(1) In multi-room houses, such a design is probably more difficult to incorporate and tends to reduce the number of compartments. It would be difficult to reconfigure those Trio houses to vertically deep compartments and the same situation would exist with other commercial brands. Also, homemade wooden houses may become heavier with vertically deep compartments. (2) If using SREHs in aluminum, plastic and painted wooden housing, then such deep cavities may present martins with some difficulties exiting as there would no rustic surface to aid the martins in pushing themselves through the tighter holes. Using internal porches or cling rods at the entrance hole from the inside may resolve any potential problems. (3) Such cavities may permit longer and more intense intra-specific competition battles between martins. The losing martin may have a more difficult time exiting from the bottom of a deep cavity. However, I have not noticed such problems in my natural gourd colonies.
Okay, Should We Incorporate Entrance Holes In Our Martin Housing That Are Vertically Deeper?
Yes, I think we should consider such vertically deep housing where practical, as the pros seem to outweigh the cons. Here are some suggestions:
(1) For natural gourds, you can cut the entrance holes as high up on the gourd’s surface as appropriate. The hole should be nearly flat against the outer surface of the gourd to reduce rainwater inflow. If you use rain canopies above the entrance, then there can be some variation here in entrance hole positioning. I have had excellent results with holes placed anywhere between 3 and 8 inches above the lowest point on the gourd’s bottom. An average range of 5 or 6 inches in vertical depth has worked well. (2) For commercial plastic gourds, there is not much you can do to modify them. However, the excellent SuperGourd already has its nest hole placed around 4 inches from the bottom, so there is vertical depth present in this brand. (3) If you are building a multi-room martin house, you could certainly place the entrance holes higher than the standard one inch above the compartment bottoms. I have used nest holes as high as 3 or 4 inches and the martins readily nested in these compartments, though I can’t say for certain that these houses were anymore attractive. But there are some advantages for the vertically deeper holes as previously discussed. (4) As with the plastic gourds, you are limited in what you could do to increase the vertical depth of entrance hole placement on most commercial aluminum houses like the Trio brand. It would rather difficult to cut holes higher up on door panels with the existing holes in place. These houses are more suited for reconfiguring the compartments into double nest chambers.
Martins may instinctively prefer a nest site that maximizes darkness and seclusion to give greater protection from predators to themselves and their offspring. The vertical depth perception of a gourd or house compartment is largely a function of the placement of the entrance nest hole. The farther away the nest hole is from the bottom the greater is the seclusion and darkness of the nest chamber. Most of purple martin nesting history has been in the vertically deep and rustic nest cavities prepared by woodpeckers or in similar natural ones. We partially changed this behavior with multi-room houses incorporating entrance holes around an inch above the compartment bottoms. However, in the Deep South, we somewhat continued the concept of greater vertical depth and rugged interiors by using natural gourds. Natural gourds today provide secluded, dark interiors and their entrance holes are usually cut significantly higher above the nest bottoms than the ones found in multi-room martin houses. Natural gourd colonies are often the largest. I believe we can learn from the successes of gourds in attracting and raising martins.
Steve Kroenke, Tallahassee, Florida
