I first started using natural gourds for purple martins in the mid-60s when I was just a teenager. One gourd style I used that was well like by martins was a natural gourd with the entrance cut high up near the gourd neck. I call this design a woodpecker gourd because it somewhat replicates the natural vertically deep cavity that woodpeckers excavate and martins nested in such cavities for thousands of years. Martins still nest in woodpecker cavities out west.
Martins have short legs and sharp nails which may have evolved in part due to martins nesting for eons in vertically deep woodpecker cavities. The martin’s short legs and sharp nails are ideal for climbing down into a rustic woodpecker cavity and then climbing out.
The internal substrate of a natural gourd is coarse and martins can easily climb up the surface. Think of a natural gourd as a woodpecker cavity extracted from a tree!
Woodpecker cavities are not large inside. They are vertically deep and woodpeckers do not build any nest. The females lay their eggs on a bed of wood chips. I have measured the insides of old woodpecker cavities of red-bellied and red-headed woodpeckers and flickers. They tend to be oval shaped and vary considerably in size. I have seen some that were around 4 inches wide and 6 inches long and others that were about 5 inches wide and around 7 inches long. The entrance holes may be anywhere from 8 to 12 inches or more from the bottom and the area around the entrance tends to be tubular. A woodpecker cavity can resemble a boot depending on the diameter of the nest tree.
However, my natural gourds were much larger inside than a woodpecker cavity and varied between 9 and 11 inches in diameter. The gourds I selected as a woodpecker gourd tended to be more elongated in shape or have a longer/larger neck area.
There are a number of benefits for using vertically deep cavities. 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 probably would NOT stop adult martins from panicking and trying to fly out the entrance hole, particularly if the owl is hanging on a gourd or house. Probably nothing would. But the babies may be more protected. Also, the inside nest contents in vertically deep cavities are not nearly as visible to the eyes of owls as the nests in horizontally shallow compartment/gourds with a straight line view to the back. Owls use both sight and sound to predate martins.
(2) Would eliminate the chances of eggs accidentally rolling out the entrance hole as the martins exit their nests. This can occur if martins are fighting inside the nest and the eggs are pushed around.
(3) May encourage young martins to remain deeper in their nest cavity as they wait for their parents to bring food; this is particular true of smaller nestlings. 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. I saw this positive attribute of vertical deep natural gourds/wooden single unit houses in my colonies.
(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 eliminate the mud dam barrier sometimes seen bordering nests in the shallow house compartments. In my vertically deep natural gourds/single unit wooden houses with entrance holes cut 6 or more inches above the bottom, the nests were typically 2 to 3 inches thick. No mud dams were ever built in such cavities and no nests were built all the way up to the bottom of the entrance hole.
(7) Vertically deep cavities provide protection to small martin nestlings and minimize the chances that SY males can remove them. In my vertically deep gourds, I have never seen SY males succeed in carrying martin nestlings from the nest bottom out the nest entrance.
(8) Deep vertical cavities may activate stored genetic memories from the martin’s ancestral past and thereby function as an attractant. This may be just an extension of the darkness and seclusion factor.
Though I no longer use any natural gourds in my martin colony, I still remember how successful these gourds were in attracting and raising martins. And I will always like my vertically deep woodpecker gourds and the martins liked them too. I always had 100% occupancy levels in these gourds.
Here are some photos showing martins using a variety of vertically deep woodpecker gourds in my martin colony. These gourds had entrances cut in the necks anywhere from about 8 to 10 or more inches from the bottoms. The gourds were suspended at about a 45 degree angle or less with the entrance pointing slightly downward. The martins entered and then dropped down into the deep nesting chamber. I usually built a pre-nest of pine needles that was about 2 or 3 inches thick depending on the vertical depth. I tried to keep the depth from the entrance to the nest at least 5 inches and preferably more so the martins were not in view from the entrance. The female could be hidden as she incubated her eggs or brooded her small young. On some of the gourds I attached small cane perches just under the entrances and these helped the parent martins to feed their babies when they got larger and climbed up to the entrance to be fed.
Male ASY martin exiting his woodpecker gourd
Female ASY martin peering out of her woodpecker gourd
Male ASY martin peering out of his woodpecker gourd
Male ASY martin peering out of his woodpecker gourd
Pair of ASY martins at their woodpecker gourd
Male ASY martin peering out of woodpecker gourd
Steve
I can maybe see how the troyers got the idea for the plastic mold...lol