Woodpeckers do it vertically deeper for good reason and woodpeckers have been doing this for thousands of years. That should tell you something! Such nests are more protected from predators than a shallow cavity with the entrance hole cut only an inch or less above the nest chamber. When woodpeckers excavate their nest site, they dig in for a few inches, depending on the diameter of the tree, and then go sharply downward in a gradually enlarging chamber until reaching the bottom. The area around the entrance hole is narrow and gradually enlarges with greater vertical depth. These cavities can resemble a boot in shape. Such cavities may drop vertically downward a foot. The nest site may be oval or elliptical in shape and not as large as the woodpeckers that create them. I have measured the inside dimensions of red-bellied and red-headed woodpecker and flicker cavities and these are often about four to five inches in width and five to sometimes seven inches in length. Woodpeckers sacrifice nesting chamber girth for deeper vertical depth. Purple martins nested successfully in such cavities in the past in the east and are still raising families in them out west. The martin?s short legs and sharp nails are perfect physical adaptations for climbing in and out of rustic sided cavities. Other cavity nesting birds like bluebirds, great crested flycatchers, tree swallows, titmice, woodpeckers, and others successfully nest in such environments.
You can create a somewhat similar replica of a woodpecker cavity by using more elongated shaped gourds with shorter necks. Finding suitable candidates for ?woodpecker? gourds can be difficult with most martin gourds, since many are round in shape. These same type gourds make excellent horizontal martin cavities with offset entrance holes cut in the neck area to produce a 90 degree turn. Most woodpecker gourds can be turned on their side to create a horizontal design.
This year I used four woodpecker gourds that maximize vertical depth from the entrance hole and provide a very secure nest site for purple martins and three were occupied. However, my gourds are much larger in girth at the gourd bottoms than a woodpecker cavity. In these gourds, the entrance is cut on or just below the neck, creating vertical depth of around eight or more inches. The immediate around the entrance is narrow like a woodpecker cavity and creates privacy and protection. Martins like this. Then the gourd is tilted forward at about a 45 degree angle on the hanging crossbar so that the hole is pointing downward, producing a gradual sloping vertical descent to the bottom of the nesting chamber. This type of gourd has both vertical and horizontal depth and somewhat replicates a natural woodpecker cavity, which greatly protects any nesting birds from the reach of predators and rain inflow. When I add one of my pre-built nests, the vertical depth is reduced to about six or seven inches from the entrance hole.
Martins have NO difficulty reaching the entrance hole because the gourd inside substrate is coarse and martins are perfectly adapted to climb such material with their short legs and strong nails. This is much better than having the nest level with the entrance hole, which will often occur in vertically shallow cavities with entrances cut only an inch or less above the bottoms.
Also, purple martins do NOT build their nest all the way to the entrance hole in vertically deep ?woodpecker type? cavities and I have never seen a mud dam in one. Building the nest to the entrance hole bottom in vertically deep cavities would be biological/evolutionary suicide and expose themselves and young to predators. Martins don?t do it in woodpecker cavities and neither do other cavity nesting birds. Humans have offered vertically shallow cavities to martins and this has resulted in martins building their nests up to the entrance holes and then possibly using mud dams to reduce visibility inside and protect the eggs/young. When a martin only has an inch or less from the bottom to the entrance, then the martins have no choice but to build their nests up to the hole.
I hope to use more vertically deep woodpecker gourds in the future. I just need to find suitable candidates! Below are some pictures of martins using woodpecker gourds. On the last picture you will see where the previous entrance hole was cut before transforming the gourd. That hole was on the front and not as vertically deep from the bottom. I covered the entrance with a piece of aluminum using adhesive.
Steve
