Richard & Diane Van Vleck - Personal Pages
Naturalist's have long preached the value of "edges", such as where forest meets meadow or lawn meets field. In developed areas, the most important edge is the fencerow. This is where the most species of birds are found, because of the variety of vegetation present. In contrast, large fields offer a monoculture attractive to only a small number of species. Predators also frequent fencerows, creating well established trails in their nightly sojourns. The vast majority of bluebird nestboxes are erected along fencerows simply because they are out of the way of farm machinery and the boxes can be conveniently mounted on fence posts. However, many fencerow bluebird trails suffer a high rate of nest predation by house wrens, raccoons, opossums, flying squirrels, chipmunks, and black rat snakes. Where predation is a serious problem, nestboxes moved out into the open field may be less frequently discovered by predators. And an open field may be the only site away from wren habitat in some areas.
Since posts cannot be set in the middle of a field where machinery must pass, a movable nestbox is required. A post mounted on a crossed plank base can be easily moved by one person and will last for many years if the parts touching the ground are made from cedar or locust or are well sealed. In many crops, the box may need to be moved only once or twice during the nesting season. Moving the box aside for a tractor to pass and returning it to its same position and orientation moments later will not bother bluebirds or tree swallows any more than a nest check. The base of the portable nestbox will leave a footprint in the soil, making it easy to replace the box precisely in it's original position.
The base of the portable nest box must be wide and heavy enough to prevent the box from blowing over in a strong wind. Crossed planks 3' long should be adequate. Also, placing a couple pieces of scrap iron on the base will anchor it securely. If you are putting the box in a neighbor's field, make sure a piece of the iron doesn't end up in your neighbors mower or bailer!
A 5' perch should be located a few feet in front of the nextbox. Even a single tall perch should be avoided in order to discourage house wrens from using the box. I used a 6' pointed stake tapped into the ground. Unlike the nestbox, if the stake falls over, nothing is lost. But, if driven in too deeply, it may be difficult to move when the box is moved. If you are relying on someone else (the tractor operator) to move the box and stake, make it as easy as possible.
I chose a field of barley to test my first portable nestbox in the 1993 nesting season. Barley is about as tall a crop at maturity as I would care to test. It is also easy to walk through without doing much damage, by stepping between the drills. My millet plot would have been an even better choice, but I wanted to test the taller grain. The winter barley would provide a good bluebird habitat early in the breeding season, but would soon outgrow the bluebird's interest. I placed the box within 50 feet of the edge of the field so that feeding trips would not be unduly long. The box would only need to be moved once, when I cut the barley. In checking the nestbox, I approached from a slightly different angle each day, thus not establishing a beaten path through the barley. While it is often stated that predators will follow a human scent to bird nests, I think this is not an important factor in the home habitat, where humans are wandering about daily. Predators are much too busy to retrace our steps unless we are carrying bait. However, everything from mink to groundhogs follow established paths, for the same reason that we do. It's easier. Nestboxes located along fencerows are almost always on such wildlife trails.This first test box was immediately claimed by one of our 5 pairs of tree swallows and a brood was fledged with no problems. Later, a pair of bluebirds successfully raised 3 young in the test box, but, this was after the barley had been cut, creating good bluebird habitat once again. The tree swallows are primarily aerial hunters and spend a great deal of time over our grain plots even when their nestbox is elsewhere.
The purpose of the portable nestbox located in fields is an attempt to lower nestbox predation, not to provide additional nest sites. Bluebirds still require shorter vegetation than most field crops provide at maturity. While bluebirds may be attracted to boxes in such crops in the spring, the crop will quickly outgrow their needs, forcing them to fly to the nearest good hunting habitat. Therefor, portable boxes should be placed around the periphery of a large field, not in its center.
The portable nestbox is an idea worth further testing. It is NOT yet a recommended technique. While its use in low vegetation would pose no problem, use in medium height crops must be further tested. For instance, might fledglings, landing in a grain crop, have trouble relaunching themselves? Careful observation of the family after fledging will reveal if all the young are present. Fields that will be sprayed are best avoided, but a nestbox in the fencerow beside a sprayed field is at the same risk as one in the field. The adults will still catch poisoned insects in the same place. While the arsenal of farm chemicals has grown substantially, modern applicators cause less drift, so, nestlings in the field should get no greater dose than those along the fencerow.
Addendum: Since this first test in 1993, we have used the same portable box every year, in a variety of crops. The feet on the post have not needed repair, but are beginning to weaken from rot and will soon need replacement (but 6 years on the ground isn't bad for scrap 2x6's). The box has been brought inside each winter. Both bluebirds and tree swallows have used the box. In two years, the box was used first by tree swallows and then by bluebirds for their second brood. There have been no nest failures in 8 or 9 nestings.
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