Reduce Insects Organically With Birds

With a resident entomologist as its owner, Windrose Stables, a boarding facility in central California, was armed against insects, but biting stable, horse, and deerflies continued to be a serious problem.

The number of manure-associated flies was also increasing. Manure flies serve as intermediate hosts for roundworms (Habronema muscae) and can transmit diseases. As Windrose is an organic farm, owners Erin Borden, PhD (entomology), and Mark Borden, MD, were unwilling to use pesticides. They tried sticky tape, attractant traps, parasitoids (insects whose larvae are parasites that eventually kill their hosts--in this case, the unwanted flies), and manure management strategies, but frequent application of citronella spray was still required to prevent hair loss and continuous irritation to the farm's horses and mules.

In their fourth year at the farm, the Bordens erected a free-standing nesting box intended to attract bluebirds. Before the box was introduced there were no cavity nesting birds on the property, although one pair of Western Bluebirds (Sialia mexicana) had been using the tack shed as a "base of operations," and they had been seen eating insects around the pipe pens.


Figure 1

Figure 1

Figure 2

Figure 2

The Bordens made the box from some remnant vinyl fence post, and erected it on a 10-foot section of one-inch galvanized electrical conduit (see figure 1). The box was seven feet above the ground. On the same day that it was erected, a pair of Tree Swallows (Tachycineta bicolor) moved into the box. The box was located a few feet from the riding arena, and it was easily observed. The bird fledged three sets of nestlings in the first season.

A dozen Tree Swallows were in competition for the nest box the following spring. The Bordens constructed six other identical boxes, assembly-line style, and erected these in a line, about ten meters apart. Swallows were entering the boxes as the owners were pushing the conduit into the soft Davis, Calif., clay soil. All seven of the nestboxes were occupied within a week. Tree Swallows eat only insects, and at this point the Bordens had recruited quite a squadron of insect-eating birds.

The pair of Western Bluebirds was seen entering the original nestbox the next spring (the Borden's sixth year on the farm), but the Tree Swallow pair was also attempting to occupy the box, and an intense competition arose. The vinyl arena fence was only a few feet away, and, although lower to the ground, an existing hollow post was easily modified to form a comfortable "fencebox." Within a day the Bluebirds were busy constructing a tightly woven grass nest within the post. Construction of this fencebox was much quicker than the pole type, at less than five minutes per box, and the Bordens constructed 27 additional boxes within the next week (see figure 2). The boxes were spread across the pasture and perimeter fence as well. Within the first week three pairs of Tree Swallows, and another Bluebird pair had taken up residence. These boxes transformed an attractive--but environmentally neutral--vinyl fence into a lasting, wildlife-enhancing asset.

That year and the following year, without changing the number of horses on the property or introducing other fly control measures, the aerial fly population was visibly reduced.

Several of the boarders noticed the decrease in flies, and one asked the Bordens what had been done to cure the fly problem.

Nestbox Design


Screws will support the floor.


The location and size of the entrance hole will determine what species utilize the box.

The size of the entrance hole is possibly the most important factor in determining which species of bird will occupy fenceboxes. For Western Bluebirds and Tree Swallows, a 1.5-inch diameter hole is ideal. If the hole is 1 5/8 inches or larger in size, a European Starling (Sturnus vulgaris) may be able to enter. If Mountain Bluebirds (Sialia currucoides) are found in your area, a 1 9/16-inch hole might be ideal. House Sparrows (Passer domesticus) can also enter the 1.5-inch hole and might occupy some boxes.

The sparrows will likely occupy the boxes that are closest to the barn, because, unlike the swallows, they cannot outfly an aerial predator such as a Coopers Hawk (Accipiter cooperi), and they must rely on cover to escape. Sparrows also prefer a box with a larger volume, so placing the opening slightly lower on the post, or decreasing the box volume by using more cedar shavings or a thicker floor material will also decrease their tendency to compete with the swallows and bluebirds. Having a few pairs of House Sparrows around the barn is likely to be beneficial, as they will eat insects and clean up spilled grain, thus competing with any rodents that might try to move in.

Nestbox Construction

Constructing a nestbox within a hollow vinyl fence post is a quick and easy task.

The caps along the top of the fence posts are glued in place, but a gentle tap will generally remove them without damage.

First create a fencebox floor by using two, two-inch small-diameter stainless screws at the level of the top of the second rail. Drill before inserting the screws to minimize stress on the post. A square piece of exterior plywood cut to loosely (about �-inch clearance) within the post is then dropped in from the top. This will securely rest upon the middle rail of the fence and the screws, forming the floor.

Another way to form the floor is to cut a piece of hardware cloth to fit snugly within the post, and then use a slender piece of wood to pack it down to rest on the screws and rail. Drop in a handful of cedar shavings and replace the top. Cedar shavings have insect-repellent properties, are absorbent, and resist decay.


Nestling Bluebirds inside a fencebox.

Next, use a 1.5-inch diameter hole saw to drill the entrance hole. The hole can be located nearer to the top rail for Bluebirds, or close to the middle for Tree Swallows. Bluebirds are very agile on their feet, whereas the Tree Swallows seem to prefer the hole a few inches lower. Perches at the entry hole are not needed, and they could allow predators to access the fencebox. The boxes do not need to be cleaned, as the birds will do their own housekeeping. The fence strength and integrity is not significantly affected by this modification. The Bordens have also used a single small screw to secure the cap. This screw allows easy inspection and photos, while preventing curious humans from observing the nest too frequently.

The rail openings, entrance hole, and high ceiling allow ventilation and cooling in hot weather, and the fencebox doubles as a winter refuge from the cold and wind.

Take-Home Message

Recruiting attractive insect-eating birds to a horse facility is an easy way to increase insect control. In areas such as California's Central Valley, where removal of the native oaks has decreased available nesting holes, the cavity-dwelling species are in desperate need of nestboxes, and they will quickly occupy available housing.--Mark and Erin Borden


Mark Borden is an MD, and Erin Borden has her PhD in entomology. Erin's research is in the area of integrated pest management and biodiversity. Both are lifetime riders and active horse enthusiasts. Erin rides stadium jumpers, and Mark competes in endurance races on his mules.

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