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Using Barn Owls for Rodent Control

by Tom Hoffman

About the Author

Tom is a wine grape grower in the Lodi District in California. He has ten years of experience designing owl nest boxes and working with owls in his family vineyard. His brochure, Using Barn Owls for Rodent Control in Vineyards and Orchards has received national attention. Tom has spoken on the subject at various symposiums in California and was featured in the July 1997 issue of Grape Grower Magazine. In 1997, he was also awarded the Lodi Chamber of Commerce Agribusiness Award for his work promoting owls.

Table of Contents
General Information About Barn Owls
Nesting and Mating
Hunting Habit
Placement of Nest Boxes
Figuring Boxes Per Acre
Managing Resident Owls
Special Considerations When Dealing With Barn Owls
Getting Help When You Need It
Considering Nest Box Designs


Using barn owls to fight rodent populations is an old idea that is getting a second look by many sectors of the agriculture industry. This is, in part, due to pressure from environmental and consumer groups to reduce chemical use in the field. But credit must also be given to the notion that nature can often be a farmer's ally in his battle against pests.

Recognizing the barn owl's value as expert rodent hunters, farmers can easily encourage their presence by providing nesting sites as the birds are attracted to almost any snug, dark cavity. As well, the birds will tolerate a fair amount of noise and commotion around their nest as long as they are not directly threatened. While the food supply remains dependable, the owls will return season after season.

The information on this web page is intended to provide the reader with a basic understanding of the barn owl, and also enable him or her to attract barn owls by constructing and locating nest boxes. It must keep in mind, however, that barn owls will not the ultimate solution to a farmer's rodent problems. Instead, they represent one out of many tools a farmer has in his disposal in the fight against these pests.

General Information About Barn Owls

The family of barn owls, known as Tytonidae, is found world wide. Exceptions are in regions of high latitudes or high elevations where extremely cold climates prevail. The North American species, known as the Common Barn Owl (Tyto alba pratincola), is found across most of North America, ranging from the Guatamalan peninsula to the northern frontier of the United States. Within this range, this owl will inhabits anywhere open areas for hunting can be found as long as locations for nesting and roosting are available.

Barn owls are now considered rare in many states, (Nebraska, North and South Dakota, Minnesota) and are listed as endangered in others (Illinois, Iowa, Indiana, Missouri, Michigan, Ohio and Wisconsin). While much of this decline in their population can be attributed to predation by Great Horned Owls, another major factor is the loss of adequate nesting sites due to urbanization and the development of American agriculture.

The Common Barn Owl is, on the average, a short lived creature. Studies indicate that about 60% of all barn owls die before completing their first year. The causes of death include accidental pesticide poisoning, starvation, human predation, accidents with moving vehicles, fences and power lines, and, the most common cause, attacks by the Great Horned Owl. Of the remaining 40%, studies in New Jersey show that the average life span is 18 months to two years. This is just enough time for the young owls to mature and reproduce. Only one percent of barn owls live to reach the ripe old age of ten.

Barn owls are noisy birds, making a wide variety of distinct calls. These range from soft chirps and clicks to a characteristic metallic, raspy screech. When threatened or alarmed, they emit a violent hiss not unlike the sound to steam escaping under pressure. Upon occasion, barn owls are even known to hoot.

Typically, barn owls weigh about 15 ounces, are about 17 inches long and have a 3.5 foot wingspan.

Nesting and Mating Habits

B arn owls belongs to a group of birds known as cavity dwellers, and when it comes to choosing nesting sites, these owls are not picky. In a natural setting, they will inhabit tree cavities, crevices between the fronds of palm trees or small caves in cliffs or banks. As well, they readily accept artificial cavities and have been found to nest in any snug, quiet enclosure, ten feet or more off the ground. These might include rafters, spaces between bales of hay, attics and unoccupied rooms in upper stories of buildings. Other acceptable nesting sites may be barrels, steel drums, cat litter boxes, or specially designed boxes for owls.

With a shortage of nesting or roosting locations available, they will sometimes surprise you with the locations they choose. A friend of mine didn't get around to erecting the nest boxes he purchased from me and left them laying on the ground in a quiet corner of his shop yard for several months. When he decided to put them up, he was surprised to find that owls had already moved in.

No preference is shown by the owls for boxes made specifically for them. The chances of owls accepting a new nest boxes is roughly 50%, depending on the availability of other more familiar locations, the density of the resident barn owl population and the dependability of the food supply in that vicinity. In general, they tend to return to the same site, but not always. If you have owls nesting in your hay loft, the chances are, they will probably choose to stay there. Simply putting up a new nest box inside the barn does not mean the owls will nest in it. It is more likely that another pair will move into it.

Because of the barn owl's short life span, they have developed a tremendous reproductive capacity. In some regions, barn owls have been known to nest all year round.

Barn owls in northern and central California begin selecting nesting sites in December or January. The nesting season is from February to May, with peak hatches in April. Occasionally new nests may be started as late as March. By July, most nest boxes have been vacated by the young, who have flown to nearby trees or buildings for the final stages of their development.

A second nest for the season with the same mate may be started in the same or in different locations. I have found, however, that my nest boxes aren't used a second time. This may be because all my boxes are out in the vineyards and the birds may look for a cooler location for their second brood. They probably find locations in the trees or in farm buildings where there is more protection from the hot California sun.

The owls may have different mates during subsequent mating seasons. Interestingly, males may have two concurrent mates nesting as much as a mile apart during a single season if there is a shortage of males in the vicinity.

The clutch size varies, and commonly may be up to eight eggs, although typical clutches are from 3 to 6. In one unusual case in Texas, 27 young were hatched in a single nest box, and all survived to fledge.

The hen lays one egg every two or three days and begins incubating immediately after the first egg is laid. The eggs are incubated for 30 to 33 days. The chicks hatch in the order they were laid, which results in siblings with as much as two weeks age difference between them.

During the incubation period, the female remains on the eggs almost continually. She is fed by the male, but nevertheless, loses much of her stored fat. During this time, the hen becomes skittish and cranky, as would any mother confined to the home for such a long period. While she is reluctant to leave the nest unguarded, if she is forced to flee in a state of panic and fear, she may not return and the nest will be abandoned. For this reason, it is wise not to disturb a nesting hen during the early part of the breeding season.

This is not true while the chicks are growing. A parent frightened away from the nest during this stage will instinctively return to continue caring for his or her young. However, inspection of the box during the day in April or May will likely as not find the young home alone. The parents will best resting in a quiet location nearby. Having worked all night to feed the hungry chicks, they no doubt want some quiet time for themselves.

While an adult may eat one rodent a night, each chick may eat from two to five, depending on the size of the chick and the size of the rodent. During the course of the breeding season, as many as 3000 rodents may be consumed by the parents and their family of five young. That's impressive, especially if you have a rodent problem. Imagine what a dozen barn owl families can do for you.

Young leave the nest after approximately eight weeks of age. If all goes well, they have made their first flight to a nearby tree or building. At this stage, they begin final preparations for life on their own: mastering their skills flying and hunting, while learning how to avoid predators like the great horned owl. The parents still have an active role in this development as they continue to feed the young for another 5 or 6 weeks. Offspring are sexually active after 18 months.

Hunting Habits

As hunters, barn owls are highly adapted creatures. The design of their wings renders them almost silent in flight, and their highly developed sense of hearing enables them to hunt in total darkness.

They will fly as far three and a half miles in search of food, routinely flying as much as a mile. Some growers have commented to me that while the owls do an admirable job controlling gophers and other rodents across their fields, the pests seem to thrive in the area immediately below the nesting box. One explanation for this is that animals instinctively protect their young by not drawing attention to themselves at the nesting site. To the owl, this means not hunting in the immediate area around the nest box, since predators may observe the activity and follow the parent's return flight home.

While most of their hunting takes place during flight, barn owls may hunt also from a perched position. This is useful information, since a grower can affect the level of rodent control in a specific area by installing several 10 to 15 foot high perches along with nesting boxes.

Rodents are their preferred food, but small birds roosting in trees or bushes frequently become victims of the barn owl. Cats are not threatened, and ground squirrels, not being nocturnal, are unfortunately not controlled.

Placement of Owl Boxes

Barn owls are not known to have strong territorial instincts, and will even nest in colonies where food supplies are abundant. This is useful for the farmer to know because it means several owl boxes can be erected as close as several hundred yards apart in fields with higher rodent populations.

There are several approaches to the placement of owl boxes. Each farmer, of course, should consider what best serves both his or her needs as well as the needs of the owl. In any case, where ever the nest boxes are place, I recommend it be an area of low human activity. For the comfort of the owls inside, I recommend facing the opening way from any prevailing winds. If they are to be erected on a post, it is preferrable to be within 100 years of a large tree to provide refuge for the young after leaving the nest. I don't recommend puting nest boxes near areas where vehicles are parked since the owls fecal material is very corrisive to any metal surface.

One approach is to build boxes that will fit into existing silos, barns or other farmyard buildings. This is a location where owls may already be living and will therefore be attracted to these additional nesting sites easily. In a nest box, the nestlings would receive protection from falling and remain out of sight and be less likely to be startled when visitors enter the building. As barn owls have never been tidy animals, attracting owls to a barn may be fine as long as whatever is on the floor is not needed to be kept clean or is covered.

A second approach is to place the owl boxes either in or under trees adjacent to the rodent infested fields. The theory behind this approach is that while the owls will be close enough to take advantage of the rodents in the fields, the young owls will be able to enjoy the protection from the hot sun provided by the surrounding trees while being able to use the trees as refuge during those first weeks after leaving the nest.

However, this approach also has its drawbacks. A wide variety of animals, such as cats, opossums, raccoons, squirrels, and great horned owls may prey upon both the young and the adults if these predators are in the vicinity. Measures need to be taken to protect the young if they are to survive

A final method is to install the boxes in the fields where the food supply is found. I install my boxes at the end of vineyard rows on 16 foot 4x4's, buried 3.5 feet in the ground. The top of the box is positioned at the top of the post, leaving the bottom of the box at about eleven feet. This allows for convenient inspection and cleaning, while providing enough height to attract the owls.

The debate continues as to whether or not the young owls need special protection from the sun. With a body temperature of 104 degrees F., some say it is doubtful that springtime temperatures will rise to a level that could have an effect on the young birds. In vineyard settings around Lodi, California, the owls have finished their breeding by the end of May and the young have left the nest. Never have I found a second brood in any of my boxes. Hot weather is therefore not a major threat, unless something unusual happens during the early spring months. The need for shade, apart from that which is provided by the design, does not appear necessary.

Figuring Boxes Per Acre

Growers always want to know how many boxes per acre will be needed. I have never seen a specific guideline with this information. It depends on how many gophers you want to get rid of, and how many boxes you want to make.

In placing my boxes, I tend to space the boxes around the rodent infested areas, figuring about four to six will handle 50 acres. Where rodents are not such a serious problem, the same number of boxes will work for 100 acres.

The biggest mistake a grower can make is to not providing enough sites. Those who have multiple nest boxes in place, find that 40 to 70 percent of them are used by the birds, so installing just one box may or may not work. Keeping in mind the success rate of box inhabitation is about fifty percent, put up two, three, or more boxes to increase your chances of attracting some. From then on, build several boxes each year to keep ahead of the population. When you can keep 70% tp 80% of your boxes filled, you know you have attracted all the owls you land will support.

Managing Resident Owls

Owl boxes require a minimum of bother after they are first installed, but inspecting and cleaning out the boxes may be necessary at certain times of the year. (Health issues stemming from cleaning the owl boxes are a concern, however. Please read the following section, "Special Considerations When Dealing with Barn Owls" to fully grasp the health issues involved.) It is therefore advisable to build into the design a means for both inspection and housework. This is usually done by installing either a drop-away floor or a clean-out flap on the side. I definitely prefer the flap on the side. With a drop-away floor one can never really be certain what is inside until whatever it was is dumped on the ground.

Cleaning should also be taken into consideration when mounting the box. A box installed too high will be difficult and dangerous clean, where a box 11 to 12 feet from the ground will be more accessible and just as readily acceptable by the owls.

Box inspections may be done as much as twice during the year, in June and November. Additional inspections may occur at certain times.

June inspection:
Cleaning should be done after the last chick leaves the nest in late spring. Remove the remains of any dead animals, and the old wood shavings. If you are so inclined, save any owl pellets because schools and universities will be most eager to take them off your hands. At this time, the interior of the box can be disinfected with a solution of 2% household bleach sprayed into the box.

Fall inspection:Inspection in November or early December, before the adult owls return for the breeding season, is also necessary to insure that paper wasps or honey bees have not moved into the box since the owls have left. If wasps or bees are present, they should be removed or killed with a pyrethrin based insecticide. The nest should be removed and destroyed.

Optional inspections: Inspection in January may occur if a grower wishes to know if owls are inhabiting the box. A quiet peak will cause no harm if egg laying has not yet begun. It is just as easy, however, to tell if owls are living in the box by observing it from the outside. Signs of inhabitation are white fecal material and pellets lying on the ground around the base of the box, and an accumulation of dirt around the door, brought in as the birds enter after having caught rodents with their talons.

Another inspection may also occur in April if a grower wishes to know how many owl chicks have hatched. Frightening away the adults after the chicks have hatched does not keep them away.

Inspections during the egg laying season, from the beginning of February to the end of March, are definitely not recommended since they may frighten the mother away and she might not return.
Inspections should always be kept quiet so as not to disturb the residents. Knocking on the post to see if anything flies out is never recommended.

Special Considerations When Dealing with Barn Owls

While employing a population of barn owls for the purpose of rodent control, you must never forget that you are dealing with wild animals. Just as with any other wild beast, there are health issues to be addressed, and legal points that must also be covered.

Health Risks are Real
Hantavirus is a real danger whenever a person comes into contact with wild rodents, their hair, fecal matter or even their nesting material. This is most particularly true about deer mice, a species which is common across much of the United States and is easily identified by their white undersides. While the deer mouse carries the virus without showing signs of being infected, it contaminates almost everything it comes into contact with. Since 1993, 138 case of hantavirus have been diagnose in Arizona, Colorado, New Mexico, Idaho and Oregon. 23 people have died as a result of contact with the virus.

Barn owls cough up castings, or owl pellets, that contain the undigested hair and bones of their prey. Many casting can be found in an occupied nest box. As well, uneaten rodents may also be found in nest boxes. Debris in an owl nest box can easily be infected hantavirus.

While cleaning out nest boxes at least once a year maintains the box and helps to control wasps and disease that may affect the young, this act can expose a person to hantavirus. Certainly, the benefits of having a clean nest box are hardly worth the risks of exposure to this deadly disease. However, if a nest box is to be cleaned, the person must use rubber gloves and a dust mask. Every effort should be made to stay out of the dust.

Legal Issues Related to Handling Barn Owls

Barn owls, as are all other owls, hawks and eagles, are a protected species. Only licensed individuals may handle them or keep them. For more information, contact the US Department of Fish and Wildlife.

Getting Help When You Need It

One oddity of the barn owl is its cannibalistic nature. In the absence of food supplied by parents, it is not uncommon for older siblings to eat their smaller brothers and sisters. Later, when the younger chicks have grown and are big enough to fight back and too big to be consumed, the older ones may simply force them out of the nest when the competition for food gets tough.

Some spring morning, during an inspection of your fields, you may find young owls cowering among the rows in the vicinity of one of your boxes. You an be sure that one of two things has happened.

One assumption you could make is that the has grown to the stage where it should be able to fly and the parents have coaxed it out of the nest. Obviously, being young and unconditioned, it didn't make it to shelter. But just because it is in an vulnerable position doesn't mean that it has been abandoned by its parents. Before any measures are taken, observe it for several days. The parents will continue to provide for it where it is.

The second assumption you might make is that the young owl was forced from the nest prematurely for one reason or another. This can sometimes be obvious if the bird has not yet developed all of its adult plumage. Returning it to the nest box could result in death for the young bird, so at times like this, it is handy to have the telephone number and address of your nearest raptor rehabilitation center.

In any case, before rescue measures are taken, observe the young owl closely for a few days. If all is well, the parents will continue to care for it until it is able to reach safety on its own.

If it appears to be failing, or is in danger from the elements, (heat,cold, etc.) from other animals or humans, measures must be taken if it is to survive. Using a pair of thick gloves, grab the young owl by the legs and turn it upside down. If it doesn't want to cooperate and you can not get to its feet, try throwing a sheet or large towel over the bird first, and then taking it by the legs. Once you have ahold if it, put the bird into a paper grocery bag and fold the top over to keep it closed. It can then be easily transported to the center where it will be raised to full size and released.

Considering Nest Boxes Designs

As mentioned before, barn owls are not terribly picky when it comes to where they build their nests. Nest box designs range from elaborate, spacious multi-roomed arrangements, complete with perches and insullation, down to very simple one room constructions. The truth is that any snug, elevated cavity in a quiet area will do, and as far as the owl is concerned, a man can make them as well as nature.

Basic requirements for a man-made nest box include the following:

1. Minimum dimensions are 12 by 12 inches for the floor and a cavity depth of 16 inches.

2. The entrance should be no more than six inches in diameter to keep out great horned owls. I prefer a five inch hole. I even have several boxes with openings less than that, and the owls use them season after season. (In fact, a man I know in Stockton CA, who insists that a hole diameter greater than three and three-fourths inches puts the owls in danger!) The entrance should be located fairly near the floor of the box to provide access for the young, unless you provide a means of enabling the young to scramble up to it, such as cleats or a perch inside the box.

3. Air circulation should be insured by making vent holes are allowing an air space near the roof.

4. Water drainage must be provided for by making holes in the floor, usually near the corners.

5. A means of clean-out and inspection must also be worked into the design.

6. If space allows, partitions separating the entrance from the nesting area protect the eggs and young from predators.

Optional items include:

1. Insulating panels on the sides exposed to the sun.

2. A roosting room for the parents to perch in during the day while the young occupy the nesting area. This room usually has the same size opening as the main cavity, and is crossed with perches with 14 inches or so of head space. No floor is recommended for this room so as to permit castings and fecal material to fall to the ground.

3. Extra space in the nesting area. The boxes I make to sell commercially have a floor that is almost 16 by 24 inches, and a cavity depth of almost 24 inches. The idea behind more room is that it encourages the hen to lay more eggs. More eggs, more rodents eaten. In my opinion, bigger is better.

4. Perches for landing outside and roosting inside. These also enable the young to stretch their wings and exercise before their first flight. I recommend perches, especially if the box is not going to be place in or very close to a tree.

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