Using Barn Owls for Rodent Control (2023)

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UsingBarn Owls for Rodent Control

by Tom Hoffman

About the Author

Tom is a wine grape grower in the LodiDistrict in California. He has ten years of experience designingowl nest boxes and working with owls in his family vineyard. Hisbrochure, Using Barn Owls for Rodent Control inVineyards and Orchards has received nationalattention. Tom has spoken on the subject at various symposiums inCalifornia and was featured in the July 1997 issue of GrapeGrower Magazine. In 1997, he was also awarded the Lodi Chamber ofCommerce 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 WhenDealing With Barn Owls
Getting Help When You Need It
Considering Nest Box Designs

Introduction

Usingbarn owls to fight rodent populations is an old idea that isgetting a second look by many sectors of the agricultureindustry. This is, in part, due to pressure from environmentaland consumer groups to reduce chemical use in the field. Butcredit must also be given to the notion that nature can often bea 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 nestingsites as the birds are attracted to almost any snug, dark cavity.As well, the birds will tolerate a fair amount of noise andcommotion around their nest as long as they are not directlythreatened. While the food supply remains dependable, the owlswill return season after season.

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

GeneralInformation About Barn Owls

Thefamily of barn owls, known as Tytonidae, is found world wide.Exceptions are in regions of high latitudes or high elevationswhere extremely cold climates prevail. The North Americanspecies, known as the Common Barn Owl (Tyto alba pratincola), isfound across most of North America, ranging from the Guatamalanpeninsula to the northern frontier of the United States. Withinthis range, this owl will inhabits anywhere open areas forhunting can be found as long as locations for nesting androosting are available.

Barn owls are now considered rare in many states, (Nebraska,North and South Dakota, Minnesota) and are listed as endangeredin others (Illinois, Iowa, Indiana, Missouri, Michigan, Ohio andWisconsin). While much of this decline in their population can beattributed to predation by Great Horned Owls, another majorfactor is the loss of adequate nesting sites due to urbanizationand 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 beforecompleting their first year. The causes of death includeaccidental pesticide poisoning, starvation, human predation,accidents with moving vehicles, fences and power lines, and, themost common cause, attacks by the Great Horned Owl. Of theremaining 40%, studies in New Jersey show that the average lifespan is 18 months to two years. This is just enough time for theyoung owls to mature and reproduce. Only one percent of barn owlslive to reach the ripe old age of ten.

Barn owls are noisy birds, making a wide variety of distinctcalls. These range from soft chirps and clicks to acharacteristic metallic, raspy screech. When threatened oralarmed, they emit a violent hiss not unlike the sound to steamescaping under pressure. Upon occasion, barn owls are even knownto hoot.

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

Nestingand Mating Habits

Barn owls belongs to a group of birds known as cavity dwellers,and when it comes to choosing nesting sites, these owls are notpicky. In a natural setting, they will inhabit tree cavities,crevices between the fronds of palm trees or small caves incliffs or banks. As well, they readily accept artificial cavitiesand have been found to nest in any snug, quiet enclosure, tenfeet or more off the ground. These might include rafters, spacesbetween bales of hay, attics and unoccupied rooms in upperstories of buildings. Other acceptable nesting sites may bebarrels, steel drums, cat litter boxes, or specially designedboxes for owls.

With a shortage of nesting or roosting locations available, theywill sometimes surprise you with the locations they choose. Afriend of mine didn't get around to erecting the nest boxes hepurchased from me and left them laying on the ground in a quietcorner of his shop yard for several months. When he decided toput them up, he was surprised to find that owls had already movedin.

No preference is shown by the owls for boxes made specificallyfor them. The chances of owls accepting a new nest boxes isroughly 50%, depending on the availability of other more familiarlocations, the density of the resident barn owl population andthe dependability of the food supply in that vicinity. Ingeneral, they tend to return to the same site, but not always. Ifyou have owls nesting in your hay loft, the chances are, theywill probably choose to stay there. Simply putting up a new nestbox inside the barn does not mean the owls will nest in it. It ismore likely that another pair will move into it.

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

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

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

The owls may have different mates during subsequent matingseasons. Interestingly, males may have two concurrent matesnesting as much as a mile apart during a single season if thereis 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 inTexas, 27 young were hatched in a single nest box, and allsurvived to fledge.

The hen lays one egg every two or three days and beginsincubating immediately after the first egg is laid. The eggs areincubated for 30 to 33 days. The chicks hatch in the order theywere laid, which results in siblings with as much as two weeksage difference between them.

During the incubation period, the female remains on the eggsalmost continually. She is fed by the male, but nevertheless,loses much of her stored fat. During this time, the hen becomesskittish and cranky, as would any mother confined to the home forsuch a long period. While she is reluctant to leave the nestunguarded, if she is forced to flee in a state of panic and fear,she may not return and the nest will be abandoned. For thisreason, it is wise not to disturb a nesting hen during the earlypart of the breeding season.

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

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

Young leave the nest after approximately eight weeks of age. Ifall goes well, they have made their first flight to a nearby treeor building. At this stage, they begin final preparations forlife 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 theycontinue to feed the young for another 5 or 6 weeks. Offspringare sexually active after 18 months.

HuntingHabits

Ashunters, barn owls are highly adapted creatures. The design oftheir wings renders them almost silent in flight, and theirhighly developed sense of hearing enables them to hunt in totaldarkness.

They will fly as far three and a half miles in search of food,routinely flying as much as a mile. Some growers have commentedto me that while the owls do an admirable job controlling gophersand other rodents across their fields, the pests seem to thrivein the area immediately below the nesting box. One explanationfor this is that animals instinctively protect their young by notdrawing 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'sreturn flight home.

While most of their hunting takes place during flight, barn owlsmay hunt also from a perched position. This is usefulinformation, since a grower can affect the level of rodentcontrol in a specific area by installing several 10 to 15 foothigh perches along with nesting boxes.

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

Placementof Owl Boxes

Barnowls are not known to have strong territorial instincts, and willeven nest in colonies where food supplies are abundant. This isuseful for the farmer to know because it means several owl boxescan be erected as close as several hundred yards apart in fieldswith higher rodent populations.

There are several approaches to the placement of owl boxes. Eachfarmer, of course, should consider what best serves both his orher needs as well as the needs of the owl. In any case, whereever the nest boxes are place, I recommend it be an area of lowhuman activity. For the comfort of the owls inside, I recommendfacing the opening way from any prevailing winds. If they are tobe erected on a post, it is preferrable to be within 100 years ofa large tree to provide refuge for the young after leaving thenest. I don't recommend puting nest boxes near areas wherevehicles are parked since the owls fecal material is verycorrisive 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 owlsmay already be living and will therefore be attracted to theseadditional nesting sites easily. In a nest box, the nestlingswould receive protection from falling and remain out of sight andbe less likely to be startled when visitors enter the building.As barn owls have never been tidy animals, attracting owls to abarn may be fine as long as whatever is on the floor is notneeded to be kept clean or is covered.

A second approach is to place the owl boxes either in or undertrees adjacent to the rodent infested fields. The theory behindthis approach is that while the owls will be close enough to takeadvantage of the rodents in the fields, the young owls will beable to enjoy the protection from the hot sun provided by thesurrounding trees while being able to use the trees as refugeduring those first weeks after leaving the nest.

However, this approach also has its drawbacks. A wide variety ofanimals, such as cats, opossums, raccoons, squirrels, and greathorned owls may prey upon both the young and the adults if thesepredators are in the vicinity. Measures need to be taken toprotect the young if they are to survive

A final method is to install the boxes in the fields where thefood supply is found. I install my boxes at the end of vineyardrows on 16 foot 4x4's, buried 3.5 feet in the ground. The top ofthe box is positioned at the top of the post, leaving the bottomof the box at about eleven feet. This allows for convenientinspection and cleaning, while providing enough height to attractthe owls.

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

FiguringBoxes Per Acre

Growersalways want to know how many boxes per acre will be needed. Ihave never seen a specific guideline with this information. Itdepends on how many gophers you want to get rid of, and how manyboxes you want to make.

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

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

ManagingResident Owls

Owlboxes require a minimum of bother after they are first installed,but inspecting and cleaning out the boxes may be necessary atcertain times of the year. (Health issues stemming from cleaningthe owl boxes are a concern, however. Please read the followingsection, "Special Considerations When Dealing with BarnOwls" to fully grasp the health issues involved.) It istherefore advisable to build into the design a means for bothinspection and housework. This is usually done by installingeither a drop-away floor or a clean-out flap on the side. Idefinitely prefer the flap on the side. With a drop-away floorone can never really be certain what is inside until whatever itwas is dumped on the ground.

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

Box inspections may be done as much as twice during the year, inJune and November. Additional inspections may occur at certaintimes.

June inspection:

Cleaning should be doneafter the last chick leaves the nest in late spring. Remove theremains of any dead animals, and the old wood shavings. If youare so inclined, save any owl pellets because schools anduniversities will be most eager to take them off your hands. Atthis time, the interior of the box can be disinfected with asolution of 2% household bleach sprayed into the box. Fall inspection:Inspectionin November or early December, before the adult owls return forthe breeding season, is also necessary to insure that paper waspsor honey bees have not moved into the box since the owls haveleft. If wasps or bees are present, they should be removed orkilled with a pyrethrin based insecticide. The nest should beremoved and destroyed. Optional inspections:Inspection in January may occur if a grower wishes to know ifowls are inhabiting the box. A quiet peak will cause no harm ifegg laying has not yet begun. It is just as easy, however, totell if owls are living in the box by observing it from theoutside. Signs of inhabitation are white fecal material andpellets lying on the ground around the base of the box, and anaccumulation of dirt around the door, brought in as the birdsenter after having caught rodents with their talons.

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

Inspections during the egg laying season, from the beginning ofFebruary to the end of March, are definitely not recommendedsince they may frighten the mother away and she might not return.

Inspections should always be kept quiet so as not to disturb theresidents. Knocking on the post to see if anything flies out isnever recommended.

SpecialConsiderations When Dealing with Barn Owls

Whileemploying a population of barn owls for the purpose of rodentcontrol, you must never forget that you are dealing with wildanimals. Just as with any other wild beast, there are healthissues to be addressed, and legal points that must also becovered.

Health Risks are Real
Hantavirusis a real danger whenever a person comes into contact with wildrodents, their hair, fecal matter or even their nesting material.This is most particularly true about deer mice, a species whichis common across much of the United States and is easilyidentified by their white undersides. While the deer mousecarries the virus without showing signs of being infected, itcontaminates almost everything it comes into contact with. Since1993, 138 case of hantavirus have been diagnose in Arizona,Colorado, New Mexico, Idaho and Oregon. 23 people have died as aresult of contact with the virus.

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

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

Legal Issues Related toHandling Barn Owls

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

GettingHelp When You Need It

Oneoddity of the barn owl is its cannibalistic nature. In theabsence of food supplied by parents, it is not uncommon for oldersiblings to eat their smaller brothers and sisters. Later, whenthe younger chicks have grown and are big enough to fight backand too big to be consumed, the older ones may simply force themout of the nest when the competition for food gets tough.

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

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

The second assumption you might make is that the young owl wasforced from the nest prematurely for one reason or another. Thiscan sometimes be obvious if the bird has not yet developed all ofits adult plumage. Returning it to the nest box could result indeath for the young bird, so at times like this, it is handy tohave the telephone number and address of your nearest raptorrehabilitation center.

In any case, before rescue measures are taken, observe the youngowl closely for a few days. If all is well, the parents willcontinue to care for it until it is able to reach safety on itsown.

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

ConsideringNest Boxes Designs

Asmentioned before, barn owls are not terribly picky when it comesto where they build their nests. Nest box designs range fromelaborate, spacious multi-roomed arrangements, complete withperches and insullation, down to very simple one roomconstructions. The truth is that any snug, elevated cavity in aquiet area will do, and as far as the owl is concerned, a man canmake 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 acavity depth of 16 inches.

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

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

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

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

6. If space allows, partitions separating the entrance from thenesting 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 daywhile the young occupy the nesting area. This room usually hasthe same size opening as the main cavity, and is crossed withperches with 14 inches or so of head space. No floor isrecommended for this room so as to permit castings and fecalmaterial to fall to the ground.

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

4. Perches for landing outside and roosting inside. These alsoenable the young to stretch their wings and exercise before theirfirst flight. I recommend perches, especially if the box is notgoing to be place in or very close to a tree.

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