Natural Resources Section
Proc. Fla. State Hort. Soc. 125:366–369. 2012.
Use of Barn Owls for Sustainable Rodent Control in Agricultural Areas
Richard N. Raid* University of Florida, IFAS, Everglades Research and Education Center, 3200 East Palm Beach Road, Belle Glade, FL 33430-4702
Additional index words. Tyto alba, biological control, wildlife, mammalian pests In the Everglades Agricultural Area (EAA) of south Florida, rodent pests annually inflict millions of dollars in crop losses to vegetables, sugarcane, and rice. For decades, attempts were made to control rodent populations through the sole use of chemical rodenticides. However, rodenticides are costly, need frequent re-application, and pose risks to non-target species. With sustainable rodent control as the ultimate goal, University of Florida (UF) extension special- ists have implemented a program encouraging agricultural producers to erect nesting boxes for barn owls (Tyto alba) on agricultural lands. Seeking to satisfy multiple objectives, the UF Barn Owl Project utilizes these prodigious rodent predators as a model system for instructing youth and the general public about symbiotic relationships that frequently exist between the environment and agriculture. Student and adult volunteers enthusiastically support the project by assisting in the construction of nesting boxes while learning about this highly beneficial raptor. Due to a paucity of suit- able natural nesting sites, the nesting boxes are quickly colonized. Research indicates that barn owl density in the EAA is now among the highest in North America. A number of growers report that they no longer use chemical rodenticides. Perhaps the greatest beneficiaries of the UF Barn Owl Project are students and teachers. Owl pellets (regurgitated masses containing the undigested remains of rodents and other owl prey) are routinely collected, heat-sterilized, and used to provide hands-on lessons in predator–prey relationships. The personal involvement facilitated by the Barn Owl Project provides all participants with a sense of contribution and accomplishment.
Small mammalian pests such as mice, rats, moles, shrews, considered to be the most widely distributed raptor in the world, and rabbits inflict millions of dollars in damage to agricultural occupying all continents except Antarctica (Taylor, 1994). These crops throughout the world. In the Everglades Agricultural Area birds are highly evolved for preying on rodents, equipped with (EAA) of south Florida, at least five different rat species have powerful talons, a sharp beak, fluted feathers for silent flight, been identified as causing damage to sugarcane (interspecific nocturnal habits, excellent night-time vision, and some of the hybrids of Saccharum spp.) (Martin et al., 2006). Extrapolating most acute hearing of any animal on earth. Prodigious rodent from damage figures reported by Lefebvre et al. (1989), rats were predators, barn owls thrive in open agricultural areas that fre- responsible for some $30 million in losses to Florida sugarcane quently support abundant rodent populations. The EAA is one annually. Losses may be direct, such as damage to the crop such area, consisting of nearly 700,000 acres of sugarcane, rice, itself, or indirect, such as damage caused by rodents gnawing vegetables, and turfgrass sod. However, although already present through irrigation tubes, hydraulic hoses, or even electrical wires in the Everglades, barn owl populations were sub-optimal due on equipment such as tractors, or refrigeration units at packing to a paucity of nesting sites. Characterized as “cavity nesters,” houses. Formerly, management of these pests focused on the use barn owls prefer to nest in dark, protected areas. Frequent natural of chemical rodenticides, primarily zinc phosphide (Holler and nesting sites include caves, cliff faces, and tree hollows, while Decker, 1989), and/or anti-coagulants, which cause the rodents favored man-made sites commonly include barn lofts, nooks, to bleed to death. Rodenticides have several drawbacks, however. and silos, and even church steeples (Taylor, 1994). A flat, nearly They are costly, they need frequent re-application, and they may treeless terrain devoid of man-made structures, there were few pose a threat to non-target species through secondary poisoning, such sites in the Glades. Since nesting boxes had been utilized to e.g., predators feeding on poisoned rodents and in turn being provide nesting habitat for barn owls in other parts of the world poisoned (Martin et al., 2007). In addition, rodents may become (Colvin, 1984; Duckett, 1991), this technique was considered in bait-shy, rendering the bait completely ineffective. south Florida.
Rationale for the UF Barn Owl Program Barn Owls and Nesting Boxes
In an effort to establish a more sustainable form of rodent A field trial was conducted during a 2-year period starting in control in the EAA, the University of Florida established the UF 1994 to investigate the acceptance of nesting boxes by barn owls Barn Owl Program in 1994. Barn owls (Tyto alba) are generally in the EAA. Paired nesting boxes (eight each of two designs) were placed throughout the EAA at widely distributed sites and were monitored for barn owl activity. By the end of the 2-year period, barn owls had established themselves in all eight of the *Corresponding author; email: [email protected]; phone: (561) 993-1564 boxes using the design described herein (Fig. 1), but owls had
366 Proc. Fla. State Hort. Soc. 125: 2012. 2002, a second study was conducted, placing 120 nesting boxes throughout the Glades. Within 6 months of placement, 100% of the boxes exhibited at least roosting activity, and more than 50% of the boxes supported active nesting. Within 1 year, more than 90% of the boxes supported nesting, with vacancies found only in those boxes that were loose and not securely fastened to their mounting posts. This result demonstrated the rapid acceptance of nesting boxes by the barn owls in the EAA. Presently, now that a majority of barn owls in the EAA have been reared in boxes, it is not uncommon to erect a nesting box one day and find a barn owl already roosting in it the next day. The owls find and accept them that quickly. Subsequent field studies have been established to examine the influence of box height and box color on colonization. In a comparison of three different box heights (4, 8, and 12 ft above ground level), when provided a choice, barn owls first occupied the 12-ft boxes, then the 8-ft boxes, and finally, the 4-ft boxes. However, at the minimum height, barn owls only colonized those in areas where they were not disturbed by human activity. At present, the UF Barn Owl Program recommends using a 12-ft pressure-treated post for mounting, positioning the box 8 to 10 ft above the ground. At this height, boxes are less likely to be blown over by strong winds, yet are high enough that they are not as frequently disturbed by people peering in. It made no differ- ence whether a box was painted white, brown, or green, or was left unpainted; all supported owls. In our program, we currently Fig. 1. Nesting box design (lower left) and box plans for the barn owl nesting box paint our boxes white using recycled paint. This color provides used in the EAA. Materials are 5/8-inch exterior-grade plywood and 1-inch pine board. Note that the position of the hole (off to one end) creates a dark corner a light reflective surface, a lower temperature in the box, and at the opposite end that the female will use for laying and incubating eggs. Face greater longevity because the wood is somewhat protected from opening to the north if possible to minimize entry by sunlight. the elements. Optionally, roofing material can also be used to increase the level of weather protection. These studies demonstrated the predator’s adaptation to local established themselves in only two of the alternate design, a design conditions. The most important criteria are darkness and privacy typically targeting barred owls (Bierregaard, 2012). Thus, there during daylight hours. Given a suitable structure, barn owls will was evidence of a particular preference in design. The described colonize whatever is available, as long as they do not feel threat- model provides greater floor space and a darkened corner, pre- ened. Researchers have also been successful with boxes mounted ferred by females for laying and incubating her clutch of eggs. inside of old buildings (Fig. 3), such as in the rafters of pole sheds. Furthermore, it was observed that barn owls preferred nesting These sites provide some protection for the boxes from weather- boxes placed out in the open on posts (Fig. 2) rather than boxes ing and they offer protection to the owls during the vulnerable placed in the few trees that dot the Glades. Presumably, a box out fledging period when they are first learning to fly (Fig. 4). in the open decreases the threat of predation. During 2001 and
Fig. 2. A typical barn owl nesting box in the EAA. Most are now erected on Fig. 3. A barn owl nesting box located inside an old pump house in the EAA. 4-inch × 4-inch × 12-ft pressure-treated posts, with the box positioned out in Placement of boxes inside of a traditional nesting site (barns, church steeples, the open 8 to 10 ft above ground level. Barn owls prefer open flight into and etc.) provides added protection. Be aware that nesting and roosting sites can out of their nesting box. be messy.
Proc. Fla. State Hort. Soc. 125: 2012. 367 With rodents making up more than 90% of a typical barn owl’s diet, that figure accounts for a lot of small mammalian pests. In the EAA, adult barn owls may bring as many as two dozen rat and mice carcasses back to their nest each night as chicks ap- proach maturity, as well as for their own needs (two to four prey per owl per day). The female or mother owl initially tears the prey into small pieces to feed the chicks, but once owlets reach an age of 3 to 4 weeks they are fully capable of swallowing an entire rat, whole. After their bodies digest the muscle and flesh of their prey, they regurgitate the undigested remains (typically bones and fur) in the form of a compressed pellet (Fig. 5). These pellets aggregate in nesting boxes and leave a record of what the owls have been eating.
Owls as Sustainable Rodent Control
During the course of a year that typically includes two nesting cycles supporting anywhere from four to twelve young, a barn owl nesting box in the EAA will accumulate the remains of several thousand small rodents and other barn owl prey remains. Dissec- tion of 100 owl pellets collected throughout the EAA revealed a predominance of cotton rat (Sigmodon hispidus) and house mice (Mus musculus) remains, with the remaining prey being other rat, shrew, and lizard species, as well as large insects (large beetles, dragonflies, grasshoppers, etc.) (Raid et al., 2004). Cotton rats have been identified as the most abundant, and hence most dam- aging, rodent pest of sugarcane in the EAA (Martin et al., 2006). Therefore, there is abundant anecdotal evidence that barn owls are having a significant impact on mammalian pest populations, although the absence of an owl-free check makes it impossible to ascertain (Martin et al., 2007). However, the value of barn owls as a means to control rodents is supported by the fact that rodenticide Fig. 4. A brood of five owlets nearly ready to fledge from the nesting box where usage in the EAA has decreased substantially (Wayne Boynton, they have been reared. Approximately 7 to 8 weeks old, they are standing on Boynton Farms, Inc., personal communication, 2004) without a the remains (mostly bones and fur) of hundreds of prey items that have been concomitant increase in rodent damage. Correspondingly, there devoured during the course of the nesting cycle. have been fewer predatory raptors and scavengers (e.g., eagles, hawks, vultures) from the EAA delivered to animal rehabilitation centers suffering from secondary poisoning (Diane Suave, Palm Barn Owl Biology Beach County Animal Rescue, personal communication, 2000).
Barn owls are considered to be medium-sized owls, standing about 16–18 inches tall and weighing approximately 1 lb as an adult. Most are thought to mate for life and they are classified as habitual nesters, meaning they return to the same nesting site year after year if left undisturbed. Possessing a high rate of metabolism, barn owls require a lot of prey for self-sustenance and to support their young. In Florida, T. alba is typically double-brooded, with fall and spring nesting seasons. Barn owls are unique among rap- tors in having large clutch (numbers of eggs laid) sizes, normally ranging from four to seven eggs. Most raptors, such as hawks or eagles, only lay one or two eggs. Owl eggs are laid at 3-d intervals to ensure that at least the eldest make it to fledge from the nest if there is insufficient food for the entire brood. Incubation is typi- cally 32 d from laying to hatching. The adult male, differentiated from the female by a slightly smaller size and usually whiter breast coloration, is the main provider during nesting, with the female staying in the nest to incubate eggs. Almost totally nocturnal in their activity, hunting commences at dusk and ends at dawn. An active nest requires a high number of food forays, particularly after the chicks begin to hatch. Capable of devouring up to 150% Fig. 5. A display of barn owl pellets collected from nesting boxes in the EAA. of their weight in prey per day just weeks after hatching, barn Note the range in size and shape. Pellets can be collected, sterilized, and used owl chicks (Fig. 4) require a lot of food, and hence dead prey. for educational purposes.
368 Proc. Fla. State Hort. Soc. 125: 2012. In the EAA, the barn owl population has risen substantially since the initiation of this project, and existing box numbers (cur- rently estimated at 300 to 400) support nearly 100% occupancy. As a result, barn owl populations in the EAA are among the highest densities in North America (Mark Browning, Pittsburg Zoo, personal communication, 2009).
Outreach and Education
The UF Barn Owl Project offers several unique opportunities with regard to outreach and education (Raid, 2010). The first is the use of barn owl pellets for dissection. Students in grades K–12 frequently dissect owl pellets as part of a biological unit on predator–prey relationships. Using a simple bone or skeletal chart, students can easily find and identify nearly all of the bones within a rodent’s body (Fig. 6), learning anatomy while at the same time realizing the benefits of certain wildlife species to mankind. A survey of students revealed that 100% of those who Fig. 6. Dissected and whole barn owl pellets, showing the undigested remains of barn owl prey. Bones of prey, typically mice and rats in the EAA of south have dissected owl pellets distinctly remember that very lesson Florida, are easily identified using a bone chart. and more than 95% of teachers surveyed revealed that the owl pellet dissection lesson is definitely the most talked about lesson of the year. Through the collection and donation of owl pellets, UF Barn Owl Program facilitators have assisted hundreds of Literature Cited schools, 4-H clubs, and nature centers in educating thousands of youth in their lessons. Costing as much as $3 to $4 per pellet Bierregaard, R.O., Jr. 2012. Barred owl box plans. Univ. North Caro- when purchased from scientific suppliers, such lessons would be lina–Charlotte. Verified 12 Nov 2012.
Proc. Fla. State Hort. Soc. 125: 2012. 369