Spring 1984, Volume 1
Carl D. Marti
Ghoulies and Ghosties and Ecological Studies
Barn owls are among those ghostly creatures that go bump in the night. Theirs is a spectral reputation enhanced by secretive, haunting habits. For biologists, the adaptability of barn owls is interesting to ecologists while physiologists are fascinated by the incredibly accurate ability to locate prey acoustically. Zoogeographers puzzle over reasons for the widespread distribution, and animal behaviorists find their nocturnal activity a challenge to study. Even non-biologists find barn owls interesting; a recent monograph' on the species lists many references to barn owls in folklore and literature.
In this paper I will present major reasons why people of such divergent backgrounds find the barn owl interesting and will discuss some of my own research on the species. Unlike some who study more esoteric aspects of biology, I am rarely asked why I study barn owls. Any encounter with the barn owl, whether by children in a classroom or by a farmer in his barn or by a biologist in the lab or field, is stimulus to know more about the animal. There are, however, many aspects worthy of understanding that are not immediately self-evident.
Barn Owl Biology
Officially known as the common barn-owl (Tyto alba) by the American Ornithologists' Union, most taxonomists place it in the smaller of the two families of owls. Only ten species make up this group; the other family, the "typical" owls, has over 130 species. One divergent theory, however, has barn owls more closely related to falcons.
The barn owl is the most widespread of owls, being found on every continent and many remote islands. This distribution implies versatility in resource use, i.e. that barn owls can successfully catch and cat many kinds of prey and use a variety of sites for roosting and nesting. Also implied by this distribution is an efficiency and strength of flight for crossing wide water barriers.
One noticeable aspect of the barn owl's range is its lack of occurrence either far to the north or south of the equator; in North America the northern limit lies just over the southern border of Canada. The explanation is that barn owls are poorly adapted to very ccld winter cl mates. Compared to other similar-sized birds the barn owl has a higher metabolic rate requiring more food relative to body size. It also has less efficient insulation causing greater loss of body heat. Abnormally severe winters in areas where barn owls normally survive can cause extensive mortality. Such an event occurred in northern Utah in 1981-1982.
Reproduction in barn owls is characterized by a flexibility not seen in most other vertebrate predators. No nest is built by the owls themselves but many kinds of cavities are selected for egg laying. Natural sites range from tree cavities and crevices in rock cliffs to burrows in river banks which the owls sometimes excavate themselves. A higher tolerance for human presence than most other birds of prey has allowed barn owls to utilize a wide variety of artifical cavities. Open buildings, such as barns, are often used. Although most egg laying is done in the spring, barn owls have been known to breed in every month of the year if food is abundant. A yet unidentified mechanism allows them to adjust the clutch size (range is from 3-12 eggs) to density of available prey-more prey, more eggs. For birds, young barn owls are dependent upon their parents for a rather long time. From egg laying to becoming fully independent requires over three months although second broods are sometimes begun before young of the first are independent. There is a distinct division of labor in nesting by this species. Females do all of the incubation and brooding while males provide food for the whole family until the young can be left uncovered (about three weeks of age). Males are then joined by their mates in capturing prey for the young. Thus, both parents are essential and nesting will fail if either adult dies.
Barn owls feed only on prey they capture alive and are specialists in capturing small mammals-mice, rats, gophers, and shrews. Birds normally make up only a small percent of the diet. In sects are rarely taken because the cost/beneflt ratio is unfavorable for a large bird which hunts on the wing to eat very small prey; too little energy is obtained to compensate for that used in searching and capturing. Barn owls, like many other birds, swallow prey whole. Nondigestable material (hair, feathers and bones) is compressed into compact pellets in the stomach and regurgitated several hours after eating.
The excellent night vision of owls is well known, but there are circumstances in nature when even the visual adaptations of owls would not be able to detect small, concealed prey. Relatively recently it was discovered that barn owls can locate and capture prey acoustically, allowing them to take prey in very dark conditions or under vegetation and snow.2 The silent flight of owls, also well known to most people, must now be reevaluated in terms of the owl's auditory prey detection. Long thought to be useful in preventing the owl's prey from hearing the approaching predator, it is now believed that quiet flight may be at least as useful in preventing interference with the owl's own hearing for prey detection.
Barn owls are rather sedentary birds and no truly migratory populations are known. There is, however, a dispersal of young owls shortly after they become independent of their parents. Records from banded barn owls show that this many occur in any compass direction and may range from a few to over a thousand miles in distance. Having completed their dispersal, barn owls select a home range and usually remain there for the rest of their lives. The first few months after independence from the parents are also the most hazardous-about 70 percent of all barn owls die in this period. Even though most have a rather short life, the oldest known wild barn owl lived almost 18 years and some have survived 20 years in zoos.
Personal Research on Barn Owls
My interest in barn owls has been primarily ecological which translates as the study of the barn owl's interactions both with other species and its physical environment. Ecological studies must be long-lasting to produce true understanding of how organisms interact with their environment and adapt to changes. Several factors contribute to this necessity. Field research, especially, is faced with difficulty in controlling variables. A further problem is that several important variables have long cycles. An example is in the yearly seasonal change; interactions of barn owls and their prey may be very different in summer than they are in winter. Thus, data on owl diets gathered in summer may be meaningless in describing the owls' winter adaptations. Futhermore, some important barn owl prey undergo cyclic population changes taking from three to five years to complete. During these cycles, prey densities may range from tremendously abundant to very scarce. Once again, data gathered when major prey are abundant may fail to help us understand how owls cope with lean times.
My work with barn owls began in dissertation research where I compared the feeding adaptations of four owl species. Not long after my coming to Utah, an opportunity to continue studies on barn owls presented itself serendipitously. In 1976, Phillip Wagner, a biologist with the Utah Division of Wildlife Resources (a graduate of Weber State College in zoology), and I investigated reports of strange owls at a farm in Davis County. We found that they were barn owls, thought at that time to be uncommon in Utah .3 This discovery led us to investigate similar farmlands in Davis, Weber and Box Elder Counties. There we found what appeared to be excellent barn owl habitat but little evidence of barn owl presence. We hypothesized that lack of suitable nest sites was the primary factor limiting barn owl populations and began an experiment to test this. To do so, we built and installed 30 nest boxes in abandoned silos on farms in the threecounty area. This plan met with considerable support and interest by farmers who recognized the value of having efficient "mouse-eaters" on their premises. It also met with considerable success for the owls; half of the boxes were used for nesting in the first year. During the following six years occupancy has averaged over 80 percent annually.
Because we could gather data simultaneously on diet, reproduction, movements and survival of barn owls, it would be possible to study the interactions of those factors as well as the contributions of weather and changes in prey populations. Since most of the boxes were occupied all year, both as nest sites and roosts, we could collect data yeararound. In 1977, we began the research by collecting regurgitated pellets each month at each site. By analyzing remains in pellets it is possible to determine food habits both qualitatively and quantitatively. Doing this for numerous sites in all seasons and over many years will allow insight into variables that otherwise escape understanding. Despite the many published studies on barn owl foods from around the world,4 it has not been possible before to study variables of barn owl predation, i.e. the roles of weather, vegetational and prey changes and individual learning by owls.
Each year we record reproductive data: number of nests, number of eggs laid per nest and number of fledgling per nest. We also band all the young and as many adults as possible for individual recognition. These banded birds provide information on mortality and direction and distance of movements. By analyzing returns of owls found dead and those we recapture, we are beginning to understand causes of mortality, which way and how far the birds move. For example, during the abnormally severe winter of 1981-1982, we were able to document and quantify the tremendous die-off of barn owls caused by the combined stress of very low temperatures and deep snow-the mice were safely under the snow.
A sabbatical leave in 1979 allowed me to add another important dimension to this study. During this time, I began an association with the U.S. Bureau of Land Management working with their Birds of Frey Research Project in southwestern Idaho. A unique place, the Snake River Birds of Prey Area has one of the densest concentrations of nesting birds of prey in the world; fourteen species nest there. Several conditions, however, are different for barn owls than in Utah. One in particular, the concentration of other predators (hawks, eagles, falcons, other owls, coyotes, badgers and snakes) is very different than in the farmlands of northern Utah, where there are few other predators to compete for food. Thus, an excellent opportunity exists to test several hypotheses on the role of competition by comparing diet characteristics and changes in Utah and Idaho barn owls.
The overall plan for this research is to gather data as described above, year-around for Utah and two or three times a year in Idaho for a ten-year period. This interval should be sufficient to contain most cyclic or unusual events that may affect the owl populations. The data will then be searched for patterns and correlations between, for example, diet composition and reproductive success. Effects of weather (temperature, snow cover, etc.) will also be examined in relation to diet, reproduction and mortality.
Benefits of the Research
What good is it? First is a promise of new knowledge, chiefly that of long-term information of ecological interactions. There is a great need to know all that is possible about our environment especially because humans have become such a significant, but often careless, ecological force. Searching the ecological literature in conjunction with this study also is a great benefit to teaching of ecological principles. Students benefit from it in other ways, too: seeing local examples of ecological relationships; having material for laboratory analysis; and participating in a research project. Additionally, they may have the opportunity of meeting and working with potential employers in natural resource agencies.
Weber State College benefits from national and worldwide exposure through dissemination of the results of this work (two papers have been published, two are in preparation and six papers have been presented in scientific meetings to date). An additional benefit to Weber State College is the cooperation with local farmers and landowners and another state agency, the Division of Wildlife Resources.
1 thank the following for contributions to this study: Utah Division of Wildlife Resources; U.S. Bureau of Land Management; the Research and Professional Growth Committee, Weber State College; and the landowners who have allowed access to their property.
1 D. S. Bunn, A. B. Warburton and R. D. S. Wilson, The Barn Owl (vermillion, SD: Buteo Books, 1982), pp. 228-251.
2 R. S. Payne, "Acoustic Location of Prey by Barn owls (Tyto alba)," Journal of Experimental Biology 54 (1971): 535-573.
3 W. If. Behle and M. L. Perry, Utah Birds (Salt Lake City: Utah Museum of Natural History 1975), p. 23.
4 R. J. Clark, D. G. Smith and L. It. Kelso, Working Bibliography of Owls of the World (Washington, DC: National Wildlife Foundation, 1978), pp. 306-308.