- Definition of impact
- Reference to the 19 species and appendix A
- Discuss what the paper will entail
- Shooting, trapping, poisoning
- Barbed-wire fences
- Other impacts not of great significance in population disturbance
- How chemicals kill owls
- Why do detrimental
- Secondary Poisoning
- Direct killing
- Non-agricultural chemicals
- Naturally occurring elements
- Random species and the effects toxicology may have on them
- Barn owl example of structure usage
- Grazing and Ranching
- Benefits and negative impacts
- Short-eared owl and Burrowing owl
- Barn owl population changes throughout the 20 th century due to agriculture practices in Ohio
- Idaho Study on developed vs. undeveloped agriculture lands
- Habitat Alteration
- Describing the significance of this impact and why it is the most harmful of all impacts by humans
- Discussion of each species, habitat requirements and the impact of habitat alteration on each of them
- Migratory Bird Act
- Endangered Species Act
- Other significant governmental actions
- Broad overview
Humans have long roamed the earth interacting with other species and the habitat in which they are found. These interactions or impacts can be considered harmful or beneficial. There is concern because some of the impacts are harmful. Many people interpret the word "impact" as having a negative connotation. In this paper, the word will be used in another context. Impact will imply both positive and negative aspects of the interactions between humans and owls. This is necessary to clarify because humans cause both harmful and beneficial impacts to owls.
One group of species being impacted by humans is owls composed of the families Tytonidae and Strigidae. Owls are fairly secretive and many people do not realize the impacts which humans impose on them.
Within the United States there are 19 species of owls that are found year-round. Appendix A gives a brief synopsis of their natural histories (Johnsgard 1988, Stokes 1996). This information introduces the reader to the species and gives brief examples of how diversified owls are. Many people think that all owls live in trees. Some species such as the Burrowing Owl (Athene cunicularia) live predominantly underground and the Short-Eared Owl (Asio flammeus) nests directly on the ground. The appendix will also show that not all owls are large. The Great Grey Owl (Strix nebulosa) stands approximately 74 cm in height, while other owls such as the Elf Owl (Micrathene whitneyi) stand only 15 cm in height. With such diversity, a single human interaction can have varying outcomes on different species.
Impacts to owls can occur at two levels: individual and population. The individual scale involves impacts, which effect only select individuals in a given geographic area. Most of the time, these impacts do not have a drastic effect on an overall population dynamics of owls in a geographic range. The population scale impacts are of more concern to biologists because they usually affect most if not all owls threatening their survival. This paper will focus primarily on population impacts.
Some impacts only affect individual owls. Individual impacts against owls include deliberate shooting, trapping and poisoning (Postovit 1987). Although impacts such as these have decreased due to education and legislation, it continues to be a problem. Individuals who feel that the birds are posing a threat to their livelihood are more likely to take actions to eliminate the birds. There also have been reports of killings to collect the feathers or for sport (Postovit 1987). According to Gutierrez (1995), there have been documented killings of Spotted Owls in areas where logging is a vital part of the economy. There have also been reports of shootings of Long-Eared, Burrowing, and Short-Eared Owls. Since most species of owls are nocturnal, they are not facing impacts like diurnal bird species. There has been debate over the effect that these killings have on owl populations. Some researchers have concluded that these killings may cause declines in local populations of owls while others say there are no long-term impacts to the population as a whole (Postovit 1987).
Power lines are both beneficial and detrimental to owls. The beneficial impact that owls gain from power lines and poles is their availability for perching and roosting (Postovit 1987). The areas around power lines that are cleared provide good habitat for hunting. The owl can sit on the power line or pole and scope out the surrounding area for prey. The main detriment that power lines cause is electrocution. Various owl species have been found electrocuted including Great Horned Owls (Bubo virginianus) and Short-Eared Owls (Asio flammeus) (Fitzner 1975, Postovit 1987). Owls probably come in contact with power lines more often than reported. Some individuals may be injured and crawl away from the area never to be found. Owl populations probably are not significantly reduced due to electrocution (Postovit 1987). With power and utility technology increasing, negative impacts from power lines may continue to decrease.
Of all the individual impacts that owls face, automobiles appear to be the most detrimental. Many people drive at night when owls generally hunt. Many owls use highways and nearby areas for hunting. Owls perch in trees and other structures to watch for their prey. When an owl sees their prey, they move in for the kill flying close to the ground. Since owls have frontally situated eyes, they tend to have a narrow visual field of approximately 110 degrees (Sparks 1970) resulting in "tunnel vision". If an owl is flying perpendicularly over a road, it may not be able to see oncoming automobiles. In a study done in Hawaii from 1992-1994, 81 Barn Owls (Tyto alba), and five Short-Eared Owls (Asio flammeus) were evaluated to determine their cause of death. Of the 40 that died from trauma, 29 appeared to have collided with automobiles (Work 1996). Devine and Smith (1985) performed a study on Eastern Screech Owl (Otus Asio) mortality. They found that Screech Owls are frequently killed by vehicles while hunting near roads. The number of deaths by automobiles may not be considered detrimental to a specific population of owls.
Another cause of random killings of owls is barbed-wire fences. The number of owls killed by barbed-wire fences probably is not significant, but it has been reported numerous times and probably occurs more often than reported. Throughout the United States, Great Horned, Burrowing, Short-Eared, and Barn Owls have been found entangled in fences (Allen 1990). On 16 July 1993, a dead Long-Eared Owl (Asio Otus) was discovered hanging from a barbed-wire fence in Colorado (Tischendorf 1997). Raptor rehabilitator Heather Best of the Western North Carolina Nature Center stated that a majority of the Great Horned Owls that they receive are injured from being entangled in barbed-wire fencing.
The list of human caused impacts that effect owls on the individual scale also include dams, research, railroads, airplanes, recreation, and mass noise pollution such as sonic booms. There have even been reports of owls being caught in abandoned fishing line.
These individual impacts are detrimental to owls, but impacts that affect owls on a population scale are of more concern to scientist. Population scale impacts cause greater impact to owls, so they generally are studied more and are ecologically more important.
Population Impacts : Toxicology
Chemical contamination is considered to be one of the most detrimental threats to owl populations. Whereas most human impacts on owls are on a local scale, pesticides and other chemicals affect owls on a global scale. Chemicals are not confined to any geographic area because they can travel through the air, soil, and water. One reason that chemical contamination is so detrimental to owls is because they can not recognize and avoid pesticides (Postovit 1987). With other human impacts, the owls are able to recognize the changes and move to a different location or adapt to the changes. Owls are not able to avoid chemical pollution because it is a global problem.
Most of the time chemical contamination does not directly kill owls. Instead, chemicals usually affect owls through bioaccumulation. Lower levels of the food chain do not consume high levels of pesticides. As you move up the various trophic levels, the accumulation of chemicals increases. The species at the bottom of the food chain take in low concentrations of the chemical. When the predator eats the prey, the toxins from the prey accumulate in the predator. This build up of chemicals moves up the food chain. Since owls are at the top of the food chain, they tend to accumulate large amounts of the chemicals from their prey. Retention of the chemicals in their bodies is what causes the problems with bioaccumulation. A problem with many of the pesticides, rodenticides and other chemicals is that many of them are resistant to breakdown in the environment. Chemicals which do not break down can cause problems years down the road after they have been put in the environment. DDT is a good example of a resistant chemical that has affected owls and other raptors (Weidensaul 1996).
Scientists have realized the importance of studying the impacts that chemicals have on various birds of prey. Various methods can be used in determining pollutant levels in owls, such as the evaluation of road kills, infertile and fertile eggs, eggshell fragments, and performing feather, blood, fat, liver and muscle biopsies (Peakall 1987).
Chemicals also cause secondary poisoning in owls. Owls receive secondary poisoning through prey which have ingested a chemical that may or may not break down in the prey or owls body into another chemical (such as DDT breaking down into DDE). Hill and Mendenhall (1980) completed studies on the effects of Famphur on Barn Owls. Famphur, an organophosphate, is not considered as dangerous as other chemicals because it is inactivated rapidly by biological processes. Due to the possibility of Famphur being accumulated prior to its breakdown and killing potential owl prey, it was assumed that there was a possibility that prey which have been in contact with Famphur may cause secondary poisoning to birds of prey. In Hill and Mendenhall's (1980) study, they found that owls and other raptors that eat prey poisoned by Famphur could experience secondary poisoning.
Another instance of secondary poisoning was a study of the impact of Volid on owls. In this study, Volid, a rodenticide, was used to control the vole population in an apple orchard. Radio transmitters were attached to 38 Eastern Screech Owls, 5 Barred Owls, 3 Red-Tailed Hawks, 2 Great Horned Owls, and 2 Long-Eared Owls. After treatment of 57% of the orchard, 6 of the Screech Owls were positively identified as being killed by Volid. One Long-Eared Owl was found to have died from secondary poisoning. Many species of owls prey on small rodents such as voles. From eating voles poisoned by Volid, the study concluded that there are significant hazards to Eastern Screech Owls and other raptors due to the bioaccumulation of Volid (Hegdal and Colvin 1987).
There are recorded instances of direct killings to certain species of owls from chemicals. In Canada, Carbofuran has been blamed in the reduction of Burrowing Owl populations by being applied directly to nest sites (R.A. 1994). Carbofuran is used as a pesticide to kill insects, mites, and nematodes. It can be used against pests in field, fruit, vegetable and forest crops. This pesticide is known to be highly toxic to birds because of secondary poisoning, but also because of the impact birds face with direct contact. In the United States, granular Carbofuran was banned on 1 September 1994. It is still legal for use as a spray, but is classified as a Restricted Use Pesticide (RUP) (http://ace.orst.edu/info/extoxnet/pips/carbofur.htm 1996). With its wide usage throughout North America, it poses a potential hazard to owls.
Not all pollutants are from agricultural practices or rodenticides. A study conducted in the late 1980's found Lead from mining in Great Horned Owls and Western Screech Owls in Northern Idaho. Although no deaths were recorded and the concentrations in these 2 species of owls were relatively low, lead in high concentrations can impact owls negatively or even cause death (Henny 1993).
Some toxins occurring naturally in the environment are lethal. Mercury is a naturally occurring element that is found in most environments. Humans release mercury into the environment in large concentrations through industry (Postovit 1987). Large amounts of mercury are dangerous to owls.
Other studies are being conducted and have demonstrated impacts to owls from chemicals. "Carbonyl-based insecticides destroy non-target moths, which may be critical prey early in breeding season" for Flammulated Owls (McCallum 1994). These moths are one of the prey of Flammulated Owls. Predation on moths which have been in contact with specific insecticides may cause secondary poisoning to Flammulated Owls and other species of owls. Great Grey Owls prey primarily on small rodents, many which are being poisoned with rodenticides. Barn owls have residues of pesticides and egg shell thinning due to pesticides used in agricultural practices (Marti 1992). Burrowing Owls nest on the ground so they are directly effected by application of insecticides, pesticides and other chemicals applied on the ground (Haug 1993).
Population Impacts : Agriculture
The primary way that agriculture impacts owls is by habitat alteration. Farming and ranching are a primary land use in the United States. Since agricultural practices often modify landscapes, this can have both beneficial and harmful impacts on owls (Postovit 1987).
The most beneficial aspects of farming to owls are the planting of trees and the building of structures such as barns (Postovit 1987). One species of owl that benefits from barns and other buildings on agricultural lands is the Barn Owl. The Barn Owl inhabits countryside with open fields and hedges for hunting and uses old buildings for breeding sites (Johnsgard 1988). Throughout the early-mid 20 th century, many barns and farmhouses were built. These provided habitat for Barn Owls to thrive in.
Grazing has both beneficial and detrimental impacts. In some circumstances, grazing may have beneficial impacts on Burrowing Owls. The grazing of lands with tall grasses can create suitable habitat for burrowing mammals. Humans therefore provide nesting sites for Burrowing Owls. There are two primary ways that owls are impacted by grazing. The first is the alteration of nesting site availability. "Intensive grazing in meadow and marsh habitats may adversely affect the nesting habitat of ground nesting raptors such as the Short-Eared Owl (Kochert, et al. 1988)." In North Dakota, Short-Eared Owls tend to avoid grazed habitat.
The second way owls are impacted by grazing is through prey abundance and vulnerability. Abundance of small bird, mammal and reptile populations is reduced because of habitat alteration. Grazing can reduce the vegetation cover. Prey species that require low levels of cover are favored over those that need dense or tall cover. Lower ground cover makes prey more easily seen and captured by owls. In South Dakota, young Great Horned Owls hunt mainly in areas with little or no cover (Kochert, et al. 1988).
There are also indirect impacts that livestock-grazing pose. One is the development of water ponds. Stock ponds in Arizona have provided conditions suitable for mesquite forests which provided habitat for Long-Eared Owls which were previously not there (Kochert, et al. 1988).
Colvin (1985) studied how agriculture effected owls over the past century in Ohio. Prior to 1920, the Barn Owl was extremely rare in Ohio. The clearing of the countryside in the 1920's and 1930's greatly increased the owl population. Since then, the Barn Owl population has declined overall. This decline has been correlated with the availability of grasslands. As the grasslands have declined, so has the Barn Owl population. The population increase began in the 1920's and the major decline started in the 1960's (Colvin 1985).
In Idaho, a study was performed on the influence of agricultural development on raptor species. The study was done along the Snake River Plain in Southern Idaho. From 1986-87, studies were conducted via road surveys. Of the 11 species of raptors found, 2 were owl species. The Burrowing Owls were the only resident that flourished in the agriculturally developed areas. The Short-Eared Owl was also seen during the study. They were less abundant in developed areas. Owls may have also been secondarily effected by a decline in prey densities. Differences in prey populations and vegetation between developed vs. undeveloped rangelands may have a drastic beneficial or detrimental impact on owl densities. Although the Burrowing Owl flourished in the study area, all other raptor species appear to have been negatively effected by agriculture (Leptich 1994). Agricultural practices cause habitat alterations, but agriculture is not the only way that habitats are altered.
Population Impacts : Habitat Alteration
Human alteration and destruction of habitat causes significant impacts on owls. Loss of habitat is the primary factor in the decline of raptor populations throughout the world. In addition to agriculture, logging, urbanization, recreation, energy and mineral development are altering owl habitats throughout the United States. In the United States, "98% of the tall-grass prairies have been plowed, half of the wetlands drained, 90-95% of old-growth forests cut, and overall forest cover reduced by 33% (Miller 1994)." Many species of owls have already begun to feel the tightening grip of altered habitats.
One way that owl habitat is altered is by urbanization and building of man-made structures. Urban areas have a population greater than 2,500. Approximately 75% of Americans live in urban areas and the remaining 25% live in rural areas (Miller 1994). This can be beneficial or harmful to owl populations. Owls could benefit from urbanization because it moves people into a condensed area and keeps them from being spread out. The city removes habitat in a small area, keeping other areas from being directly effected. Urbanization can also be harmful. Altering habitat drastically can make it unsuitable for most owl species. Some species such as Screech Owls and Great Horned Owls are more tolerant of urbanization, but most species are negatively impacted.
Some owl species tend to be habitat specialists. They have specific habitat requirements in which they need to survive. Pima County, Arizona is considering designating habitat for Northern Pygmy Owls (Glaucidium gnoma). In an article published in July 1999, there was concern by builders. The concern is that only 16% of the county is taxable. Designating 731,000 acres of habitat as critical for the Pygmy Owl could be harmful to the economy. If the land is designated, private landowners will need a federal permit to develop their land. In addition, taxes may have to be increased to compensate the loss of revenue. This in turn may result in fewer people being able to afford housing in the area. One person stated, "We have to ask, are we putting the owl before the people (Sodoma 1999)?" This is a controversial problem throughout the United States and will likely continue to be for some time to come.
In California 's Silicon Valley, the Burrowing Owl is being affected by development. Land prices in the area are high because of the computer industry, so developers are the only people able to purchase and develop the land. This alters Burrowing Owl habitat. There is an effort throughout the region to try and preserve as much habitat as possible to save the decreasing population of Burrowing Owls. For example, Mission College has implemented a plan to preserve some owl sites. They installed artificial burrows and keep the grass cut low which the owl prefers. Although conserving the Burrowing Owl habitat has been difficult, the region is in the developmental stages of implementing a plan to conserve the Burrowing Owls and their habitat (Holmes 1998).
The most well known habitat alteration that effects owls is logging. The impact of logging on each species varies. The method of harvesting trees can determine that impact. The four main harvesting methods are selective cutting, shelterwood cutting, seed-tree cutting, and clear-cutting (Miller 1994). Selective cutting is the removal of intermediate-aged or mature trees in an uneven-aged forest (Miller 1994). This harvesting style can be both beneficial and harmful to owls. It may be beneficial to owls that do not prefer dense forests and those that can live in both old and intermediate-aged forests. This style of harvesting does not remove all the trees and leaves the forest in an uneven aged state with young, middle aged, and mature trees. It may be detrimental to owls that prefer a specific age group of trees. Some owls such as Great Horned Owls are generalists. They are able to live in a variety of habitats. Others like the Spotted Owl are specialists and need old-growth forest to survive.
Shelterwood cutting is another form of logging. This style of harvesting involves removing the mature trees usually in two cuttings (Miller 1994). Most of the mature trees are removed in the first cut leaving some smaller trees to shelter the seedlings. When the seedlings are well established, the remaining trees are cut. This style of logging can have a drastic impact on owls that prefer mature or old-growth forest. The benefit of this logging style is that it does not remove all the older trees at once. They are removed over a period of time, which may be beneficial in allowing owls to adapt to the altered landscape more easily.
Seed-tree cutting involves the removal of nearly all the trees in one cutting (Miller 1994). A few of the best trees are left for producing seedlings. This style of cutting may be beneficial to species that do not require a developed forest because the trees are removed. The new forest is an even-aged forest. Spotted Owls and Great Grey Owls would most likely be harmed by such logging practices.
Clear-cutting removes all trees in a given area in one cutting. There are variations of clear-cutting such as strip logging and whole-tree harvesting. The cutting may be in a patch, strip, or a whole stand. In the United States, nearly 66% of the annual timber harvest is attained by clear-cutting (Miller 1994). The practice of clear-cutting has a negative impact on owls that depend on trees for nesting. Clear-cutting can also be beneficial. It creates foraging habitat. Many owls such as Great Grey Owls prefer to hunt along forest margins which provide open areas for visual searching for prey (Johnsgard 1988). Also, clear-cutting can create suitable habitat for owls that do not nest in trees such as the Burrowing Owl and the Short-Eared Owl.
Studies on the Spotted Owl, Great Grey Owl, and the Barred Owl are presented in the following section to illustrate the impact of human-altered habitat on owl populations. These species were chosen because of the abundance of information on them.
Spotted Owl : The Northern Spotted Owl has become the symbol in America with the Jobs/Logging vs. Owl debate. In a study by Forsman, Meslow, and Wight (1984), 98% of the sites with Spotted Owls were either old-growth forests or a mixture of mature and old-growth forests. They also prefer to live in multi-layered canopies and uneven-aged forests. The Spotted Owls do not build their own nest. They utilize natural sites of broken tree tops and cavities (Beck and Gould 1992). The average home ranges of Spotted Owls range from 400 to 2776 hectares (Johnsgard 1988). The main habitat alteration that seems to effect Spotted Owls is the logging of old-growth forest in the western United States (Forsman and Bull 1989). The concern for the Spotted Owl began back in the 1970's when the logging vs. habitat debate in the Pacific Northwest was on the rise (Beck and Gould 1992). Some studies have indicated that Spotted Owl populations have decreased with the elimination of old-growth forests and the replacement by younger forests. For many decades throughout the 20th century, the Pacific Northwest was heavily logged because of the ever increasing need for timber in the United States. Much of the old-growth forests in the United States have vanished, but most of what remains is located in the Northwest. Less than 10 percent of the old-growth forests that once stood in the Northwest are still standing. Many people in the Northwest depend on logging for employment, so it is understandable that there is much opposition to the preservation of old-growth forests for Spotted Owls (Gup 1990). Although there have been some concerns about the effect that preservation of owl habitat may have on jobs and the economy, the economy in Oregon is currently on the rise. In 1993, President Clinton's forest plan caused many job losses due to lumber trade regulations, but it included occupational retraining. Timber harvesting is not the only reason for concern with the Spotted Owl. The California Spotted Owl is also losing habitat for breeding and foraging. New residential developments, fires, recreational activities, and the "mining" of water from streams in the owl's habitat are impacting the California Spotted Owls population (Beck and Gould 1992). This species of owl will continue to be one of concern in the United States because of their dwindling numbers.
Great Grey Owl: The Great Grey Owl prefers mature or old-growth forests throughout the Pacific Northwest. They nest in abandoned nests or on top of tree trunks. Logging effects on Great Grey populations can either be beneficial or harmful. Depending on the extent, type and time of logging, it can either benefit or be detrimental to the species (Johnsgard 1988). "In northeastern Oregon, owls spend the majority of their time foraging in selectively logged stands", and in southeastern Idaho, the owls prey on gophers in clear-cut areas (Forsman and Bull 1989). The reduction of dead trees used for nesting, and dense canopy trees used by juveniles are problems that logging causes with Great Grey Owls.
Barred Owl: The Barred Owl live primarily in mature forests and swamps. They nest in tree cavities but sometimes will use abandoned nests. Logging of mature and old-growth forests has been the primary habitat alteration that affects this species (Smith et al. 19__).
In Connecticut and New Hampshire, Barred Owls avoid developed areas. However, there is evidence that Barred Owls are able to sustain human development more readily than other owls (Smith 1975).
Although there are many other species of owls, this is a sample of some of the impacts caused by habitat alteration and destruction.
One of the beneficial impacts humans have on owls is through legal protection. In the United States, there are serious penalties for harming all birds of prey including owls. Legislation has helped to increase public awareness, and reduce shootings, poisoning, and habitat destruction (Postovit 1987).
An early law, the Migratory Bird Treaty Act of 1918, prohibited the shooting of all raptors. Though it took many years for the public to become conscious of the act, it has had a positive impact on owls and other raptors throughout the United States (Postovit 1987).
In 1973, the United States Congress approved the Endangered Species Act. This law restricts the possession of any endangered or threatened species in the United States. Not only does this include species found throughout the United States, but also those found world wide (Weidensaul 1996).
The ban on DDT was an extremely important piece of legislation. This chemical has had an impact on raptor species reproduction throughout the United States since its use began during WWII (Postovit 1987). Without this federal legislation, the populations of owls throughout the nation would be far more negatively impacted by deaths. Further legislation is needed to protect owls from habitat alteration, poisoning, and other negative human impacts.
Humans impact owls in many ways. Some impacts are beneficial while others are harmful. Some of the impacts only affect a few individual owls, while others may effect the entire population of a particular geographic region. Harmful impacts are being addressed by scientist to find a way to minimize or eliminate them. Overall, the status of owls in the United States is rather stable. Most owls are remaining steady in numbers while selective species like the Spotted Owl are declining in number. The outlook for owls in the future is a bright one. Now that the United States population is being exposed to environmental issues, the general population will most likely want to protect their natural resources and the species that live in them such as owls. This paper was written to give the reader an overview of some of the greatest concerns for human-owl interactions. It is recommend the reader to research more information on each of the topics spoken of in the paper. There also needs for further research on many of the topics mentioned. This paper is meant to be an introduction to such topics, but is not to be considered a compilation of all the impacts of humans on owls. Much of the study of raptor biology and human impacts on raptors is a fairly new area of research.
It is probable that impacts humans have on owls and other raptor species will continue to be of great concern. Humans are continuously altering owl habitat. With continued population growth and development, owls will continue to be at risk. Continued research is needed to help protect owls from the impacts of humans and how to maintain stable owl populations.
http://ace.orst.edu/info/extoxnet/pips/carbofur.htm Revised 1996.
Basic natural history of the owl species found within the continental 48 states of the United States of America
|Strix nebulosa Great Grey Owl||73||Coniferous forests, stunted forest, hardwood forest||Small rodents||Abandoned nest or top of tree stump|
|Nyctea scandiaca Snowy Owl||60||Open tundra, beaches, marshes||Rodents, hares, fish birds, lemmings||Nest on Tundra|
|Bubo virginianus Great Horned Owl||56||Extremely varied: coniferous forests, deciduous forest, deserts, suburbs||Small rodents, rabbits, skunks, porcupines, cats, songbirds, osprey, hawks, other owl species||Abandoned nest, hollow tree or cliff edge|
|Strix varia Barred Owl||53||Prefer mature forests, swamps, hardwood or coniferous forests||Small rodents, small birds||Primarily tree cavity, but also will use abandoned nest|
|Strix occidentalis Spotted Owl||46||Prefer old-growth forests and/or mature forests||Flying squirrels, small rodents, rabbits, reptiles, birds||Cavities in old growth trees|
|Tyto alba Common Barn-Owl||46||Country sides, open fields (hunting), agricultural grasslands||Small rodents||Barns, old buildings, tree hollows, old burrows|
|Surnia ulula Northern Hawk Owl||40||Coniferous and/or deciduous forests bordering open areas||Primarily voles, but also mice, lemmings, birds, rabbits||Tree cavity, hollow end of dead tree|
|Asio otus Long-Eared Owl||38||Edges of dense, coniferous or deciduous forests||Mammals, amphibians, reptiles, insects||Abandoned nest or tree cavity|
|Asio flammeus Short-Eared Owl||38||Open habitats such as fields, marshes, dunes, grasslands||Mammals and songbirds||Nest on ground: surrounded by grasses|
|Aegolius funereus Boreal Owl||25||Boreal, coniferous forests||Voles, lemmings, mice, other small rodents and mammals||Tree cavity, nest box|
|Athene cunicularia Burrowing Owl||23||Open grasslands, prairies, plains||Insects, small mammals, amphibians||Abandoned burrows|
|Otus asio Eastern Screech Owl||23||Woodlands, parks, suburbs, swamps||Amphibians, small fish, invertebrates, small rodents, flying squirrels, other mammals||Tree cavity, bird houses|
|Otus kennicottii Western Screech Owl||23||Wetlands, lowlands, deserts, rainforests, deciduous and coniferous forests||Mice, crayfish, invertebrates||Tree cavity, nest box|
|Aegolius acadicus Northern Saw-Whet Owl||20||Coniferous or mixed woodlands||Mice and voles||Tree cavity, nest box|
|Glaucidium gnoma Northern Pygmy-Owl||18||Woodlands and forests within low and high mountainous terrain||Small mammals, birds, reptiles, amphibians||Tree cavity (natural or caused by another species of bird)|
|Otus trichopsis Whiskered Screech-Owl||18||High elevation dense oak forests||Crickets, caterpillars, moths, grasshoppers, centipedes, arthropods||Abandoned flicker or natural cavities|
|Otus flammeolus Flammulated Owl||18||Pine/oak forests, coniferous forests in high elevations (seem to prefer yellow pines)||Arthropods: beetles, moths, caterpillars, crickets, centipedes, arachnids||Abandoned woodpecker holes, natural cavities|
|Glaucidium brasilianum Ferruginous Pygmy-Owl||18||Desert habitats, riparian timberlands||Small mammals, birds, lizards, invertebrates, scorpions||Abandoned woodpecker hole, natural cavities|
|Micrathene whitneyi Elf Owl||15||Upland deserts, low plains near river bottoms||Mice, shrews, insects, scorpions, spiders, centipedes||Abandoned woodpecker hole, saguaro cactus|