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Human Impacts on Owls Within the United States

Paper by Jeremy Benjamin Updated 2006-03-23 Created 2005-06-15
Page 1 of 5

I  Introduction

  • Definition of impact
  • Reference to the 19 species and appendix A
  • Discuss what the paper will entail

II  Human impacts effecting owls on an individual basis

  • Shooting, trapping, poisoning
  • Powerlines
  • Automobiles
  • Barbed-wire fences
  • Other impacts not of great significance in population disturbance

III  Human impacts affecting owl populations

  • Toxicology
    • 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
  • Agriculture
    • Intro
    • 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
    • Logging
    • Urbanization

IV  Actions Taken to Help Protect Owls

  • Migratory Bird Act
  • Endangered Species Act
  • Other significant governmental actions

V  Conclusion

  • Broad overview
  • Recommendations

Introduction

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.

Individual 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.

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