Spotting the Spotted Owl

Deep in the forests of the Pacific Northwest, lives a nocturnal carnivore of unparalleled repute. Some say they’ve seen it swoop down on its victims in the dead of night and feast on the bodies among the branches of knotted, old trees. Others say its haunting call can be heard from a mile away. The mere mention of the creature’s name brings shudders to loggers and some local inhabitants. Fear over its existence has incited rallies, garnered the attention of three government agencies, and caused people to tie themselves to trees.

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owl lunch
Flying squirrels and wood rats are a spotted owl's favorite prey. (Photograph courtesy Jerry Mires)
single owl

To scientists this animal is known as Strix occidentalis caurina, but it is commonly referred to as the northern spotted owl. For more than ten years environmental groups and logging corporations spent millions of dollars in attempts to control the fate of the bird’s old-growth forest habitat. Hardly a week went by in the late 1980s where there wasn’t a story about a protest rally attempting to save the owl or news of lawsuits being filed to stop the harvest of the owl’s habitat.

A northern spotted owl at home in the old-growth forests of the Pacific Northwest. As part of the plan to protect the spotted owl, biologists and land managers are using satellites to map potential habitat. (Photograph courtesy Janice Reid)
At the end of the decade the U.S. Fish and Wildlife Service put the owl on the Federal Threatened Species List, and the federal government devised a strategy to conserve the old-growth forests in western Oregon, Washington, and Northern California. While these government actions quelled the conflict between loggers and environmentalists, they created a whole new set of challenges for the scientists studying the owl’s habitat. The problem was that no one in the state or federal governments had a way to determine how many birds or how much old-growth forest existed in an area as vast as that specified by the government plan. For four years scientists from three government agencies toiled over a number of approaches.

Earlier this year a method to monitor the owls was finally decided on. With the help of Landsat 5 and the newly-launched Landsat 7, researchers plan to locate areas where owls are likely to live. They will then use this information, along with ground surveys, to map out the owl’s habitat and create a method for assessing the health of the owl population in the Pacific Northwest. Whether this strategy will succeed remains to be seen.

ornamentnext The Spotted Owl’s Checkered Past

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

Because of the link between spotted owls and undisturbed forest, the threatened birds have been caught up in the conflict between environmentalists and the timber industry. (Photograph courtesy Sierra Club)
The Spotted Owl’s Checkered Past   Page 1 Page 3
"These old forests in the Pacific Northwest were thought to be of little value to anyone but loggers not more than thirty years ago. People were simply unaware that the spotted owls were out there," said Eric Forsman, a research wildlife biologist with the United States Forest Service. Forsman’s graduate work in 1972 is part of the reason people are aware of the owls today. He and researchers at Oregon State University discovered that a majority of spotted owls reside in old areas of the forest where the trees are typically more than 100 years old.

Spotted owls evolved over eons to thrive in the old-growth conifer forests. Forsman said everything about their lives depends on these types of forests. They build their nests in large tree cavities and on thick branches in dense tree canopies. The owls hunt for animals found in abundance in the decaying logs and leaves of the old forest. They even developed coloring that camouflages them in the shadows created by the big trees. When the old forest is chopped down, the birds have trouble nesting and feeding their young, Forsman said.

  old growth forest
The predominant ecosystem in the Pacific Northwest is temperate rain forest. Mature forests are characterized by a full range of tree ages, from saplings to decomposing logs. This diversity in age results in a complex canopy and a layer of detritus on the forest floor. (Photograph courtesy U.S Fish and Wildlife Service)
By the early 1970s, logging companies had already reduced the old forest in the Pacific Northwest by 85 to 90 percent. "After making the discovery about the owls, we looked at the area planned for harvest by logging companies in the future and noticed that a majority of it was the remaining old forest - spotted owl habitat," said Forsman. "Fifty percent of the area belonged to the government."

Over the next decade news of the owl’s plight spread. Eventually concern for the owl caught the attention of the media and mainstream environmental groups. Clashes erupted between environmentalists and loggers, whose livelihood was being threatened. The northern spotted owl practically became the mascot for the environmental movement in this country. Extremist groups even tied themselves to marked trees and damaged logging equipment. "I was amazed to see how the pendulum had swung. It got to the point where you couldn’t even go into the woods to cut down so much as one tree," said Forsman.

The U. S. Fish and Wildlife Service made the concern for the northern spotted owl official in 1990 when they placed the bird on the Federal Threatened Species List. Little was done about the owls until a year after President Clinton took office. His administration established the Forest Ecosystem Management Team (FEMAT) in 1993. The team was made up of personnel from the Bureau of Land Management (BLM), the United States Forest Service (USFS), the U. S. Fish and Wildlife Service (USFWS), and regional universities (U.S. Fish and Wildlife Service, 1996).

Together they drew up a plan that reduced future timber harvests on federal property by 80 percent and created a number of guidelines to manage the remaining older forest. One of the primary goals of the plan was to develop a model to determine the well being of spotted owls and their habitat, strewn out across 25 million acres of land in western Oregon, Washington, and Northern California (Bureau of Land Management, 1993).

ornamentnext The Problem with Spotting Owls
back Spotting the Spotted Owl

clearcut forest
A cheap and effective method of harvesting lumber is to clearcut. Unfortunately, clearcutting destroys the diversity of the forest, leaving habitat unsuitable for the spotted owl even after decades of regrowth.(Photograph courtesy U.S Fish and Wildlife Service)
The Problem with Spotting Owls   Page 2 Page 4
"It may sound strange, but we have no count of exactly how many spotted owls are out there," said Joseph Lint, a wildlife biologist at the Bureau of Land Management. For the past twenty years Lint and other scientists have been keeping track of owls by first-hand observation. Lint explained that biologists pick out an area of land, go into the forest, and search for pairs of male and female owls. When the owls are spotted, researchers release mice and wait for the birds to snatch up a free meal, said Lint. The birds usually take the food back to their nests and the biologists follow. If any young are found in the nest, the scientists band them. The banded, spotted owls are then tracked down several years later to see how many of them survived and if they are having young of their own.

During the course of one of these surveys, the researchers typically track down a few hundred owls over a range of tens of thousands of acres. Lint said the scientists use these numbers to infer the survival rate of the owls, the total acreage of owl habitat in the area, and the structure of that habitat. They’ve found over the past ten years that the number of owls they've monitored have been decreasing at a rate of about 4 percent every 5 years. "So there isn’t a sharp decrease in the survival rates of the owls tagged. The population is probably stabilizing," said Lint.

  banding an owl
Traditional methods of monitoring spotted owls are time and labor intensive. Individual birds must be caught and banded by hand. (Photograph courtesy U.S Fish and Wildlife Service)
spotted owl rangeWhile this mark and recapture method is the only one proven to work, it would be very difficult to use it exclusively to monitor the 25-million acres specified in the federal plan. The costs to maintain personnel and equipment would likely run to $3 million a year, and it would require a very large number of personnel (Lint et al., 1999). Instead, a model was needed that could accurately predict the health of the owl population in the Pacific Northwest using a minimum of groundwork.

"One of the first steps [in developing a predictive model] is to know the extent of spotted-owl vegetation and the range of the spotted owl," said Jim Alegria, the manager of the Interagency Vegetation Mapping Project for the Bureau of Land Management in Washington and Oregon. He said the BLM and USFS agreed that the solution to their problem lies in Earth-imaging satellites. Over the next few years they hope to have old-forest growth and owl-habitat maps for Oregon and Washington.

The advantage of satellites is that they can map out an area of landscape in a fraction of the time it takes to chart the area on the ground. More importantly, advanced imaging satellites such as Landsat 5 and Landsat 7 render images that can be used to identify large expanses of vegetation on the Earth. The satellites' sensors can highlight where one type of vegetation begins and another ends (Townshend et al., 1993).

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back The Spotted Owl’s Checkered Past

Burgundy colored regions on this map represent the historical range of the northen spotted owl in the United States (it also extends north into British Columbia). One hundred-fifty years of logging, agriculture, and urbanization have reduced the amount of old growth forest (potential spotted owl habitat) by 85-90%.
Color Coordinating Satellites   Page 3 Page 5
TM bands As can be seen through a prism, sunlight contains many different colors (wavelengths). When sunlight strikes objects, certain wavelengths are absorbed and others are reflected or emitted. The Landsat 5 and Landsat 7 satellites are sensitive to these different types of light, including infrared wavelengths that cannot be seen with the naked eye. Each satellite has several separate light detectors (photoreceptors) on board, which measure the energy reflected or emitted by the Earth. One light detector records only the blue light coming off the Earth (band 1). Another observes all the yellow-green light (band 2), and still another picks up on all the near-infrared light (band 4). The Landsat satellites move in circular orbits very nearly from pole-to-pole around the Earth and scan strip after strip of our revolving planet. Their data are beamed back to the surface, where they can be processed into pictures.

To differentiate between types of vegetation and their attributes, scientists take an image of an area using the Landsat satellites’ light detectors. The researchers then compare the different colors of light reflecting off a patch of land. They mix and match these colors until they come up with the combinations that most distinguish the types of vegetation and vegetation characteristics such as the density of foliage. The combination is processed and made into an image that can be easily analyzed (Townshend et, al., 1993).

Percent Coniferous vs Percent Broadleaf

  labels The Thematic Mapper instrument aboard several of the Landsat satellites detects light in seven distinct bands. Red, green, and blue bands can be combined to create pictures as we would see them Different band combinations and comparisons reveal unique aspects of the Earth. (Image by Jesse Allen, GSFC Viualization Analysis Lab)
coniferous and broadleaf coverage

An example can be found in the way satellites are used to separate areas of conifer trees from hardwood trees. Researchers know that evergreen conifer trees absorb more infrared light (light to the right of red on the color spectrum) and reflect more red light than hardwood trees (Parkinson, 1997). This information provides them with a system to tell the two tree types apart. If the difference of red to infrared light recorded by the satellite (bands 3 and 4) is high, then they know a patch of trees viewed by the satellite is hardwood. If the difference is low, then the trees are probably conifer (Parkinson, 1997). The scientists can create a map by highlighting the variation between the two wavelengths.

ornamentnext Searching for Spotted Owls from Outer Space
back The Problem with Spotting Owls

Vegetation maps derived from satellite data can do more than show the presence of vegetation - they can also distinguish between conifers (left) and broadleaf trees (right). Since spotted owls prefer nesting in douglas fir and other conifers, it is necessary to determine vegetation type to map suitable habitat. (Image by Janine Savage, Global Change Master Directory, based on data from the Interagency Vegetation Mapping Project)
Searching for Spotted Owls from Outer Space   Page 4 Page 6
vegetation mapDiscerning the amount of potential owl habitat throughout the Pacific Northwest is not as simple as gauging whether or not a group of trees is conifer. "There are many layers and levels to this project," said Alegria. The researchers are now simply attempting to create a reliable satellite vegetation map of the 25-million acres of land specified by the FEMAT plan. When complete, the map should help biologists identify areas of old-growth forest and potential owl habitat.

Karin Fassnacht, a research forester at the U.S. Forest Service, works with Alegria to develop these maps. She explained, the scientists’ goal is to use Landsat satellites to map a number of attributes of forest vegetation across the Pacific Northwest. These attributes include the density of forest vegetation, the amount of conifer trees, the amount of broadleaf plants, the average size of the trees, and the complexity of the forests’ structure. Each of these cover variables will appear on the finished Landsat maps in terms of percentage, she said.

  This simple vegetation map (a portion of Washington State) only shows the amount of vegetation present. Clearcuts and barren mountaintops (shown as gray and brown) are easy to spot, but there is no way to distinguish conifers from deciduous trees or to determine the age of the forest. Click to see a 3D flythrough. (2.1MB) (Image and animation by Robert Simmon, based on North American Landscape Characterization data)
Quadratic mean diameterSo far, the scientists have found ways to combine the colors from the Landsat images to locate four out of five of these forest attributes on a given strip of land. "We are currently unable to map structure at all [using the satellites]," said Fassnacht.

When the maps are fully developed, they may help scientists determine if the trees in a section of forest are old. High percentages of large conifer trees with greater structural complexity usually indicate old-growth forest in the Pacific Northwest. By looking for these attributes on the satellite map, researchers should be able to locate areas likely to contain old-growth forest. "Additional ground-based or photo-based measurements must then be made to determine if any old-growth exists," said Fassnacht.

In the same manner, the maps may assist researchers in determining where owls and other animal species might exist. Those who know how the owls live could use the map, in part, to pick out those areas that have the percentage of conifer growth, structural complexity, and tree sizes required for the bird to survive Fassnacht said. These areas of potential habitat could then be verified on the ground.

With enough effort, the scientists should be able to map out all the areas where northern spotted owls could possibly live in the 25-million acres. The same could be done for other threatened species in the old-growth forest, such as the marbled murrlet. "There are several ongoing projects aimed at mapping habitats," Fassnacht said.

The BLM and USFS have just recently put out their first satellite-imaging map of the forests along the Coast Range, said Alegria. Though the map does not contain the structural complexity of the forests, it shows promise. Within the next few years the team hopes to have mapped the entire 25-million-acre plot of land specified by the federal plan. Not long after that, a map of all the potential owl habitats of the Pacific Northwest should be complete.

ornamentnext Putting It All Together
back Color Coordinating Satellites

Dark green areas on this map exhibit the characteristics of old growth forest, which are large trees, dense vegetation and a make-up of at least thirty percent conifer. The final feature - canopy complexity - may be measured by NASA's upcoming Vegetation Canopy Lidar mission. (full size map) (Image by Janine Savage, Global Change Master Directory, based on data from the Interagency Vegetation Mapping Project)

coverage map

landcover palette

Putting It All Together   Page 5
Yet, even with a map outlining the likely habitat of the owls, scientists will still not be able to tell if the owls are surviving or not. One last step is required. "We have to combine the remote-sensing data with the demographic owl data," said Bill Ripple, a member of the of the owl-habitat modeling team at Oregon State University. In basic terms, the information collected about the owls on the ground has to be matched up with the satellite maps.

Ripple said knowing the sheer amount of owl habitat is not enough to determine whether the owls will survive. Other factors such as the fragmentation of the old forest, the geometry of the owl habitat, and the type of old forest all contribute to the owl's ability to survive and reproduce.

The trick is in discerning just how these features affect owl health and reproductive success. "We will first bring together years of field data collected by field crews monitoring owl offspring," said Ripple. He and his team will comb through years of ground-based studies to see what features of the landscape in an owl's hunting range cause the owls to increase or decrease in number.

Once these patterns have been found, the Oregon State University team will try to integrate them with the owl-habitat map. For instance, if large fragmentation of the owl’s habitat causes a decline in the population, then the researchers can rule out the areas on the map that are highly fragmented. "We have just begun this five-year study," said Ripple. "For all we know we may be unsuccessful." If they are successful, only minimal ground surveys will be needed to see whether the model continues to work. Cost will drop and the area could be surveyed quickly.

Of course, this entire plan may be thwarted by natural selection before these models can be completed. The barred owl, a close relative of the spotted owl, has invaded the Pacific Northwest, said Forsman. This owl can out hunt the spotted owl and is generally more resilient. Worse than that, the barred owls can breed with the spotted owls. Together they create a hybrid owl that can reproduce. "With the barred owls out there, the data are getting really hard to interpret. The associations [between the many variables] are becoming really muddled," said Forsman. "In the end the spotted owl may get kicked out of the forest anyway."


  1. U.S. Fish and Wildlife Service, 1996: An Environmental Alternatives Analysis for the 4(d) Rule for the Northern Spotted Owl, Portland, Oregon, 2-3.
  2. Bureau of Land Management, 1993: Standards and Guidelines for Management of Habitat for Late-Successional and Old Growth Forest Related Species Within the Range of the Spotted Owl, Portland, Oregon, A1-A7.
  3. Lint, J., B. Noon, R. Anthony, E. Forsman, M. Raphael, M. Collopy, and E. Starkey, 1999: Northern Spotted Owl Effectiveness Monitoring Plan for the Northwest Forest Plan, United States Department of Agriculture, Portland Oregon, 27-29.
  4. Townshend, J. R. G., C. J. Tucker, and S. N. Goward, 1993: Global Vegetation Mapping. In R. J. Gurney, J. L. Foster, and C. L. Parkinson, (Eds.),Atlas of Satellite Observations Related to Global Change, Cambridge University Press, London, 301-310.
  5. Earth From Above: Using Color Coded Satellite Images to Examine the Global Environment, C. L. Parkinson, 1997: University Science Books, Sausalito, California, 95-118.

ornament back Searching for Spotted Owls from Outer Space

baby owls
Although the population of northern spotted owls has stabilized for now, long-term threats remain, not all of them due to human activity. (Photograph courtesy Jerry Mires)

 Quadratic Mean Diameter back Searching for Spotted Owls from Outer Space
Tree Trunk Size