Jeff Koenings, Ph.D.
Director

Jerry Gutzwiler
Chairman
Washington Fish &
Wildlife Commission
 

Scientists Seek Reasons for Declining Ferruginous Hawk Population
Posted June 2003

Summary
WDFW recently completed a second season of ferruginous hawk nesting surveys, one of four projects under way since 1999 to investigate an apparent decline in the population of this state threatened species. Using satellite telemetry, scientists have found the non-breeding period is a critical time for ferruginous hawks, when they travel widely outside of Washington and many die. Those that survive return in spring to the same nesting territories in eastern Washington. The first season of intensive breeding surveys in 2002 were designed to update historic nesting records and to compare results of air and ground nest observations for efficiency and accuracy. Unhatched eggs collected during surveys are currently being analyzed to assess contaminant levels. At two nest sites on the Hanford Reservation in Benton County, nest platforms have been installed on transmission line towers to reduce nestling mortality and improve productivity.

By J.W. Watson, Wildlife Research Scientist
WDFW Wildlife Program

The ferruginous hawk, so-named for its iron rust or rufous coloration, is the largest soaring hawk in North America and occupies the most arid regions of eastern Washington. This raptor was classified as a state threatened species in 1983 due to a decline in the breeding population. Since that time, surveys indicate the percent of occupied nesting territories has continued to decline. In 1987, 54 percent of 118 territories surveyed were occupied by ferruginous hawks; in 1995, 36 percent of 179 territories were occupied, and in 2002 only 20 percent of 241 territories surveyed were occupied. Although loss of pre-settlement shrub-steppe habitat certainly reduced the ferruginous hawk population, more recent vacancy of nesting territories is a bit puzzling. In 1999, WDFW scientists initiated a multi-faceted study to better understand the factors affecting the population, a first step in the effort to stabilize or increase the population.

The recent decline in nest occupancy prompted several questions related to the non-breeding period when most ferruginous hawks leave Washington for six to seven months. Do ferruginous hawks return to nest at established territories? Do they experience high mortality in winter that reduces the number of adults returning to nest? Where do they go in winter and what do they eat? To answer these questions, biologists deployed the first of 28 satellite transmitters or PTTs (platform transmitter terminals) on adult and juvenile hawks captured in Benton and Franklin counties in 1999. The birds are captured using fine mesh netting that entangles the birds harmlessly, allowing biologists to handle them and place PTTs on their backs.

PTTs allow long-range tracking of hawks for up to two years. PTTs transmit a signal every four days to a low-orbiting NOAA (National Oceanic and Atmospheric Administration) weather satellite, which transmits the location information to ground stations accessed by biologists via computer. Preliminary results indicate ferruginous hawks from Washington traveled widely in winter, moving an average of almost 3,900 kilometers (2,418 miles). Many hawks migrated to the northern plains of Montana and Canada after nesting and remained there through early fall. Most of these hawks migrated again in September to California, where they occupied winter ranges. One in three adults did not survive the winter, but those that did returned to the same territory to nest. Only one in four juvenile hawks survived after migrating from their nests and they traveled twice as far as adults on average during their first year. Clearly, the non-breeding period is a critical time for ferruginous hawks, affecting their health and survival and thus breeding status in Washington. A final report from this satellite project, conducted in cooperation with the Woodland Park Zoo and U.S. Fish and Wildlife Service, is due later this year.

Nesting surveys are a vital tool for assessing the status of breeding ferruginous hawks. Aerial surveys are the most efficient means of surveying ferruginous hawks, but are considerably more expensive than ground surveys. Ground surveys are time consuming because of the remoteness of many nests. A second project, initiated in 2002, is designed to more efficiently and effectively survey ferruginous hawks and improve the statewide database of historic nests. By conducting air and ground surveys at each nest for a minimum of two seasons, biologists hope to be able to determine the biases of each survey method and develop the most efficient, inexpensive way to monitor ferruginous hawks. The task of organizing the nesting survey effort has been headed by WDFW Survey Coordinator Eric Cummins and uses staff from WDFW regions and Olympia headquarters, along with cooperators from Woodland Park Zoo.

Nesting survey results from 2002 were revealing: Of 241 historical territories surveyed, only 20 percent were occupied by ferruginous hawks, 41 percent were unoccupied or had unknown occupancy, 20 percent had no nests, and 20 percent were occupied by other species including red-tailed hawks and Canada geese. Final results of recently-completed 2003 surveys are being compiled, but preliminary results suggest continued declining nesting activity in Yakima and Benton counties, with several nesting pairs in Franklin, Adams and Grant counties where there has been considerable spring rain.

Surveys conducted by air and ground yielded different results. Ferruginous hawk activity was classified differently between air and ground surveys at 27 percent of 147 territories surveyed. At 17 nests, the species was inconsistently identified as ferruginous hawks. There was a difference of 15 young or 23 percent of all young counted between air and ground surveys, which were separated by an average of four days. Preliminary results show there are important differences in the results of air and ground surveys. Dual surveys were conducted again in the spring of 2003 to further identify these differences.

During ground surveys biologists collected unhatched eggs from ferruginous hawk nests to establish baseline information on contaminants including lead and pesticides. Contaminants have the potential to reduce productivity, or the average number of young produced in nests; they affect embryo survival or reduce eggshell thickness resulting in cracked eggs. Two previous studies in western states over 20 years ago found few contaminants in ferruginous hawk eggs. However, ferruginous hawks in some areas of eastern Washington feed on or near agricultural land where pesticides are sprayed, eating pocket gophers that live along edges of irrigated fields. Egg samples were collected in both 2002 and 2003 and are currently being analyzed.

Ferruginous hawks will nest on artificial platforms, especially in areas where there are no other elevated structures for nesting. The platforms provide a safe haven from ground predators. On the Hanford Reservation in Benton County near the Tri-Cities, transmission towers on a line built in the 1970s have provided nesting habitat for nine pairs of ferruginous hawks that have been quite productive for several years. Unfortunately, each summer one or two broods of these nestlings die because, as the young grow and become active, their nests fall apart and they fall to the ground before fledging. In winter 2003, in cooperation with the Bonneville Power Administration, biologists erected two custom nest platforms on transmission towers and built mock nests to entice nearby hawks to breed. Platforms were built by two students from Prescott High School in Walla Walla. Both pairs of hawks were near the nests early in 2003, and biologists are keeping their fingers crossed for positive results. Success may result in installation of additional platforms.

Future ferruginous hawk studies may include video monitoring of nests to assess reasons for nestling mortality during the later stages of nesting. Based on surveys and nest visits, it is not uncommon to find 24- to 26-day-old young that have died of apparent starvation. Recent years of drought in eastern Washington may be responsible for reducing small mammals upon which the hawks feed. Reduced prey may result in reduced attendance of eggs and young by adults, as well as reduced food delivered to nests.

Related Research

Migration and Winter Ranges of Ferruginous Hawks From Washington Progress Report -- January 2001
http://wdfw.wa.gov/wlm/research/papers/ferrhawk/

Management Recommendations for Washington's Priority Species, Volume IV: Birds - Ferruginous Hawk
http://wdfw.wa.gov/hab/phs/vol4/feruge.htm

WDFW Raptor Research: Ferruginous Hawk
http://wdfw.wa.gov/wlm/research/raptor/ferrhawk/ferrhawk.htm

Related Web Links

Ferruginous Hawk Field Study: Woodland Park Zoo
http://www.zoo.org/educate/hawks.html

Hawks Aloft
http://www.hawksaloft.org/

Birds of Prey - Buteo.Com
http://www.buteo.com/

Author Biography