Rawlins Field Office

Ferruginous Hawk

close-up or young hawk

natural nest on rock pinnacle

artificial nest structure

nestlings on artificial nest

The western two-thirds of Carbon County in the Rawlins Field Office boasts one of the highest nesting densities of ferruginous hawks within Wyoming, and possibly throughout its historic range. The largest of North American hawks, "ferrugs," as they are affectionately known, are designated a sensitive species by Wyoming BLM. The listing means that for one reason or another, the species is rare, sensitive, or otherwise vulnerable, but not enough information exists about it to list it as threatened or endangered under the Endangered Species Act. Throughout the 1980s, the number of nesting ferruginous hawks declined substantially. The reasons for this decline are unknown, but it is suspected that both natural conditions (fluctuations in the numbers of their primary prey species) and increases in human disturbance were the major factors.

Ferruginous hawks nest on a variety of natural substrates, such as rock outcrops or pillars, high points on open ground, on juniper trees, and on large shrubs, such as serviceberry and sometimes sagebrush. However, because of the extreme vulnerability of most of these nest sites to predators, nesting mortality has probably always been high, and ferruginous hawk population levels have probably always been fairly low.

For the past 14 years, the Rawlins Field Office has been very actively involved in a cooperative project to improve the nesting habitat for the ferruginous hawk. Never abundant, probably because of its habit of nesting on the ground where it is much more easily preyed upon, the ferruginous hawk has benefitted from the placement of 92 artificial nest structures throughout the field office as mitigation for proposed and actual development projects. Of these 92 structures, 51 were erected by BLM, 30 by Energy International, Inc., four by the U.S. Air Force, three by the Wyoming Highway Department, two by Exxon, and two by Amoco. A cumulative total of 1,140 artificial nests have been available to the hawks over the past 14 years. Of these, 566 nests have been used, producing 1,307 young ferruginous hawks to fledgling age. It is hoped that the efforts of the BLM to improve nesting opportunities and nesting security for ferruginous hawks will help to keep this species from being listed as threatened or endangered.

In addition to the erection of the nesting structures and the monitoring work that has been done for the past ten years (1993-2002), several biologists have participated in a research project designed to determine a reliable method to census ferruginous hawks. The project involved banding young hawks on the nest in order to monitor their activities during subsequent nesting years. During the period, 1,278 young ferruginous hawks were banded with U.S. Fish & Wildlife Service metal leg bands, and colored plastic leg bands depending on which type of nest the bird hatched from. The biologists hope that these banded birds will be seen in the future in the central Wyoming area and will help researchers determine more of the biology of this rare and magnificent bird.