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Wednesday, February 12, 2014

Wildlife Wednesday: Golden Eagles


A golden eagle perched in Monte Field. Photo courtesy of  Laura Pavliscak

Those of you who have spent some time on Tejon Ranch know that this property is replete with golden eagles (Aquila chrysaetos). Indeed, it is not uncommon to see 5, 10, or even 15 eagles in a given day! Although golden eagles can be found throughout the West, Tejon Ranch is a particularly reliable place to see them. Why is that? What about the ranch is so conducive to their survival? While we do not have the data necessary to reach a definitive answer for these questions, if we look a little bit at golden eagle life history, it becomes clear that Tejon Ranch meets many of their necessary requirements.



HABITAT
One of the biggest factors that has contributed – other than the poisoning that led to their protection in 1962 – to the decline of golden eagles over the last century is the destruction of their habitat. These raptors prefer two broad categories of habitat for their daily activities- canyons and rugged mountains for roosting and breeding, and flat, open country for feeding. Contrary to Youtube accounts, golden eagles mainly prefer small mammals for their prey. According the Cornell Lab Birds of North America Online, such food consists of 80-90% of their diet (Kochert, Steenhof, Mcintyre, and Craig 2002). A quick trip to the San Joaquin or Antelope Valley grasslands on Tejon Ranch will confirm that California ground squirrels (Otospermophilus beecheyi) abound. While some may consider ground squirrels to be a scourge on grasslands, we like to think of them as golden eagle (and red-tail hawk, wintering ferruginous hawk, coyote, and bobcat) food.
A closer inspection reveals that it is mealtime. Photo courtesy of Laura Pavliscak

NESTING
As mentioned above, golden eagles prefer to set up their nests in trees atop rugged canyons. A look at the hillshade relief map of the ranch (below) reveals a plethora of such habitat. In fact, it appears that golden eagles create pair bonds for life (or at least a very long time) and prefer to use the same nest over and over again. Raptor researchers have told Conservancy staff not to consider each nest individually, but as neighborhoods, where established pairs have established nests and home ranges for as long as decades!  Golden eagles also maintain alternate nest sites in the vicinity of the active nest.
One can imagine how the history of land use changes and habitat destruction in southern California have made the vast Tejon Ranch a “refuge” for these birds. Conversely, the abundance of golden eagles on Tejon Ranch may indicate what their numbers should be like throughout the region. These are the kinds of questions we like to ponder around here.
This hillshade map of Tejon Ranch highlights the property's mix of rugged terrain and open grassland



WINTERING
Although golden eagles tend not to travel the inter-continental migration routes as raptors like the Swainson’s hawk (Buteo swainsoni), some do migrate over shorter distances (while others are thought to maintain their territories through the year). Tejon’s resident eagles are likely joined by wintering birds from northern latitudes, the Conservancy’s Christmas Bird Counts have documented some of the highest golden eagle numbers of any count in North America!

Tejon Ranch is an incredible place to see golden eagles. As we continue to observe and learn about these amazing creatures, we are particularly interested in finding out where they nest on the ranch and how neighboring wind farms affect their populations.
To find out more about golden eagles, including identification tips and life history, check out these great resources

http://www.allaboutbirds.org/guide/golden_eagle/id

http://animals.nationalgeographic.com/animals/birds/golden-eagle/

http://peregrinefund.org/explore-raptors-species/Golden_Eagle#sthash.RGfN4FQn.dpbs

REFERENCES
Kochert, M. N., K. Steenhof, C. L. Mcintyre and E. H. Craig. 2002. Golden Eagle (Aquila chrysaetos), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/684
doi:10.2173/bna.684