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Posted 3/19/2014

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By Randy Gon
South Pacific Division Public Affairs

It sounds like something out of “Jurassic Park”, but kleptoparasitism by bald eagles or stealing prey from other species, has never been witnessed in combination with cooperative hunting, until now.

In a recently published article in “The Journal of Raptor Research” http://www.bioone.org/doi/full/10.3356/JRR-13-45.1, U.S. Army Corps of Engineers (USACE) members working at Lake Sonoma in Geyserville, Calif. in 2013 describe what is considered the first officially documented occurrence of a pair of bald eagles stealing a fish from an Osprey employing cooperation as a tactic.

Bald eagles are well known scavengers and very opportunistic in that they never pass up an opportunity for food according to Wade Eakle, U.S. Army Corps of Engineers, South Pacific Division ecologist. “They don’t hesitate to steal food from each other; known as ‘piracy’ and engaging in ‘kleptoparasitism’ or stealing from another species,” said Eakle. “Rarely have humans seen them cooperating to hunt as a pair, but this was the first time we witnessed them actually cooperating to steal, from another species.” Hence the hybrid term, “cooperative kleptoparasitism”.

Once the two eagles forced the Osprey to drop the trout on the ground, the larger female immediately seized the fish and flew away.

USACE Park Rangers discovered the pair’s active nest in 2001 and have monitored the bald eagles and their offspring ever since. The two raptors benefit from a healthy fish population in the lake and mild winter habitat.

Prior to the completed construction of Warm Springs Dam by USACE in 1983, “there are no records that we were able to find of bald eagles or Osprey nesting in the Dry Creek watershed,” said Eakle. “This is a good example of ‘if you build it, they will come.’ The creation of the lake has created a suitable habitat.”

Monitoring species at the top of the food chain can provide a good picture of the whole ecological health of a region and other species that may be on lower tropic levels, according to Eakle.  

USACE actively participates in recovery efforts for the bald eagle and the conservation of many other endangered or threatened species at USACE projects.

Accept for the American Southwest, the raptor's population has trended upward nationally according to results from a 25-year analysis of bald eagle counts from 1986 through 2010. The Midwinter Bald Eagle Survey, coordinated by the U.S. Army Corps of Engineers, is an annual event where several hundred individuals count eagles along standard, non-overlapping survey routes. For more details on the history of the count, see http://ocid.nacse.org/nbii/eagles/

To learn more about areas seeing a growth in population of the America's national mascot, visit the USA Today story here: http://usat.ly/1cZpOWN