What’s not to love about the recovery of America’s iconic bald eagle populations? Well, plenty, if you happen to be an endangered seabird or fox.
According to a new study out in PNAS, the growing population of Haliaeetus leucocephalus on California’s Channel Islands could put unwanted pressure on birds and island foxes (Urocyon littoralis).
Seth Newsome, lead author and a researcher at the University of Wyoming, Laramie, and his colleagues reconstructed the diets of past eagles using isotope analysis of bones and feathers from a historic nesting site.
In the millennia before they were wiped out from the islands by use of the pesticide DDT, the eagles fed mainly seabird carion. From around 1850 to 1950 they switched to eating sheep. However, the sheep have now been removed and the seabird numbers are not what they were, so the re-established eagle population may be about to embark on a change of diet.
“Because there are no sheep on the islands today and the seabird populations are diminished, we think that the introduced eagles could scavenge seal or seal lion carrion, exert predation pressure on a threatened but recovering local seabird population, or even prey on the endangered island fox,” says Newsome (press release from Carnegie Institution, his alma mater).
“Each of these sources has its challenges for wildlife managers. Several studies have shown that seals and sea lions are contaminated with pollutants, and a growing bald eagle population could potentially exert significant predation pressure on the fragile fox and seabird populations.”
But Peter Sharpe, of the Institute for Wildlife Studies in Avalon, California, told National Geographic that while pollutant levels in marine mammals are a worry, the eagles will feed on whatever animal is most numerous, so rare beasties are unlikely to be particularly targeted.
Image: bald eagle on Santa Catalina Island in 2008 / Peter Sharpe, Institute for Wildlife Studies.