After decades of scrambling on the underside of California bridges to pluck endangered peregrine falcon chicks from ill-placed nests, inseminating female birds and releasing captive-raised fledglings, wildlife biologists have been so successful in bringing back the powerful raptors that they now threaten Southern California's endangered shorebird breeding sites.
As a result, the U.S. Fish and Wildlife Service says it will no longer permit peregrine chick rescues from Bay Area bridges, a move that they concede will likely lead to fluffy chicks tumbling into the water below and drowning next spring.
"It's a paradox," said Marie Strassburger, chief of the federal agency's division of migratory birds and habitat in Sacramento. "Yes, chicks are cute. I won't deny that for a second."
But she said the loss of chicks that fledge from the nest too early is a natural part of life.
Peregrines nest high on cliffs, trees, buildings and bridges because they hunt by diving, at speeds topping 200 mph, at wild birds they like to eat.
When fledging, young peregrines fly well and land poorly. On cliffs, there are plenty of easy spots for a crash landing. On buildings, they scramble back onto window sills or ledges when their first flights go awry, or they hit the sidewalk and can be carried back to their nests.
But on bridges, with smooth steel or concrete supports, chicks find no perch and often just hit the water.
"We see the loss of a chick by natural causes as an educational moment, as this happens in nature all the time," Strassburger said. "The peregrine falcons on the bridges in the Bay Area just happen to be in a very visible spot, so the public is more aware of it."
The recovery of peregrines, and now their potential threat to other species, underscores the fragile balance of nature that biologists have struggled with in recent years.
Saving bighorn sheep in Yosemite National Park meant hunting protected mountain lions; reintroducing gray wolves in the Rockies brought a backlash when ranchers complained they were killing livestock; and bringing golden eagle populations back on California's Channel Islands nearly devastated the island fox, one of the world's smallest canines.