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," said Strassburger. "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.
The decision to stop saving peregrine chicks is strictly local, says U.S. Fish and Wildlife Service migratory bird specialist Alicia King at their Arlington, Virginia headquarters. She said she didn't know of any other place where this was happening, and there's no national position. She noted that in many communities the peregrines are beloved and their chicks are treasured.
"But birds sometimes nest in places that are not the best places for them to nest, and while it's hard to watch, sometimes nature has to take its course," she said.
No one is suggesting that the drowning deaths of a dozen or so chicks taken from Bay Area bridges is going to tip the entire species back into a risky situation. Nor is anyone suggesting that allowing a few birds to be saved would actually damage the dwindling population of at-risk shorebirds hundreds of miles south.
But there are two very different sentiments about how to proceed.
For wildlife biologist Glenn Stewart, who directs the University of California, Santa Cruz Predatory Bird Research Group, allowing baby birds to topple into the choppy, frigid San Francisco Bay and drown is an indefensible approach.
"Yes, peregrines are recovered, but should we let this sometimes vigorously protected and sometimes left-to-drown resource be squandered?" said Stewart, who wrote "Eye to Eye with Eagles Hawks and Falcons" published earlier this year.
And conversely, the U.S. Fish and Wildlife Service says it makes no sense to permit chick rescues in one part of California when they are busy having to trap and move them away from threatened species habitat elsewhere in the state.
Thus Stewart was informed this year, as he applied for his annual springtime permit to remove chicks, that this would be his last.
Peregrine falcons were listed under the Endangered Species Act in 1973; at the time, there were just 11 of the birds known to be living in California and about 100 nationwide. Over the next three decades, independent biologists working with federal and state researchers successfully rescued the species, largely by releasing more than 4,000 captive-raised peregrines in 28 states, but also through meticulous conservation, ranging from chick rescues to incubating and hatching eggs.
Today there are around 2,000 in California, and as many as 10,000 more across the U.S., where they've become wildly popular, thanks to live, streaming webcams above their nests and annual media accounts of their rescues from New York to Portland, Ore. KathyQ, a peregrine nesting in downtown Indianapolis, has her own Facebook page.
Falcon fans from around the world log in regularly to watch peregrines perched in their nests below bridges or on ledges of tall buildings, commenting excitedly when the 1 ounce chicks hatch, and weeks later, gathering in crowds below as they attempt their first flights. In many places, the birds even have names: Diamond Lil and her mate Dapper Dan have nested on the 33rd floor of the Pacific Gas and Electric Co. building in downtown San Francisco since 2008.
This spring was poignant for Stewart, who began working with peregrines in the 1970s, at a time most thought they would go extinct. For what may well be his last time, in April he plucked four soccer ball-sized chicks from a nest below the Richardson Bay Bridge that spans an inlet in the north end of the San Francisco Bay.
"They look soft and fuzzy, but they have a very dense coat of down, their feet are heavy and they bite," he said.
The chicks were taken to the University of California, Davis, veterinary school where they were cared for before being moved to a building top where they were released. Costs are covered by the research groups.
Bill Heinrich who directs Interpretive Center at the Boise, Idaho-based Peregrine Fund said after so much effort, it makes no sense to halt rescues, especially since taxpayers no longer cover any of the costs. He hadn't heard of similar decisions anywhere else, and the U.S. Fish and Wildlife Service has issued no national directives.
"These are the most beautiful birds of prey in the world and also the fastest," he said. "We spent millions of dollars and decades bringing them back from the brink of extinction. I can understand why they don't want to pay for their rescues, but it makes no sense not to allow it."
But in recent years, Fish and Wildlife official Strassburger said, biologists have had to move peregrines from several Southern California breeding areas for endangered gulls called California least terns and threatened Western snowy plovers, sensitive little shorebirds that nest on beaches.
"It's difficult to think that sometimes we end up in that place, where you recover an endangered predator to the point they become a threat," said National Wildlife Federation senior scientist Douglas Inkley in Reston, Virginia. "Predators must be part of the picture. The long term answer is that we need to recover those prey species so neither population is at risk."
Biologist Stewart understands the long-term goals, but says the decision to ban him from saving a handful of chicks from Bay Area bridges next year is "dumbfounding."