One minute it's a pigeon picking debris off the sidewalk, the next it's a feathery splatter of red surrounded by a startled onlookers.

Peregrine falcon at work.

Virtually extinct 50 years ago, the fastest animal on Earth was in a population tailspin, and by 1975 the North American peregrine population was 324 nesting pairs. The birds were protected and restocked -- some in Pittsburgh and other urban centers. The program worked so well peregrine falcons have become one Pennsylvania's most visible wildlife success stories. Their population has grown, as city pigeons will attest, and some people want the bird taken off the state's endangered species list.

Not so fast, says Game Commission executive director Carl Roe.

"There are 29 active peregrine nests in 14 counties and they have drawn great interest. They truly are an attractive, charismatic species and their recovery from the brink of extinction has everyone pulling for them," he said, in a written statement. "However, peregrines need to occupy more of their historic nesting sites before we can remove them from the state endangered species list.".

The urbanization of the falcons has succeeded, but Game Commission biologists are concerned that more young birds haven't moved out of town and nested in their traditional habitat: cliff faces. It's believed that the species' population stability is significantly increased when they reside on cliffs, rather than exposing themselves the "unique mortalities" associated with city living.

"This year's 29 nests are great news when you compare them to the 40 or more sites they used annually in the early 1900s, because it represents substantial progress," said Roe. "But only four of the 2010 nests are on cliffs, which was where almost all of their nests were located prior to their dramatic decline after World War II. So we're looking for peregrines to occupy more river bluffs and precipices. Then we'll relax a little more."

Art McMorris of the Game Commission's peregrine management program calls the birds' comeback "substantial."

'But we still are sorely lacking in cliff recolonization," he said. "The reasons for this are undoubtedly complex, and not totally understood. But they may be related to predation and competition for nesting areas from great horned owls, overgrown vegetation on historic nesting ledges, and attractions in urban areas, such as the abundance of prey species."

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.