The talons were out and flashing like knives during a rough and tumble mating season brawl between two male peregrine falcons on a 40th-floor ledge of the Cathedral of Learning in Oakland.
The Sunday morning battle high above the University of Pittsburgh campus lasted for 20 minutes and was recorded by a Web camera set up to monitor the nest of Dorothy and Erie, the falcons that have made the Cathedral their home since 2002.
Images of the fight show a third falcon, a young male, invading the nest where Dorothy is getting ready to lay her eggs. Erie is shown wrestling with the interloper as they lock talons and strike at each others chests with their beaks. Erie eventually turns the other bird over on its back before both birds tumble over the edge of ledge.
"It was scary because if it got really bad, one of them was going to die," said Kate St. John, a Western Pennsylvania Conservancy volunteer who watches the nest and took still photos of the Web camera feed. "The fact that they fell off the nest probably saved a life."
Peregrine falcons often fight during the March nesting season, when the females are fertile and younger birds cruise for older falcons to overthrow, said Anthony Bledsoe, a lecturer of biological sciences at Pitt's School of Arts and Sciences. Rarely are the often fatal battles seen, let alone captured on film.
Mr. Bledsoe said the footage could help falcon researchers understand how a new male, or sometimes a female, takes charge over an existing nest.
The Sunday fight was not the first defense of his territory for Erie and probably won't be the last. A couple of years after the nest was established on the Cathedral, a two-hour battle was joined with two other falcons before the intruders were driven off.
And among the 18 fledglings sired by Erie was a bird named Louie that in 2003 successfully challenged and decapitated the longtime male peregrine on the Gulf Tower Downtown.
Charles Beir, the Conservancy's natural heritage director, said the territorial fights show that the peregrine population is rebounding, possibly to the point where it is exceeding available nesting sites.
"We've all been focused on seeing the peregrine population rebound and get out of the endangered zone, but now it may be constrained by habitat," Mr. Beir said. "There's plenty of food around, but finding a nest site in a high place where there's plenty of room, and it's not disturbed and the eggs are protected from predators isn't that easy. Those sites are limited and the best ones are taken."
He said discussions have begun about finding another nesting site in the area. Peregrines are territorial and the birds will not tolerate other nesting pairs within their space. The distance from the Cathedral to the Gulf Tower is about 21/4 miles, "a good minimum distance," Mr. Beir said.
He said the Gulf Tower peregrines have already produced three eggs this spring with at least one more to be laid. The eggs hatch 30 days after the last egg in the clutch is laid.
To see a slide show of Sunday's fight, visit the Conservancy Web site at www.paconserve.org/rc/peregrine-07-images/index.html.
Don Hopey can be reached at firstname.lastname@example.org or 412-263-1983.