A newcomer has arrived at the White House, provoking fierce turf battles and carrying out more than one lethal confrontation in broad daylight. But this is no political power struggle; it is nature. A red-tailed hawk is prowling the lush White House lawn and perching just above the second-story window of President Obama’s East Wing residence, lured by a booming population of gray squirrels and undeterred by the harassment of smaller birds that have tried in vain to displace it. The hawk has captivated those who frequent the White House, from visitors to journalists to staff members, some of them...