Stephen Vedder used to enjoy peaceful lakeside runs near his Marlborough, Mass., home. This spring, after years of coexisting with an ornery neighbor, those tranquil outings came to an end. It started with a whack to the back of the head. Next, Mr. Vedder was divebombed. Then clawed. The culprit was a red-winged blackbird. More than 250 million of the birds live across North and Central America, and this summer some are feeling extra aggressive toward human neighbors—driving them to change walking routes, wear protective headgear or furiously wave arms above their heads as they jog. “You talk to people...