As slight and zippy as its titular bird, “The Hummingbirds´ Gift: Wonder, Beauty and Renewal on Wings” is a fine way to spend an hour or so this spring. Sy Montgomery fans may have already read it as a chapter in “Birdology” (2010), but if you´re coming to it cold, the rescue and rehabilitation of Maya and Zuni will warm your heart while making you into an armchair ornithologist. The true story takes place in the summer of 2008, when Montgomery travels from Manchester, New Hampshire, to San Francisco to meet the tale´s hero, Brenda Sherburn. “A 5-foot-3 powerhouse in dark bangs and a pageboy,” Sherburn dedicated part of her life to hummingbird rehabilitation. That´s right – those tiny, buzzing birds, the smallest of which weighs just a gram and the largest, classified as an Andean “giant,” measures only 8 inches long – are the most vulnerable birds in the sky and there are people like Brenda who take in abandoned or injured babies in an attempt to nurture them back into the wild.