Seventeen northwestern pond turtle hatchlings, each about the size of a walnut, are making themselves at home at the Oregon Zoo this summer.  Zoo conservationists are “head-starting” the endangered reptiles, caring for them until next spring when they’re big enough to have a fighting chance in the wild.  Last month, the turtle hatchlings were retrieved from sites in the Columbia Gorge, transporting them to the zoo conservation lab.  With heat lamps and plentiful food, the turtles experience summer year-round so they don’t go into hibernation. Once they reach about 50 gram, they are returned to their ponds and monitored for safety.  Guests can watch the tiny turtles grow at the conservation lab inside the zoo’s Nature Exploration Station.  The American bullfrog, native to the eastern United States but considered invasive here, can tip the scales at more than a pound and has been driving pond turtles and a host of other small, vulnerable aquatic species to the brink of extinction.  The northwestern pond turtle, also known as the western pond turtle, is listed as an endangered species in Washington and a sensitive species in Oregon.