At just 28 years of age, Bertie Gregory has emerged as a top wildlife filmmaker. But his career didn’t come out of nowhere: years before he was traveling the world, documenting everything from jaguars to polar bears, he was already capturing footage of the nature that was nearest to him. “As a 13- or 14-year-old, instead of going to football practice or whatever, I would sneak off and jump in the river and try to get close to swans,” said Gregory. “That was a bit odd.” He soon discovered that his footage could prompt other people to care about what he saw. After winning a competition for young wildlife photographers, he had an opportunity to work with a legend in the field, big cat photographer Steve Winter, on a shoot for National Geographic. “It was one of those Willy Wonka, right place-right time moments,” Gregory explained. “He offered me a …