The river that flows between the United States and Mexico, constituting half of the border between the countries, has two names: To the north, residents call the watery divide the Rio Grande, and to its south, the Río Bravo. But in the new documentary “Birders,” released by Netflix on September 25, the distinctions and divisions marked by this border fade into the background. Instead, the river is portrayed as a convening point for migratory birds and a place of unity and common ground for those who love them. While the initial goal of the film was to tell a story about the U.S.-Mexico relationship, director Otilia Padua decided to approach the topic in an unconventional way. Instead of emphasizing the politics, she sets the main focus on birds and their natural habitats in an ecologically significant region, where migratory flyways and climate zones converge. “The way you create interest is by making people fall in love with things, and I think it’s really easy...