It's one of the bird world's most epic journeys. After dodging predators and fiercely guarding their nests in the North American Arctic tundra, the American golden-plover begins one of the longest migrations of any shorebird. The journey to their wintering grounds, primarily in the pampas grasslands of Brazil, Argentina, and Uruguay, is thousands of kilometers. Many don't survive, but scientist don't know exactly why, or where they get lost along the way. An estimated 75 percent of bird and mammal species (including the golden-plover) and all insects are too small to track over long distances, because the tags scientists typically use are too large and heavy for these smaller species to carry. The majority of the weight comes from the battery, but the tag must also have room to hold a transmitter strong enough to communicate with a satellite that can receive and relay information back to scientists on the ground.

Read more at motherboard.vice.com