A hummingbird gorges on insects and spiders — almost doubling its weight — before undertaking the 500-mile flight across the Gulf. Layers of fat build up on the bird’s back, belly, and throat. During migration, fat reserves get depleted. A hummer can lose up to half its weight and must find food to refuel as soon as it reaches land. A hummingbird bander uses a straw to blow away feathers and check for fat reserves on this bird.