At this time of year the numbers and variety of birds appearing in your yard can actually change every morning as many small songbirds migrate through the night. Species such as warblers, vireos, orioles, grosbeaks, tanagers, buntings and sparrows all migrate overnight.
Some reasons are to avoid the heat of the day, windy conditions, and predators. Many species use the stars in the night sky to guide them, and there is some evidence that, perhaps, magnetic fields at the poles help guide them.
Other large-winged birds such as hawks, eagles, vultures and even pelicans migrate during the day, as they rely on thermals and updrafts to fuel their flight. They are efficient flyers and use very little energy during migration, allowing the wind to fuel their flight.
When you think about migration facts, it is astonishing to learn of the amazing feat that many birds accomplish twice each year as they move between their summer and winter range and back again.