This bird is also known as the Swallowed-tailed Hawk, it is a raptor. There are a number of birds that qualify to be among the most difficult to photograph. Skimmers and Peregrine Falcons come to mind.
However imagine a raptor that lives by snatching Dragonflies, with their claws, in mid flight. They also skim tree tops to grab snakes, lizards, even small birds. Enlarge the image below and you can see an insect in his claws.
They live primarily in South America, Peru and Argentina have decent sized populations. Around 3% breed in Florida and South Carolina each year. Maybe more now, the population is growing. They still must winter in South America.
Above is a Kite eating in flight. They also fly low over water, dipping their beak as they fly by. Adult Kites rarely eat when not flying.
Click any image to view full size.
Kites can swoop, turn and change direction in an instant, it’s how the catch small prey flying by.
I think they might be the biggest challenge for in flight photography.