Why do great frigate birds commit robbery? Well, the motive has a lot to do with their feathers and feet. Great frigate birds lack sufficient preening oil on their feathers to make the outfit waterproof. In addition, their web-less feet are too short and too small -- virtually useless for swimming or for taking off from the water. Since great frigate birds cannot afford to get their wings wet, they need to find alternative means to gather food. Fortunately, they are superb flyers. Hence, they use their gifted skill to intimidate their fellow seabirds, compelling them to throw up the catch in their throats. Once their victims let go of their hard-won food, great frigate birds swoop down to collect their prize.