Ospreys have generally high-pitched, whistling calls compared to other many bird species. However, the exact rate of chirps and pitch of each chirp are dependent on situation and environment. Ospreys call for a variety of reasons such as to attract a mate, to warn of a threat, and to communicate amongst eachother and with young.
Osprey often call with simple peeps during flight or to communicate (Downloadable mp3 below). The video below shows a male non-urgently communicating. He may be trying to correspond with another osprey or making his territory known.
When an osprey feels threatened from an unknown osprey or another species, usually because they are getting unconformably close to the osprey's territory, their call intensifies to a fast paced squeal (Downloadable mp3 below). The video below shows a female in nest who perceives a threat, most likely intensified by her young in the nest, either from videographer or another factor.
As seen in the photograph below, during an osprey's alarm call, the chicks in the nest will often immediately lie down and attempt to keep very still. This is an instinctive response to the alarm, known as “thanatosis” or playing dead. The chicks will usually remain in this state until the alarm call stops. However, the young are usually curious as to the reason for the alarm call and sometimes poke their heads up to check out the scene.
The video below shows male performing a "sky dance" in which he flies up sharply, often with a fish or nest building material, hoovers with tail fanned in air then dives down to varying distances followed by quick ascent to repeat the process. The sky dance is a territorial display as well as a spectacle for potential mates. In the video, the female is already present and is making sounds of different pitch throughout the performance, most likely as encouragement.