How a flying intelligent race communicate?

Question

Suppose that there is a race of intelligent being (actually not as intelligent as human but it isn't important) with great bat-like wings that let them fly. They are a bit smaller than the average human and have no arms (their arms are their wings). I've yet to decide if they have a beak or a human-like face, but that's not the point. I thought that living in the open spaces of the sky the should talk in a way that let them communicate as far as possible. Turns out that low frequencies travels farther than high frequencies so It seems that they should use very low noise (like the sound a horn makes), but that's strange if you think that birds communicate with very high screech. So the question are

1. what's the most efficient way for a animal to communicate as far as possible?
2. What system could that fantasy race use?
3. Why real birds sound is as it is?

Note that they have bat-like wings but they're not bat (they can see if it is helpful)

Edit: they should be able to hear human speach and produce sound that are understandable to human. They don't need to speak fluently a human language.

Answer 1

What's the most efficient way for a animal to communicate as far as possible?

Light and low frequency sound are the most efficient ways to communicate over long distances. Light has a range limited to these bat-people's ability to resolve visual details. (The OP makes no mention of eye size so this can't be answered.) Their body size will limit the lower frequencies that they can generate and thus limit the range of communication.

What system could that fantasy race use?

Using light and sound would be advantageous in different situations. In clear skies, communicating using light has range far exceeding sound. However, in cloudy skies or in forested areas, light will be obstructed so these flyers will need to fall back to sound.

Why real birds sound as they do?

Birds sound the way they do because of the physics of their body size. Large objects are able to make lower frequency sounds. Higher frequencies come from much smaller objects. As a visible example of this relationship, consider the pipe organ's pipes.

Answer 2

Do they have bat-like vocal systems? Bats are very good at producing very short bursts of sound, which is ideal for measuring the time it takes the sound to bounce back. A very time-dependent language would seem ideal for a bat-species. Variance in pitch and length between bursts of sound could identify the phonemes. Also, although low sound travels farther, most animals have ears that are attuned to pick up higher frequencies better. Humans have an optimal range between 2-5kHz while bats have an optimal range between 15-90kHz.

Answer 3

The reason why most birds seem relatively high-pitched to us is simply that most birds are small and the pitch of a sound is limited by the size of the organ that produces it. A flying creature the size of a human could produce lower-pitched sounds than a bird, and probably would.

There are creative methods some animals use to sound deeper and larger than they are (bullfrogs, for instance, use an inflated pouch to produce sounds). If your birdfolk have a similar inflatable throat pouch (or a Parasaurolophus-like crest, perhaps) they could use it to produce foghorn-like long-distance calls. However, it would be very difficult to speak with such an organ; mammalian vocal cords and the avian syrinx organ are specialized for making a more complex range of sounds and shifting their pitch quickly, but they are limited by the creature's size.

Some more information you may find useful: How Deep is your Cheep