but I realise birds living in the rainforest have so many more chirps and calls to help them distinguish one another, and make them more noticeable from all the other noises in the surroundings. They also live in closer proximity which makes high frequency sound more effective since it can travel shorter distances. On the upside, this also allows them to make more unique varying sounds. Just like dolphins too which usually travel in packs so they benefit more using higher frequency sound to communicate, as they can form more words in rapid succession that way, compared to whales which use lower frequency sound more as it can travel farther
A friend of mine from Vietnam also once said that there are like 5 'a' pronounciations in Vietnamese (is this true?
@yaps66), and it seems apparent too that in most East to Southeast Asian countries vowels are prominent in their use to assemble words. Which is different from Arabic, Russian which use more consonants by comparison. Though there are few exceptions, such as the Mongolian with their nomadic lifestyle in the plains which require longer distance communication (check out throat singing)
Then there's also the body size factor, as it correspond to vocal pitch. Smaller body size often have higher pitch, and vice versa. I guess that may play a role too? There are so many variables! It's fascinating. I wonder which has the most influence