Talking about food for thought, there is this interesting story of the origins of the Taittiriya Yajurveda. Wikipedia states that Sage Yajnavalkya was the pupil of sage Vaishampayana. According to Wikipedia “Once, Vaishampayana got angry with Yajnavalkya as the latter displayed too much sense of pride in being abler than other students. The angry teacher asked his pupil Yajnavalkya to give back all the knowledge of Yajurveda he taught him. As per the demands of his Guru, Yajnavalkya vomited all the knowledge that he acquired from his teacher in form of eaten food. Other disciples of Vaishampayana took the form of partridge birds and consumed the vomited stuff because it was knowledge and they were very eager to receive the same. The Sanskrit name for partridge is ‘Tittiri’. As the Tittiri (partridge) birds ate this Veda, it is thenceforth called the Taittiriya Yajurveda. It is also known as Krishna Yajurveda or Black-Yajurveda on account of it being a vomited substance. The Taittiriya Samhita thus belongs to this Yajurveda”. [en.wikipedia.org]
I was looking for some Vedic scriptural references that may make some sense of how the bird song mantra (for the lack of a better term) developed.
I have not found an exact match, but the closest reference that I have found thus far is in the Devi-Bhagavata Purana, popularly known as Shrimad Devi Bhagvatam;
“21-42. Vyâsa said :-- O King! That Taksâ became very glad when he heard thus from Indra and struck off the heads of the Muni with his very strong axe. O powerful King! When the three heads, thus severed, fell to the ground, thousands and thousands of birds came out of those heads in quick succession. The three groups of birds Kalavinkas, Tittiris and Kapinjalas came out very rapidly from the three heads in due succession. The Kapinjala birds came out of that mouth that used to chant the Vedas and used to drink Soma; the Tittiri birds came out of that mouth that used to see all the quarters as if it drank them; and the Kalavinka birds came out of that face that used to drink wine. Indra became very glad to see the birds thus coming out of his mouths and went back at once to his Heavens…..”.
Note that the of Kapinjala birds are frequently rendered as "francoline partridges" or "heathcock" by translators. The Tittiris are rendered as the “common partridge”, while the Kalavinkas are most commonly translated as an “Indian cuckoo” or “sparrow”, but this bird is identified more precisely by Indian ornithologists as a type of “spotted weaver”.
The Devi-Bhagavata Purana clearly gives an association of the Kapinjala birds to the Vedas. As a matter of fact, the earliest reference to the Kapinjala birds is in the Rig Veda, and is identified not only as a bird of good omen, but as an aspect of Indra. Unfortunately, the bird song mantra chanted in Kerala was associated with Agni, not Indra. So, my research into this is off the mark.
The use of birds as omens is indicated in the Rig Veda as per the two verses involving the Kapinjala birds.
However, there seems to be another text called the Sakuna Sastra that talks about bird omens, amongst other omens. This text is attributed to the Sage Garga. I have not seen a copy of this personally, although it would be interesting to view it.
On a completely different note, there are these extraordinary bird sounds from the lyre bird, with David Attenborough narrating. Check out the video.