aeiou

As I suspected, you can collect an alphabet consisting only of birds. And thus birds would be the secret language: in one town they would make the birds fly into the given direction (but drugging it or something) and people in that city would look at what those birds be, in what order they would fly they would use it as the teletype, let's try and reconstruct the alphabet:

𓄿     [a]
𓅂    [tjw]
𓅃    [ḥrw] [nṯr] [ḥry] [nb] (𓅄 [bjk] [ḥrw] [nṯr] [nb])
𓅐    [mwt] [nrt]
𓅓    [m] [jm] [n] (𓅔 (mm, m, nm))
𓅘    [nḥ] [nḥḥ]
𓅙    [ḏb]
𓅚    [rḫyt]
𓅛    [rḫyt]
𓅜    [ꜣḫ]
𓅞    [hb] [ḏḥwtj] [jqr]   [jp]
𓅟   [dšr]
𓅠   [gmj] [gm]
𓅡    [bꜣ]
𓅣 [bnw] (𓅤 [bꜥḥj])
𓅥   [sdꜣ] [dꜣ]
𓅦    [njw]
𓅧   [ꜥq]
𓅨 [mnt] [wr]
𓅪   [bjn]    [nḏs]
𓅬   [ꜣpd]    [gb]    [ḥtm]
𓅭 [sꜣ] [zꜣ]
𓅮 [pꜣ]   (𓅯 [ḫnj] ([pꜣ, qmꜣ] (?)))
𓅰 [wšꜣ] [ḏfꜣw])
𓅱    [w]
𓅷   [ṯꜣ] [ṯꜣty]
𓅸 [sš] 𓅹[sš] these could be the prototype of Ш along with 𓆷
𓅺 [rḫtj]
𓅻 [ḥꜣm] [ḥjm]
𓅼 [snm]

I selected those which have relatively simple phonetic representations,
and here's what I got:
𓄿     [a]
𓅓    [m] [jm] [n]
𓅙    [ḏb]
𓅜    [ꜣḫ]
𓅡    [bꜣ]
𓅥   [sdꜣ] [dꜣ]
𓅧   [ꜥq]
𓅭   [sꜣ]   [zꜣ]
𓅮   [pꜣ]
𓅱    [w]
𓅷   [ṯꜣ]   [ṯꜣty]
𓅸 [sš] 𓅹[sš] (𓆷 [š] and that's a super find)
and it was 12 of them (it's hard to say was it legitimately, coincidentally or supconsciously so. After all, if I included ḏb, why not include 𓅃[nb] 𓅘[nḥ]   𓅞[hb~jp]   𓅠[gm]   𓅨[wr]   𓅬[gb]?
With these it would be 18 of them
And if we go even purer, we find ourselves with only three: 𓄿𓅓𓅱

If 𓄿𓅱𓅓 were in this order ever recorded for Aum the great Om, then 𓅱 is indeed w, otherwise it's ш.

They had a whole set of *ꜣ syllables

𓅡    [bꜣ]    𓅥 [sdꜣ] [dꜣ]
               𓅜 [ꜣḫ]
𓅮   [pꜣ]    𓅷   [ṯꜣ]   these two have their wings apart.
                  𓅭   [sꜣ]   [zꜣ]

looking at pꜣ ṯꜣ line, it makes me wonder if dꜣ is little bꜣ

If this system was developed when AEIOU B T were all the letters, ..no, it could be of AMS period, thus 𓄿𓅱𓅓 were the three letters: it's hard to tell what is the breed of the lett bird, but to distinguish Aгромную from Middle from Small

But then how many of information can one transfer in this way? šma and sam and maš (as приём, сам, помогай) but a better thought told me that the transmittion could be easily disturbed by other birds.
And then it came to me (probably not for the first time) that birds would send letters. So members of the initiation into pigeon mail can transmit news over large distances. And they wouldn't reveal their method, so people would recognize it as a divination.

Pigeon post is the use of homing pigeons to carry messages. Pigeons are effective as messengers due to their natural homing abilities. The pigeons are transported to a destination in cages, where they are attached with messages, then the pigeon naturally flies back to its home where the recipient could read the message. They have been used in many places around the world. Pigeons have also been used to great effect in military situations, and are in this case referred to as war pigeons.
As a method of communication, it is likely as old as the ancient Persians, from whom the art of training the birds probably came. The Romans used pigeon messengers to aid their military over 2000 years ago. Frontinus said that Julius Caesar used pigeons as messengers in his conquest of Gaul.[2] The Greeks conveyed the names of the victors at the Olympic Games to their various cities by this means.[3]

И это объясняет такое широкое распространение голубятен в совке.

И они исчезли с появлением сотовой связи и интернета.

Hillman said that Medea was a queen of Babylon. I corrected him, but he didn't stand corrected, replying that "Take up your arguments with the Greek historians. She was queen of Babylon."
The next morning I saw him in a dream, in which he communicated that k is for unicorn.
And I saw that top right stroke to be its horn, and the way it looks into the end of the line makes it similar to b (which is reversed 𓃀) and whole day I was thinking of whether it is k for the knight, but K is for the king, N is for the knight (in chess)
Then I thought that it is for конь, and it may imitate the hooves clacking, maybe that's why it stands side by side with L. Как конь и лошадь. и кобыла (caballa caballo)
And by the evening it came to me: if h is for horse, k must be for unicorn.
and h is for horse, yet I do not remember in which writing system it is. Some ᛸ-like shape.

(but then in that shape the horn should be higher)
so, it's just a dream, what's the big deal

𐂖 (Mycenaean Greek for wine)
酒 (Japanese for sake)
The similarity is most likely coincidental, but it may help me memorize the two.