Do you have any literature on sound changes involving ejective consonants? Specifically ejective consonants changing into something else?
I don't know of any general surveys, but several individual cases are of course found in literature in more detail. It would be worthwhile to have some compiled data on this though! For a start I'll collect some examples in this post.
The best-described case might be Semitic, where any handbook (or even just the Wikipedia article) will inform you about *kʼ > q, tsʼ > (t)s etc. being attested in Arabic / Aramaic / Hebrew. Offhand I don't know if there is a particular locus classicus on the issue of reconstructing ejectives for Proto-Semitic, though.
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Cushitic, which I've been recently talking about, has open questions remaining especially in what exactly to reconstruct for various correspondences involving ejective affricates, but at least the development of the ejective stops seems to be well-established. Going first mainly per Sasse (1979), The Consonant Phonemes of Proto-East Cushitic, Afroasiatic Linguistics 7/1, three developments into something else appear across East Cushitic for *tʼ:
*tʼ > /ɗ/ (alveolar implosive): Oromo, Boni, Arboroid (Arbore, Daasenech, Elmolo), Dullay, Yaaku and, at least word-internally, Highland East Cushitic.
*tʼ >> /ᶑ/ (retroflex implosive): Konsoid (Konso, Dirasha a.k.a. Gidole, Bussa). (As per Tesfaye 2020, The Comparative Phonology of Konsoid, Macrolinguistics 8/2. Some other descriptions give these too as alveolar /ɗ/.)
*tʼ >>> /ɖ/ (retroflex voiced plosive): Saho–Afar, Somali, Rendille.
Presumably these all happen along a common path *tʼ > ⁽*⁾ɗ > ⁽*⁾ᶑ > ɖ. Note though that Sasse reconstructs *ɗ and not *tʼ — but comparison with the case of *kʼ, the cognates elsewhere in Cushitic, and /tʼ/ in Dahalo and word-initially in Highland East Cushitic I think all point to *tʼ in the last common ancestor of East Cushitic. (As per other literature, I don't think East Cushitic is necessarily a valid subgroup and so this last common ancestor may also be ancestral to some of the other branches of Cushitic.)
For *kʼ there is a wide variety of secondary reflexes:
Saho–Afar: *kʼ > /k/ ~ /ʔ/ ~ zero (unclear conditions).
Konso: *kʼ > /ʛ/ (no change in Bussa & Dirasha).
Daasenech: *kʼ > /ɠ/ word-initially, else > /ʔ/.
Elmolo: *kʼ > zero word-initially, else > /ɠ/.
Bayso: *kʼ > zero.
Somali: *kʼ > /q/, which varies as [q], [ɢ] etc.; merges in Southern Somali into /x/). Before front vowels, > /dʒ/.
Rendille: *kʼ > /x/.
Boni: *kʼ > /ʔ/.
though some of them again could be grouped along common pathways like *kʼ > *q > *χ > x, *kʼ > *ʔ > zero.
*čʼ > /ʄ/ happens at minimum in Konso (corresponds to /tʃʼ/ in Bussa & Dirasha). Proposed developments of a type *čʼ >> /ɗ/ in some other languages could go thru a merger with *tʼ first of all.
No East Cushitic *pʼ seems to be reconstructible, but narrower groups show *pʼ > /ɓ/ in Konsoid (corresponds to Oromo /pʼ/) and maybe *pʼ > /ʔ/ in Sidaamo (corresponds to Gedeo /pʼ/; mainly in loans from Oromo).
There is also an unpublished PhD from University of California at LA: Linda Arvanites (1991), The Glottalic Phonemes of Proto-Eastern Cushitic. I would be interested if someone else has access to this (edit: has been procured, thank you!)
Secondary developments of *tʼ and *kʼ in the rest of Cushitic, per Ehret (1987), Proto-Cushitic Reconstruction, Sprache und Geschichte in Afrika 8 (he also reconstructs *pʼ *tsʼ *čʼ *tɬʼ, but I'm less trustful of their validity):
Beja: *tʼ > /s/, *kʼ > /k/.
Agaw: *tʼ > *ts (further > /ʃ/ in Bilin and Kemant), *kʼ > *q (further word-initially > /x/ in Xamtanga and Kemant, /ʁ/ in Awngi)
West Rift: *kʼ > *q (and *tʼ > *tsʼ).
(The tendency for assibilation of *tʼ is interesting; although plenty of Cushitic languages get rid of ejectives entirely, none seems to have a native sound change *tʼ > /t/.)
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The historical phonology of the largest Afrasian branch, Chadic, is much more of a work in progress, but I would trust at least the following points as noted e.g. by Russell Schuh (2017), A Chadic Cornucopia:
*tʼ > *ɗ perhaps already in Proto-Chadic (supposedly all Chadic languages have /ɗ/);
*kʼ > /ɠ/ in Tera (Central Chadic);
/tsʼ/ in Hausa and some other languages corresponds to /ʄ/ or /ʔʲ/ in some other West Chadic languages, not entirely clear though which side is more original.
Tera /ɠ/ alas does not seem to be discussed in detail in the Leiden University PhD thesis by Richard Gravina (2014), The phonology of Proto-Central Chadic; he e.g. asserts /ɠəɬ/ 'bone' to be an irregular development from *ɗiɬ, while Schuch takes it as a cognate of e.g. Hausa /kʼàʃī/ 'bone'. (Are there two etyma here, or might the other involved Central Chadic languages have *ɠ > /ɗ/?)
If Olga Stolbova (2016), Chadic Etymological Dictionary is to be trusted (I've not done any vetting of its quality) then Hausa /tsʼ/ is indeed already from Proto-Chadic *tsʼ, and elsewhere in Chadic often yields /s/, sometimes /ts/ or /h/. Her Proto-Chadic *kʼ mostly merges with /k/ when not surviving. (She also has an alleged *tʼ with no ejective reflexes anywhere, and alleged *čʼ and *tɬʼ which mostly fall together with *tsʼ, but also show some slightly divergent reflexes like /ʃ/, /ɬ/ respectively.)
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Moving on, a few other examples I'm aware of OTTOMH include the cases of word-medial voicing in several Koman languages and in some branches of Northeast Caucasian (Chechen and Ingush in Nakh; *pʼ in most Lezgic languages). Also in NEC, the Lezgic group shows complicated decay of geminate ejectives, broadly:
> plain voiceless geminate in Lezgian, Tabassaran & Agul (same also in Tindi within the Andic group);
> voiceless singleton (aspirated) in Kryz & Budux;
Rutul & Tsaxur show some of both of the previous depending on the consonant, as well as word-initially *tsʼː > /d/ and *tɬʼː > /g/ — probably by feeding into the more general shift *voiceless geminate > *unaspirated > voiced (which happens in almost all of Lezgic).
in Udi, both short and geminate ejectives > plain voiceless geminates (plus a few POA quirks like *qʼʷ > /pː/, even though *qʷ > /q/).
Again I don't know if this has been described in better detail anywhere in literature, this is pulled just from the overviews in the North Caucasian Etymological Dictionary plus some review of the etymological data by myself.
Ejectives in Kartvelian are mostly stable in manner of articulation, but there's a minor sound correspondence between Karto-Zan *cʼ₁ (probably = /tʃʼ/) versus Svan /h/ that newer sources like Heinz Fähnrich (2007), Kartwelisches Etymologisches Wörterbuch for some reason reconstruct as *tɬʼ or *tɬ. The Svan development would then probably go as *tɬ⁽ʼ⁾ > *ɬ > /h/, after original PKv *ɬ > /l/.
I do not know very much about the historical phonology of any American languages, including if there's anything interesting happening to ejectives there; if someone else around here does, please do tell!
