I am trying to find more information about the formation and pronunciation of the Greek noun Ἁμαδρυάς, taken into Latin as Hamadryas. L&S transcribes the second a of the Latin form with a macron: Hămādryas , ădis. However, I'm suspicious about whether this means the vowel is long*, or whether it just marks a long syllable before the cluster dr, which in poetry could freely be syllabified as d.r after a short vowel if necessary for the meter.
In the Attic dialect of Greek, long alpha is somewhat rare because of a historical change of aː to ɛː when not preceded by e, i or r. The α in the second syllable of Ἁμαδρυάς is not preceded by one of those sounds, so I'm not sure how it could be long. It would help to know more about the word's etymology, but I found little on this topic when I searched online. The English Wiktionary article for hamadryad, as well as the OED entry, explain the formation of the Greek word as from ἅμα 'together' and δρῦς 'tree'. Unfortunately, the length of the final alpha in ἅμα seems to be complicated: there is a Doric form ἁμᾶ that has a long vowel, as shown by the circumflex on the ultimate syllable, but as far as I can tell the Attic form had a short vowel, or at least could sometimes. This etymology, if correct, doesn't make me feel much more confident about the length of the second vowel in Ἁμαδρυάς.
Can anyone tell me about more information that is relevant to the etymology and/or pronunciation of this word? Is there anything that clearly points to a pronunciation with a long vowel aː as opposed to one with a heterosyllabic cluster d.r?
*Lewis and Short does not use the macron exclusively for long vowels: it is used to mark a short vowel in the nucleus of a heavy syllable in various cases where the spelling doesn't clearly show that the syllable is closed, e.g. in mājor (= [majjor], not [maːjor]) or cē̆dris (= [ke.dris]~[ked.ris], not [keː.dris], from Greek κεδρίς).