This is the position from a palaeontological point of view:
Perdix perdix does not occur on any mediterranean island and never has as
far as I know.
Alectoris rufa occurs on the Balearics and Corsica. These islands have a
good fossil record, but no Alectoris fossils, so they are almost certainly
introduced during the Holocene. The oldest subfossil record from Corsica is
from the 6th century AD.
Alectoris barbara occurs on Sardinia. It is normally considered a North
African species but there are Pleistocene records from Spain, France and
Italy. No fossil records of barbara from Sardinia, so it was almost
certainly introduced, probably from North Africa (not quite certainly
however, because of the fossils from Europe mentioned before). There are a
couple of records of Alectoris from Pleistocene sites from Sardinia, but
they are from old and stratigraphically unreliable contexts, so they don't
prove anything.
Alectoris graeca occurs on Sicily and has a good fossil record going back
to the Middle Pleistocene, so it's clearly natural there.
Alectoris chukar occurs on Crete. No fossils despite a fairly good fossil
record, so it is probably introduced. It should be noted that it is graeca
that occurs on the Greek mainland suggesting that the introduction came
from the east.
Except for Sicily the only galliform that is definitely natural on the
mediterranean islands is Coturnix coturnix which is of course a long
distance migrant. Both Perdix and Alectoris seems to have very limited
ability to cross water barrier.
It should be noted that it is difficult to separate Alectoris species
osteologically. I suspect that some have been identified on
in-this-area-it-should-be-that-species basis.
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