"where a bird rests," late 13c., originally only "a pole, rod, stick, stake," from Old French perche "unit of linear measurement" (5.5 yards), also "measuring rod, pole, bar" used to measure this length (13c.), from Latin pertica "pole, long staff, measuring rod," related to Oscan perek "pole," Umbrian perkaf "twigs, rods." Meaning "a bar fixed horizontally for a hawk or tame bird to rest on" is attested from late 14c.; this led to general sense of "any thing that any bird alights or rests on" (late 15c.). Figurative sense of "an elevated or secure position" is recorded from 1520s. The "land-measuring rod" sense also was in Middle English (c.1200), hence surviving meaning "measure of land equal to a square lineal perch" (usually 160 to the acre), mid-15c.
"spiny-finned freshwater fish," c.1300, from Old French perche, from Latin perca "perch," from Greek perke "a perch," from PIE root *perk- "speckled, spotted" (cf. Sanskrit prsnih "speckled, variegated;" Greek perknos "dark-colored," perkazein "to become dark"), typically in names of animals.
"to roost," late 14c., from Old French perchier "to sit on a perch" (of a bird), from perche (n.) (see perch (n.1)). Related: Perched; perching.