h₃er-
“eagle, large bird”eagle
Root for eagle. Gives Greek órnis "bird", Gothic ara "eagle", Old English earn.
Discussion
*h₃er- is a Proto-Indo-European root meaning "eagle" or "large bird," yielding the names for eagles and large raptors across the language family.
Gothic ara "eagle" and Old English earn "eagle" (preserved in the proper name Ern/Ernest and the compound erne, an archaic word for eagle) reflect the Germanic branch. Old High German arn and Old Norse ǫrn (modern Scandinavian örn) continue the form. The German Adler "eagle" is a contraction of Adelaar "noble eagle."
Greek órnis "bird" (whence English ornithology) shows a generalized sense—from "eagle" to "bird" in general. The semantic narrowing in Germanic (large bird > specifically eagle) versus broadening in Greek (eagle > any bird) illustrates typical patterns of semantic change.
Latin aquila "eagle" (whence English aquiline, eagle itself via French) is sometimes connected to this root but more likely derives from a different formation. Hittite ḫāraš "eagle" preserves the Anatolian reflex.
The initial laryngeal *h₃ colors the following vowel to *o, visible in Greek órnis. Welsh eryr "eagle" and Old Irish ilar/irar "eagle" continue the Celtic forms.
Modern descendants include English erne (archaic for eagle), the name Ernest ("eagle-strength"), and through Greek: ornithology. English eagle itself comes from Latin aquila via French aigle.
Notes
Source of "erne" (white-tailed eagle), "ornithology"