Ibn Ezra on Leviticus 21:5

לא יקרחה קרחה בראשם. על המת: THEY SHALL NOT MAKE BALDNESS UPON THEIR HEAD. Over the dead.

ופאת זקנם. על המת כמנהג מקומות בארץ כשדים והנה התברר פירוש את פאת זקנך: THE CORNERS OF THEIR BEARD. Over the dead, as is the practice in places in the land of the Chaldeans. The meaning of the corners of thy beard (Lev. 19:27) is thus clarified.²¹It too refers to an act of mourning. See I.E. on Lev. 19:27.

וטעם שרטת. אפילו אחת וכבר נזהרו ישראל על אלה. וטעם הזהירם כי ראש מוקרח וזקן מגולח ובשר שרוט לא ישמש לפני השם: Saratet (cuttings) means even one cut.²²Saratet (cuttings) is singular. Hence I.E’s. comment. Israel has already been cautioned not to do these things.²³Not to cut their flesh, to shave the corners of their beards, or to make themselves bald. See Lev. 19:27,28. The reason Scripture warns the kohanim²⁴After already having told the Israelites. about this is that one who has a bald head, or whose beard has been shorn, or whose flesh is cut shall not serve before the Lord.