Fågelholk is 'nesting box'. Fågel is 'bird' and is of course same word as English fowl, of common Germanic origin.
Holk (holken, holkar, holkarna) is something hollow; originally a wooden pail it can be a privy as well as oldfashioned knickers - byxholkar. Holkärm is a 'cap sleeve' and holkyxa is a 'socketed celt' (a bronze-age tool). Holk is related to hål, same word as English hole.