עִלַוֵיהוֹן צִפְּרֵי שְׁמַיָא יִשְׁרוּן מִבֵּינֵי זָאַזְיָא יִתְּנוּ קָל זְמָר.
1. Beside them | עֲלֵיהֶם – In other words: "beside the streams" (of verse 10). See Radak that the word "על", normally translated as "upon", can also take the meaning "beside", pointing to such usage in Bemidbar 2:20. Alternatively: "above them", with the object still being the streams, but the verse referring to water fowl who dwell on the water itself, or with the object being the beasts of the previous verse. If the latter, the verse might be emphasizing how God creates and cares for all, both animals below and birds above.
2. foliage | עֳפָאיִם – Or, similarly: "branches" (Rashi). The word does not appear elsewhere else in Tanakh but might be related to the Aramaic "עֳפִי", leaves, mentioned in Daniel 4:9,11, and see Hoil Moshe that the plural, doubled form of the word in our verse connotes layers of leaves.