“herds.” The Hebrew word eder (#05739 עֵדֶר) can refer to both flocks and herds. The NET conflates the English to “flocks and herds” for clarity.
“all the animals of the nations.” The meaning of the Hebrew is unclear because it literally reads “all animals of a nation.” Most scholars feel that the phrase means “every kind of wild animal” (HCSB), and that seems to be the most likely way to understand the verse. However, O. Palmer Robertson (New International Commentary on the Old Testament) may be correct in constructing the genitive “of a nation” (“nation” is singular in the Hebrew text) as “making up a nation.” In that case, the phrase would be something like, “And herds will lie down in the middle of her; every animal constituting a nation.” Robertson points out that the locusts are referred to as a nation in Joel 1:6, and thinks that is the case here. If Robertson is correct, the “nation” of Assyria has defied God and so become replaced with a “nation” of wild animals. What is clear is that the animals will take over the capital city of the great country of Assyria.
“desert owl and the screech owl.” The exact identity of these animals is unknown. The desert owl is some kind of unclean bird, but the screech owl may be a bird or a rodent. The Hebrew vocabulary may be unclear but the point is not: Nineveh will be uninhabited ruins.
“Devastation will be on the thresholds.” The beautiful buildings of Nineveh will be destroyed, exposing the cedar beam framework. The stonework, decorations, etc., will fall down and cover the thresholds of the buildings, so the “devastation” will be on the thresholds. For ease of understanding, some versions nuance the text, e.g., “Rubble will cover the thresholds” (NET). Also, the Septuagint reads “raven” instead of “devastation,” so some English translations have “ravens.” The fact that Nineveh will go from a city of beautiful buildings with cedar woodwork to a devastated haunt of animals and birds shows that Yahweh is the Most High God and defying Him and hurting His people will only result in destruction.