“the net is spread.” This refers to a custom, and a way of capturing birds to eat. One way was that a net was spread on the ground, covered over or disguised in some way, and some kind of feed or grain was scattered on top. When the birds came to eat, the net was yanked and the birds were caught. A similar way was that two rectangular nets were spread out and concealed, and bait was placed between them, and when the birds came to the bait they were pulled up on either side of the birds who were caught when the birds tried to fly away. But you could not spread the net out while the birds were watching, or they would not be tricked and caught. Some commentators have suggested that the verse contains an ellipsis, “in vain is the net spread [with bait] in the sight of any bird.” While that is possible, it is not necessary. Another idea is that the verb we translate as “spread” should be “lift up,” with the idea that it is in vain to lift up the net before the birds are settled, eating, and distracted. In any case, the point is that if the birds see the net, you will not be able to catch them.
The Bible is making the point that people who participate in evil by doing things like killing and robbing are more foolish than birds. The “net” and “death” that awaits them in their future should be clear to them because God speaks of it so clearly in so many places, but like foolish birds that ignore the net, they are caught in their own sin and will be destroyed in Gehenna after Judgment Day.
“bird.” The Hebrew is an idiom: “the possessor of wings.”