“their dead bodies will be food.” The dead bodies of animals and unwanted humans were left unburied and were usually eventually eaten by animals and birds. In this particular case, the Babylonians were about to kill many Judeans in their attack on Judah and Jerusalem, and those people would not be buried because their families would have been killed or captured. In a culture when family tombs and burial plots were common and it was a great curse to not be buried, most people believed (falsely, but it was a very widely held belief) that a proper burial was important for a comfortable existence in the afterlife. Thus, this verse was a horrifying threat of unspeakable loneliness and rejection (see commentary on Jer. 14:16).
“the birds of heaven.” The Hebrew is literally, “the birds of the heavens,” but the Hebrew word “heavens” is always plural, there is no singular word “heaven” in Hebrew.