From Halopedia, the Halo wiki
Flood Swarms are infected aerial life forms that have been infected by the Flood and repurposed for the parasite's use. They possess a relatively hunched body, two bony wings, a gaping mouth, two eyes, and a pair of elongated trailing limbs. Like the Coordinated Stage Flood Ranged Form, they are capable of launching barbs at potential hosts, though the exact mechanism for this is unseen in the Flood Swarm forms.
The developer's description of these creatures as "infected buzzards crossed with evil bats" implies they are in fact native organisms that have been infected by the Flood and integrated into the Parasitic collective. This is a theme and motif that dominates the Flood presence in the game, which can also be seen in the stationary Flood forms in the game, and two other mobile forms, specifically the thrasher form and the bomber form.
List of appearances