Phoenix Logs/Flood Units/Seeder Infectors

From Halopedia, the Halo wiki

Once the parasite can establish a hive it can begin to produce massive numbers of infection forms that spread out in search of new sentients to infect, and non-sapient animals on which to feed. Both ground and airborne infection forms are used as living weapons, hurling themselves at the enemy to overwhelm defenses and assimilate the unprepared.

Seeders are lighter-than-air Flood units that swim through the sky and attack aerial units. If a Seeder gets the killing blow, the crew are overwhelmed and the aircraft becomes Infected. Seeders only infect aircraft, but will "spit" shards of bone at ground targets.