In ornithology:

• Yellow-bellied Elaenia, small bird of the tyrant flycatcher family
• Yellow-bellied Fantail, fantail found in southern Asia
• Yellow-bellied Flycatcher, small insect-eating bird of the tyrant flycatcher family
• Yellow-bellied Sapsucker, medium-sized woodpecker
• Yellow-bellied Siskin, small passerine bird in the finch family Fringillidae

In other fields:

• Yellow-bellied Marmot, ground squirrel in the marmot genus
• Yellow-bellied toad, toad belonging to the order of Anura
• a colloquial term for cowardice