User blog comment:Mohrpheus/Spiritual Imbalance in Bleach - The End of the World?!/@comment-1393830-20100830223547/@comment-1268838-20100831000424

If I remember correctly, Rukia said that the spirits of animals are much weaker than those of a human (hence why that one boy's spirit had dominance over the parakeet's body, the bird's spirit got absorbed by his own). I imagine canon animal Hollows would be like that lizard thing Uryu found in Hueco Mundo, far too weak to feed on each other.