Body Language - cats and dogs
Scared, stressed, or uncomfortable: Hiding face, shivering, panting, tail between legs, wincing, retreating, uncontrollable urinating/deficating, hissing (cats), growling, wimpering, ears back.
Happy, content, or excited: Wagging tail (dogs), barking or meowing, jumping, panting, licking, playing, howling, purring, light nips, rubbing (cats).
Angry, aggressive, or territorial: Growls, yelling, scratches, bites, hissing, barking, ears back, back hair standing, puffy or wagging tail (cats).
Body Language - birds
Happy, content, or excited: Chirping, singing, chattering, toy interaction, dilated pupils
Angry, agressive, or territorial: Biting, squaking, screeching.
Scared, stressed, or uncomfortable: Retreating, plucking feathers, skin mite overgrowth, quiet, depressed.
Body Language - rodents
Happy, content, or excited: Wheel interaction, normal storage instinct, exploration, popcorning (guinea pigs).
Angry, agressive, or territorial: Biting, kicking, ears back, mounting (territorial), scratching, chattering teeth.
Scared, stressed, or uncomfortable: Retreating, submissive position on back with teeth bared, uncontrollable urinating or deficating, kicking.