This essay deals with the recognition of non-painful emotional experiences in animals, how they relate to animal wellbeing and animal welfare, and how they can be assessed, monitored and mitigated. While it is written often from a mammalian perspective, the general principles will apply to all animals that are sentient.
Morton, David B., "Distress in Animals: Its Recognition and a Hypothesis for its Assessment" (2009). SENTIENCE. 14.