Sliding Scale and Animals

I think we can place the moral status of an animal on a sliding scale. Factors for placement on the scale would include, among other things, the level of sentience, the ability to process/experience pain, and self-value.

Full grown humans, for instance, would be rather high in this scale, as they very sentient, they can process and experience pain, and, in general, they highly value their own lives. Adult parrots would also rank rather highly, as they have mental capacities similar to many three to five year olds; it is, I think, just as wrong to kill an adult parrot, as it is to kill an infant human. Many other animals have capacities similar to infant children. Some animals, like chickens, have very limited mental capacities, that is, they are not terribly intelligent. They do however, have capacities for pain, as such, I think that they do have a value significant enough to warrant not hurting them, if we can avoid doing so.

Meanwhile, shellfish and plants have no brain or central nervous system of which to speak; they are no sentient, and they cannot experience or process pain. As such, I think it is ethical to kill them so long as you don't severely limit the food source of other more intelligent animals who survive on shellfish heavy diets. 

We can use this sliding scale to determine the ethics of vegetarianism. A human, in order to be ethical, should eat things with the lowest moral value. Many humans can survive on vegetables, fruits, and shellfish. As such, they should refrain from eating animals with the sentience and capacity to process and experience pain. If a human finds that they are unable to survive without other animal proteins, they should eat the animals with the lowest moral value that would suffice to give them the necessary protein; if a human needs animal meat for protein, and chicken would suffice, they ought not to eat anything of a higher moral value than chicken.