Why should humans care about how we treat animals? To even ask the question is likely to evoke an outraged dismissal. But pursue the question and some interesting things emerge.
Most people and animal welfare organizations would say we should treat animals with compassion, empathy and respect. To the question "why?," the usual answer is "because we are ethical beings." Enculturation in our own and other societies, however, shows that, to the extent that we are ethical, it is a matter of nurture not nature. A culture's ethics evolve, and currently, the ethics for the treatment for animals is poorly articulated and in some regards, misplaced (below). Another answer to "why?" is "because we ought to," with no rationale given for the "ought."
Indeed, logic would suggest that we shouldn't care how we treat animals per se, because unlike humans, they have no notion of reciprocating our treatment of them, either good or bad. At the heart of human ethics is the notion of reciprocity, embodied in the "Golden Rule." The lack of reciprocity in the human-animal interaction is the practical flaw in the core tenet of animal welfare philosophy and the welfare movement, a rejection of "speciesism" - a "prejudice" for our own species.
Logic does suggest, however, one reason for treating animals well. If we extend the halo of concern to animals, we may enhance the core of the halo around ourselves. But treating animals well is currently based on a largely anthropomorphic view of what they are. We think they are like us.
Although the benefit to humans of a concern for animals is widely recognised, animal welfare philosophers and welfare organisations are loath to acknowledge it, other than as secondary benefit. Rather, they wish to have concern based on a "right" of animals per se. But rights, even supposedly intrinsic ones as some posit for animals, are created at the discretion of humans. They do not exist as laws of nature, like the speed of light, waiting to be discovered. And there is no logic in creating a right for an animal that does not create a net positive benefit for humans. Animal welfare has no answer to this problem other than the problematic "ethical being/ought" argument or a disguised version of the "halo" argument. As mentioned, however, only the latter has a rational basis.
But for whatever reasons humans chose to be concerned about how we treat animals, would not the discussion in our own heads as well as collectively be better informed if we could put aside some of our anthropomorphic bias?
Western concern for animals is based on the sentience of some animals, namely, that they share emotional states with humans, most importantly for animal welfare philosophers, pain. It is more than fortuitous, however, the avoidance of pain is the "interest" human most wish for themselves. It does not follow, however, that the same holds for animals. Reproduction, for example, is the main "interest" of animals and can override what would appear to humans as prolonged severe pain. Witness salmon species that ascend their natal rivers, arriving as emaciated bags of sperm and eggs to reproduce and die; domestic hens continuing to lay even when severely mistreated; male Elephant Seals and Sea Lions slashing each another in fights for harem dominance; male Antechinuses dying of stress in their one brief mating period, or a female Tasmanian Devil unable to eat due to advanced facial tumour disease but nonetheless still nursing its pouch young. It is no coincidence that one of humans' most common manipulations of domestic animals to make them more tractable is cutting out or off their reproductive organs.
While we can agree, that animals experience some basic sensations that can be discussed under very broad labels such as pain, we will never be able to tell how similar in detail or intensity they are to our own experience. This is especially true of "positive" sensations. Animal welfare philosophy and law assume, however, they can be compared with humans on both counts. Indeed, animal welfare philosophy requires concern for animal pain to be based on the equivalent intensity in humans.
A common animal welfare solution to the objective uncertainty of what an animal is feeling is to ask observers to "how would you like to be a battery chicken" or as one prominent jurist advises, "just stare into the eyes of a cow(Kirby, 2011)." But this is simply an invitation to anthropomorphise. And while we can anthropomorphise privately about an animal's emotional state without doing harm, do we want to start laying charges and casting aspersions based on our reflected projections?
A way around the uncertainty of what an animal is feeling is simply to consider animals in keeping with what they are as animals as revealed by objective observation of their physiology and behaviour. Laudably, this is the approach of animal welfare research.
The question is not whether a single parrot in a cage is experiencing pain or suffering or how we would like to be in its situation. The question is whether its treatment is in accordance with a social animal with appendages for long-distance flight and that pair-bonds for life. From this viewpoint, evidence of pain is only one indicator of mistreatment - and a rather advanced and crude indicator at that.
What an animal is varies among and within species and breeds. It will depend, for example, on whether the animal is wild, domestic, young, mature, pregnant or whatever. Most importantly, however, any question about the treatment of an animal in relation to what it is can be resolved, not by reference to what humans may think the animal is feeling but by rational observation.