I suppose I care less about the suffering of less intelligent animals than I do about more intelligent ones. All else being equal, I’d like chickens to be raised better, to the point where I pay a lot extra for it (though this is partly pure selfishness, I think chickens who’ve gotten a lot of exercise taste much better), and I think that sadism against chickens is still sadistic and evil, but even if they can suffer, I’m not going to put them on the same level as eg a suffering whale.

And babies past a few months of age are pretty remarkably intelligent, albeit still a bit uncalibrated.

It is plausible chickens have less moral weight than whales. But it's not exactly because of intelligence -- it's their capacity to suffer. I know intelligence is very tempting to use because it's a very good proxy in the extreme cases (e.g. a sponge vs. a human), so it's usually our first intuition.

But if you probe into it, I believe you'll find it's an artificial distinction that rather quickly falls apart. If your moral values ascribe negative value to the suffering of, say, both a human and a dog, then the only consistent viewpoint is to ascribe negative value to any suffering of a materially similar quality -- i.e. what matters on a moral level is the capacity of suffering for the species or individual.

There's a few uncomfortable thought experiments you can think through on this topic. E.g. you can imagine genetically engineering a human as dumb as a chicken, or perhaps a traumatic brain injury causing that, but with their mental capacity for suffering just as intact as yours or mine. Would their suffering count for less? Why would it?

As for chicken vs. whale, it may be the case that a chicken suffers less than a whale. More primitive intelligence may imply more primitive suffering. But what if it doesn't for this case? How confident can you be it does imply that, given both how chickens react to pain and stress and the neurological research that's been done on them? And even if it less, by how much? A factor of two, ten, forty, a thousand?

Those questions may seem too philosophical, but they can't be ignored in the face of a recommendation to eat 40 extremely-poorly-raised chickens per year instead of 1 reasonably-well-raised cow. And given all we know of animal cognition, it's reasonably to assume chickens have a lower capacity to suffer (and also to assume it's the same -- there's still uncertainty), but not so much lower that that calculus is anywhere close to being worthwhile.