As far as I understand it, there are three broad buckets on the spectrum of what is considered sentience:

• On the most open end (call it 0) it’s the ability to experience sensation (this might be the original definition, but language evolves); literally just being able to process and react to input by some mechanism. By the broadest, most open interpretation of this idea, even plants are sentient. Certainly chickens are, to say nothing of dogs and cats, and of course humans.

• More toward the middle (5) is the capability for experience and intent; not simply to sense and react, but to recognize a feeling and make a choice, however dimly, to pursue a course of action as a result. You might get higher-order emotions like love as a requirement for sentience with this definition. Here, plants are clearly not sentient; their reactions are simply the result of chemical processes. Chickens are on the line; I’m not an expert on chicken neurology, but as a person who lived in a rural area growing up, I think I could pretty well make a case on either side. Dogs and cats seem to certainly be sentient by this interpretation, and humans are as well.

• On the most restrictive end of the spectrum (10) is something more like human-level consciousness; tough to define without using fuzzy words like “feel” and “think,” but being able to reason and plan for the future in some way is one part. Another part I’ve seen a lot that seems like it’s in this bucket is being able to choose to follow or eschew raw impulses. This is the end of the spectrum that most sci-fi leans on when they discuss “sentience.” Chickens are probably not sentient by this measure, but dogs and cats may be. Humans are, as the prototype of our understanding of the term.

So, to answer your question, I’d say that chickens seem to be the midpoint of the spectrum. There are probably about as many definitions which include them as don’t include them. A manosphere meatfluencer would probably put sentience at a 9 or 10 on this spectrum, while some religious traditions might put it at a 1 or 2, or even as low as 0 in some cases.

Even that Wikipedia article refers to several definitions for sentience, ranging from experiences sensation through ability to make choices all the way up to human-level consciousness. Sci-fi stories have used it to mean “human-like” when the word “human-like” was inappropriate for the story, but vegan literature uses it in what might be its original definition, which could be broadly termed as “able to sense.”

Speaking of literature, the Stanford Encyclopedia of Philosophy article is a literature study, not an attempt at objective definition. I think any honest assessment agrees that there isn’t an objective definition here, partially because we don’t understand most of the questions we’re asking, let alone the answers.

I said more about it here, but in short, I think it’s perfectly reasonable to select a definition of sentience that includes chickens. I just also think it’s perfectly reasonable to select a definition that doesn’t.