Most people today just assume that chicken is a clean bird. But it was not always this way. The issue used to be hotly debated between Rabbinics and Karaites. We will look at some of the positions on this issue and try to see if any of them are true. But let us look at what Torah says about it first.
these are they which ye shall have in abomination among the fowls; they
shall not be eaten, they are an abomination: the eagle, and the
ossifrage, and the ospray,
14 And the vulture, and the kite after his kind;
15 Every raven after his kind;
16 And the owl, and the night hawk, and the cuckow, and the hawk after his kind,
17 And the little owl, and the cormorant, and the great owl,
18 And the swan, and the pelican, and the gier eagle,
19 And the stork, the heron after her kind, and the lapwing, and the bat.
And again in Devarim
Devarim(Deuteronomy11 Of all clean birds ye shall eat.
12 But these are they of which ye shall not eat: the eagle, and the ossifrage, and the ospray,
13 And the glede, and the kite, and the vulture after his kind,
14 And every raven after his kind,
15 And the owl, and the night hawk, and the cuckow, and the hawk after his kind,
16 The little owl, and the great owl, and the swan,
17 And the pelican, and the gier eagle, and the cormorant,
18 And the stork, and the heron after her kind, and the lapwing, and the bat.
19 And every creeping thing that flieth is unclean unto you: they shall not be eaten.
20 But of all clean fowls ye may eat.
What some people say is "It seems pretty clear to me. Chicken is not on the list of unclean birds so it must be clean." The problem with this position is that because Hebrew was a dead language for so long nobody really knows what birds are listed in the Hebrew text. These English translations are just guessing as to what birds are listed.
"Many of the animals on the lists of Lev. 11 are familiar, but it must be stressed that, as with plant and place-names in the Bible, a number of the precise identifications have been lost to us, and educated guesses are in order." The Schocken Bilbe Volume 1 The Five Books Of Moses page 558
The Stone Edition Tanakh did not even try to translate the names of the birds in these passages, instead choosing to transliterate them and provide some suggestions for the identities of the birds in the footnotes. Some will also say "because all the birds listed are birds of prey and scavengers this must be what makes a bird unclean." Again this position has the same problem as the last one. How can we know that all the birds listed are birds of prey or scavengers when we do not even know what birds they are? We simply can't know that. And even if this position was true, it would not help the "chicken is kosher" crowd. After all chickens are scavengers. A chicken will gladly eat another dead chicken or any other carcass for that matter. Also, we should not assume that chicken was not mentioned in the lists of unclean birds. According to a Rabbi Avraham Feld, Anan Ben David one of the founders of Karaism, and his followers believed that chicken was indeed listed as one of the unclean birds in Leviticus.
"Anan and company wrongly translated a term in Leviticus 11:19 as 'chicken' and thereby listed it with impure, non –kosher birds." Karaites-Adversaries of the Redemption, Origin of the Karaites and their Teachings by Avraham Feld
Feld is obviously against the Karaites in almost every way. And he considers this early Karaite belief to be one of their more offbeat positions. But is it offbeat? Sadly he never says why it is wrong, I guess he just wants the reader to trust him and blindly except his authority as a Rabbi. It is interesting though, that according to Strongs one of the birds listed in Lev 11:19 could be translated as "gallus montanus" in English a mountain cock. Our modern chicken is scientificaly called "gallus gallus domesticus" or domestic cock. So maybe Anan Ben David and the early Karaites were not as far off as Feld would have us believe.
As for the Rabbinic position, Daniel Frank on page 124 of the book "Jewish Biblical Interpretation and Cultural Exchange: Comparitive Exegesis in Context" says;
"Rabbanite Jews... have always declared the chicken to be a "kosher" bird, even though there are no Scriptural grounds for doing so."
Frank is correct that the Rabbis have pretty much always said that chicken is clean, but he admits that there is no Biblical proof that it is kosher. Also, according to the Jewish Encyclopedia.com under clean and unclean animals
"It was hard for the rabbinical authorities to distinguish clean from unclean birds, as the Scripture (Lev. xi. 13-19) enumerates only the birds which shall not be eaten, without giving any of the marks which distinguish them from the clean birds. This is all the more important as the names of some of the birds mentioned in the Scriptures are followed by the word "lemino" or "leminehu"—i.e., "after its kind"—and it is therefore necessary to recognize certain fixed distinguishing characteristics. The following rules are fixed by the Talmud, by which a clean bird may be distinguished. It must not be a bird of prey; it must have a front toe, if that be the meaning of ; but according to most explanations the hind toe is meant. Although most birds of prey have the hind toe, the toes of the clean bird are so divided that the three front toes are on one side and the hind toes on the other, while the unclean bird spreads his toes so that two toes are on each side; or if it has five toes, three will be on one side and two on the other (compare Rashi to Ḥul. 59a, and Nissim b. Reuben on the Mishnah to this passage).
The clean birds, furthermore, have craws, and their stomachs have a double skin which can easily be separated. They catch food thrown into the air, but will lay it upon the ground and tear it with their bills before eating it. If a morsel be thrown to an unclean bird it will catch it in the air and swallow it, or it will hold it on the ground with one foot, while tearing off pieces with its bill (Ḥul. 59a, 61a, 63a). As this distinction is not found in Scripture, opinions differed greatly during and since Talmudic times. According to the Talmud (Ḥul. 62a, 63b), only the twenty-four kinds of birds mentioned in Scripture are actually forbidden. If certain birds are positively known as not belonging to these, no further investigation as to characteristic signs is necessary, and they may be eaten. The marks of distinction are laid down only for cases in which there is doubt whether the species is clean or unclean. Authorities, especially in Germany, would only permit the eating of such kinds as have always been eaten (). Accordingly some birds are permitted to be eaten in certain countries, but not in others. There are many controversies in the casuistic literature concerning this matter. Menahem Mendel Krochmal ("Ẓemaḥ Ẓedeḳ," No. 29), for instance, declares the wild goose forbidden, while Eybeschütz ("Kereti u-Peleti," § 82) permits it. When the turkey was brought to Europe Isaiah Horwitz forbade it to be eaten; and although his opinion did not prevail, his descendants refrain from eating it even to-day."
Here we see that the Rabbinic Jews don't know much about what birds are clean and unclean. And the signs that they would look for to determine what birds were clean and unclean are not Scriptural but Talmudic. This is an important point, Scripture does not give us any physical sign to help identify what birds are clean and what birds are unclean. We just have a list of forbidden birds. All clean land animals can be easily identified even if I've never seen said animal before. It is just a matter of observing the animal's hooves and whether it chews it's cud. Not so with birds, we have to know if a bird is on the list of unclean birds in the Torah or not. We also see that the Rabbis can not even agree with themselves as to what birds are listed.
All of this to say when in doubt don't. If we can not know if a bird like chicken is unclean or not, do we eat it hoping we are not sinning? I think not. We should avoid it until we know that it is clean. But, let every man be convinced in his own mind.