Question: At one of the minyanim I attend, I am often the only kohen. Considering that lowers the level of Birkat Kohanim (=BK), should I avoid davening there?
Answer: The gemara (Sota 38a) derives from "say to them" (Bamidbar 6:23) in the context of BK that someone calls the kohanim to do BK only when there are at least two kohanim. Since a kohen violates his obligation to do BK only when he fails to do so after being "called" (Shulchan Aruch, Orach Chayim 128:2), Rabbeinu Peretz (cited by the Tur, Orach Chayim 128) says that one kohen, who is not called, cannot violate an obligation. The Tur argues, and the Beit Yosef explains that one kohen violates his obligation when he was unnecessarily called. Tosafot (Menachot 44a) raises another possibility – one kohen who is not called does not have a Torah obligation, but he still has a Rabbinic one.
You are apparently concerned by the opinions that BK is only Rabbinic with one kohen. We will start by mitigating your concern. First, not only does the Yerushalmi hold that even one kohen is called, but the Taz (128:3) and Pri Chadash (128:10) understand that one kohen has a Torah obligation without being called (see variation in Aruch Hashulchan, OC 128:9). One explanation is that calling is needed only when a kohen could pass off the obligation to his fellow kohen, whereas a lone kohen is obviously obligated (see Mishneh Halachot III:197). Admittedly, many (including the Magen Avraham 128:16; Be’ur Halacha 128:25) understand that the obligation is only Rabbinic.
Furthermore, even if the obligation/possibility of violation is Rabbinic, logic indicates that one kohen who does BK voluntarily fulfills a mitzva from the Torah according to all Rishonim. After all, he does the same action in essentially the same manner – why should the lack of prompting disqualify it. This is the approach of the Maharam Mintz (12, quoted by the Magen Avraham ibid.), who thereby explains why one kohen makes a beracha on his BK, and the Minchat Chinuch (#378). The language of the Beur Halacha (ibid.) implies there is no Torah fulfillment.
Should a kohen take steps to fulfill the mitzva specifically as an obligation? The general rule is that performing mitzvot as an obligation is better than voluntarily (Kiddushin 31a), although the extent of the preference is unclear. Arguably, the difference is smaller when one is generally obligated in the mitzva and there is also a Rabbinic obligation. (It may depend on the reasons why the reward is greater when obligated – see Tosafot, Ramban, and Ritva ad loc. Further discussion is beyond our present scope.)
Finally, we must weigh preferences in context. Even if we assume the mitzva is more complete when done with other kohanim, consider that avoiding the minyan when they do not have another kohen leaves the minyan without BK. The following halacha proves that it is proper to "compromise" other preferences to ensure a minyan has BK – apparently including greater concerns than having BK with two kohanim. The Shulchan Aruch (OC 128:20) rules that when a kohen is chazan, he does not do BK unless there are not other kohanim. This indicates that while a kohen gives up his individual mitzva of BK to enhance his ability to serve as chazan, the need for the minyan to have BK, even of one kohen, is more important. In contrast, the classical poskim are silent on making such a sacrifice to jump from BK of one kohen to two. (Az Nidberu XIII:34 believes that a kohen as a chazan with another kohen would do BK according to the Shulchan Aruch to gain the advantage, but he was unable to find a previous posek to say so explicitly.)
In the final analysis, all agree that the BK of one kohen is a mitzva (otherwise he would not make a beracha before it) and all should agree that its sanctity and value is not substantively different from that of multiple kohanim. Considering the above, you should be happy if your presence ensures that the minyan has BK.
145 - Shortening Psukei D’zimra to Catch Up
146 - Kohen Seeking a Minyan With an Additional Kohen
147 - Tallit and Tefillin During Selichot