It's the first half of an oath. Oaths are an old-time way of making extremely emphatic statements, often about facts which seem unlikely. The speaker may feel that, without the oath, listeners will assume that he or she is either lying or pulling their legs. This type of oath takes the form, "If X is (or isn't) true, let Y happen." Since Y usually involves religious figures about which taboos exist regarding speaking their names ("let God strike me down" is a common example), oaths were considered somewhat uncouth. Uttering an oath using only the recognizable first clause containing the assertion was a way of getting around that unpleasant social stigma, for statements that were too mild to justify risking it.
Your parsing of the phrase is correct. In this case, the bird is simply swearing that he really is lucky. The dash stands for the unspoken conditional portion of his oath.
(Interestingly, your other question about the same story also concerns an oath, but one in which the conditional result portion, inside which you balked at a confusing phrase, is not omitted.)