irregular and uncertain intervals

Their appearance and disappearance is at very irregular and uncertain intervals

James E. DeKay on the “wild pigeon” in Zoology of New-York, or the New-York fauna; comprising detailed descriptions of all the animals hitherto observed within the state of new-york, with brief notices of those occasionally found near its borders, and accompanied by appropriate illustrations. Part II Birds. Carroll and Cook, Albany, NY. 1844, p. 197.