As with many Neotropical birds, important aspects of the life history of Guatemalan Tyrannulet are poorly known. This species has received scant attention, since many authorities have considered it conspecific with the well studied Mistletoe Tyrannulet (Zimmerius parvus). Additional study of plumage, mensural, and song differences are needed to further distinguish Guatemalan and Mistletoe tyrannulets, and to aid in clarifying their relative ranges in Belize and Honduras. Additional data from field observation and song recordings (e.g. clarification of dawn song, descriptions of courtship) as well as museum specimens (e.g., molt timing and sequence) are needed. Most of the reported information on diet, nesting and behavior is assumed to match Mistletoe Tyrannulet.