The vocal behaviour of young warbling vireos

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Abstract
The vocal behaviour of young warbling vireos (Vireo gilvus) in southern Ontario was studied during the nestling and early fledgling periods. Six vocalizations were distinguished: screech, partial screech, beg–eeah, harsh beg–eeah, jit–eeah, and jit. All vocalizations appear to develop by a process of differentiation from the screech call. The beg–eeah, harsh beg–eeah, and jit calls of the young show clear structural and ontogenetic relationship to the eeah, harsh eeah, and vit calls of the adult. During ontogeny, three types of change were noted; calls appeared without evident transition from other forms, others disappeared from the repertoire, and some calls showed a gradual change in form. The screech, partial screech, and beg–eeah, harsh beg–eeah appear to be a part of a single series of responses associated with degrees of stimulus contrast. The ontogenetic development of the calls of young warbling vireos is compared with that of other birds.