Abstract
TEN years ago I presented a review of research on molts in tyrannid flycatchers of the genus Empidonax, in a paper that included original data for five species (Johnson 1963b). Since that time very little has been published on this aspect of the biology of these flycatchers, although Mumford (1964: 46) and Traylor (1968) have added new details to data available for the Acadian Flycatcher (Empidonax virescens). In the present paper I describe molt patterns and criteria for determining age in two close relatives in the genus Empidonax, the Western Flycatcher (E. difficilis) and the Yellowish Flycatcher (E. flavescens). The molts of these species have not been studied previously in any detail. This work was undertaken to gather background information necessary for the proper interpretation of size variation in a systematic analysis of this difficult group (Johnson MS). Finally I consider patterns and progress of cranial development in these flycatchers and offer a new hypothesis to explain retarded pneumatization of the skull roof in birds.
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