A MORPHOMETRIC EXAMINATION OF SEXUAL DIMORPHISM IN THE HYLOPHILUS, XENOPS, AND AN AUTOMOLUS FROM SOUTHERN VERACRUZ, MEXICO

--Two tropical vireonids ( Hylophilus o. ochraceiceps and H. d. " decurtatus") and two furnariids (Xenops minutus mexicanus and Automolus ochrolaemus cervi•ti,ffula•is) were examined using external measurements of museum specimens. All four species are sexually monochromatic. Degrees of sexual dimorphism are described, and discriminant equations are presented as a method for determining the sex of captured birds. Correct use of these equations is discussed. Clues to aging are offered when possible; juvenal plumages are summarized. In all four species males are larger than females in wing and tail lengths, but no significant differences were fbund in body mass. Tarsus length is longer in male A. och•vlaemus, and probably shorter in male H. ochraceiceps. Bill length is longer in male H. decurtatus. At present there are no functional explanations fbr the observed dimorphisms. The congeneric •4reonids, which show geographic and some habitat overlap, show very little overlap in morphological space. ESTUDIO MORFOMgTRICO SOBRE EL DIMORFISMO SEXUAL EN HYLOPHILUS, XENOPS, Y AUTOMOLUS EN EL SUR DE VERACRUZ, MP_. XICO Sinopsis. --Se 11ev6 a cabo un estudio morfom•trico de dos especies de vire6nidos (Hylophihts o. ochraceiceps y H. d. "decurtatus") y de dos de fitrn•cidos (Xenops minutus mexicanus y Automolus ochrolaemus cervinigularis) al sur de Veracruz, M•xico. Las cuatro especies resultaron ser sexualmente monocromfiticas. Se describe el grado de dimorfismo sexual entre las especies y se presenta una ecuaci6n discriminativa que permite identificar el sexo de individuos capturados. Se discute la utilizaci6n correcta de la ecuaci6n. En los machos de todas las especies la 1ongitud de las plumas de las alas y la cola resultaron ser de mayor tamaft() queen las hembras; aunque no se encontr6 diferencia significativa en el peso corporal. La 1ongitud del tarso es mayor en los machos de A. ochrolaemus, y probablemente mils corto en los machos de H. ochraceiceps. E1pico es mils largo en los machos de H. decurtatus. A1 presente no hay una explicaci6n funcional para el dimorfismo observado en las especies estudiadas. Los vire6nidos congen•ricos, que muestran solapamiento geogrfifico y de habitat, muestran muy poco solapamiento en espacio morfo16gico. Cryptically dimorphic bird species, those in which the sexes look alike by plumage but show small differences in size and shape, are difficult to study in the field at levels below the population because sex classes are indistinguishable during most of the year. This paper has two goals. The first is to initiate a survey of sexual dimorphism among sexually monochromatic tropical passetines. The second is to provide tropical workers with the best means possible to identify the sex of captured birds for field study (e.g., banding and behavioral studies). Workers in the neotropics lack reliable methods for distinguishing the age and sex of most resident • Current address: National Zoolo•cal Park, Conservation & Research Cente•; Fro•tt Royal, •5• ginia 22630 USA. • Current address: Burke Museum, DB-10, University of Washi•tgton, Seattle, 14•tshirtgto•t 98195 USA.