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RATES VERSUS COUNTS: FALL MOLTS OF LUCY'S WARBLERS (VERMIVORA LUCIAE)

Auk, The, Jul 2007 by Rohwer, Sievert, Navarro, Adolfo G, Voelker, Gary

ABSTRACT. -

New specimens from Sinaloa, Mexico, as well as two older specimens, show that both adult and recently fledged Lucy's Warblers (Vermivora luciae) often move far south of their breeding grounds to molt in the tropical deciduous forests of northwestern Mexico. Remarkably, the first preformative (= first prebasic) molt is complete in Lucy's Warblers, including the replacement of flight feathers, primary coverts, and rectrices; thus, previous aging criteria based on the wear and appearance of these feathers are invalid. We suggest that the recent conclusion that Lucy's Warblers molt on their breeding grounds (Voelker and McFarland 2002) is an artifact of more collecting in the southwestern United States than in western Mexico during late summer. An index of relative collecting in Mexico and the United States, as well as data from constant-effort mist netting, suggest that most Lucy's Warblers move south to molt. Received 11 November 2005, accepted 22 lune 2006.

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Key words: aging criteria, collecting effort, Lucy's Warbler, Mexican monsoon, molt migration, Vermivora luciae.

Tasas versus Conteos: Mudas de Otoño de Vermivora luciae

RESUMEN. - Nuevos especímenes de Sinaloa, México, y dos especímenes más antiguos, muestran que tanto los adultos como los juveniles recién emplumados de Vermivora luciae se mueven frecuentemente hacia el sur de su área de distribución reproductiva para mudar en los bosques deciduos tropicales del noroeste de México. Llamativamente, la primera muda preformativa (= primera prebásica) de V luciae es completa, incluyendo el recambio de las plumas de vuelo, las cobertoras primarias y las rectrices. Por lo tanto, los criterios empleados en el pasado para estimar la edad, basados en el desgaste y la apariencia de estas plumas son inválidos. Sugerimos que la conclusión reciente de que V. luciae muda en sus áreas reproductivas (Voelker and McFarland 2002) es un artefacto de una mayor colecta a fines del verano en los estados del suroeste de Estados Unidos que en el oeste de México. Un índice de colecta relativa en México y Estados Unidos, así como datos de esfuerzo constante de captura con redes de niebla, sugieren que la mayoría de los individuos de V. lucidae se mueven hacia el sur para mudar.

AT LEAST HALF the passerines that breed in western North America and winter mostly south of the United States complete all or part of their fall migration before molting (Rohwer et al. 2005). We call these movements "molt migrations." Some molt migrants winter where they molt, but most pause to molt and then move farther south for the winter. That so many western passerines migrate before molting has been interpreted as an adaptive response to complementary ecological "pushes" and "pulls." Dry late-summer conditions in the lowland west are believed to "push" western passerines away from their breeding grounds for molting, whereas abundant food resources associated with the late-summer rains of the Mexican monsoon (Comrie and Glenn 1998) "pull" these migrants to the southwest and western Mexico for molting (Rohwer et al. 2005). In some species, both adults and juveniles are molt migrants, with juveniles migrating in juvenile plumage (Bullock's Oriole [Icterus bullockii], Rohwer and Manning 1990; Ashthroated Flycatcher [Myiarchus cinerascens], Butler et al. 2006). In other species, molt migrations are undertaken only by adults, whereas juveniles remain behind in the breeding range, where they undergo their postjuvenile molt before migrating (Lazuli Bunting [Passerina amoena], Young 1991; Painted Bunting [P. ciris], Thompson 1991; western Warbling Vireo [Vireo gilvus swainsonii], Voelker and Rohwer 1998; Western Tanager [Piranga ludoviciana[, Butler et al. 2002).

Variable effort invested in collecting means that simple geographic enumerations of specimens may misinform attempts to deduce the scheduling of molt in relation to migration for species in which some individuals molt on the breeding grounds (Butler et al. 2002) or in which movement south before molting is restricted. Rates, based on collecting effort, are needed to overcome these biases. If juveniles molt on the breeding grounds, a decline in the fraction of late-summer adults shows that adults migrate before molting, provided that there are good samples of juveniles showing that collectors were active in late summer (Young 1991, Voelker and Rohwer 1998, Butler et al. 2002). But for species in which both adults and juveniles move south before molting, changing age ratios cannot establish an early departure by adults.

Voelker and McFarland (2002) predicted that Lucy's Warblers (Vermivora luciae) should depart from the drier areas of their breeding range to molt in the region of the Mexican monsoon. Because the 18 presumed molting adults that they examined were collected within the breeding range, Voelker and McFarland (2002) concluded that Lucy's Warbler was not a molt migrant. However, molt-related movements in Lucy's Warbler are difficult to assess, because its breeding range extends only a limited distance north of the northern limits of the monsoon region. These rains begin earlier and are heavier and more predictable in the south than in the north (Comrie and Glenn 1998); thus, species like Lucy's Warblers, whose southern breeding limits include only the northern part on the monsoon region, may benefit from moving south to areas of greater rain to molt.

RATES VERSUS COUNTS: FALL MOLTS OF LUCY'S WARBLERS (VERMIVORA LUCIAE)

Auk, The, Jul 2007 by Rohwer, Sievert, Navarro, Adolfo G, Voelker, Gary

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