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Seed-eating birds in temperate deserts must cope with great variation in the availability of their food resources. We studied the trophic ecology of Ringed Warbling-Finches (Poospiza torquata) in semi-arid scrublands of the Monte Desert, Argentina. We assessed seasonality in the availability of seed and in the consumption of arthropods and seeds at a regional scale, and evaluated the composition of the granivorous component of the diet and the seed dietary breadth. Ringed Warbling-Finches had a granivorous–insectivorous diet consistinglargely of arthropods in summer and seeds and arthropods in winter. The granivorous component of the diet consisted mainly of grass seeds, but with low breadth of the winter seed diet. To deal with seasonal variation in the availability of food resources, Ringed Warbling-Finches switched opportunistically between different resources, exploiting alternating seasonal patterns of food abundance. This seasonal switching is a wellestablished strategy in the behaviour of the species and was observed in all Ringed Warbling-Finch populations studied across the Monte Desert. Our results show the flexibility of foraging strategies of Ringed Warbling-Finches, an opportunist species that adjusts their relative consumption of seeds and arthropods in response to the spatial and temporal variations in these food resources. Fil: Sánchez, Rocío. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Ecología Regional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Blendinger, Pedro Gerardo. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Ecología Regional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Sap is a resource of high energy content that is usually inaccessible to birds, although woodpeckers have the ability to drill into living trees to obtain sap. Because spatial patterns of resource availability influence avian abundance, we explored how spatial patterns of sap availability determine the spatial distribution of two sap-feeding species in the semiarid Chaco of Argentina. We studied the White-fronted Woodpecker (Melanerpes cactorum), which obtains sap by drilling holes into tree trunks, and the Glittering-bellied Emerald (Chlorostilbon aureoventris), which can obtain sap only from active woodpecker holes; 12 other bird species also exploited the sap flows from holes drilled by White-fronted Woodpeckers. The abundance of tree species used for sap feeding did not explain the spatial patterns of territorial groups of White-fronted Woodpeckers. However, within each territory, the abundance of Woodpeckers was centred on a single tree from which sap was obtained. The abundance of the Emeralds was strongly associated with the availability of trees with active sap-holes. During the dry season, sap is a major component in the diet of White-fronted Woodpeckers and Glittering-bellied Emeralds. However, the spatial distribution of these two consumers in relation to the availability of sap was species-specific. This species-specific response was closely related to the ecology and life history of each species. The abundance of woodpeckers could be determined by local mechanisms, such as location of a single sap tree in their small territories, whereas non-territorial hummingbirds would be able to track sap wells at a larger scale than the territory of a single territorial group of Woodpeckers. Our results show the importance of spatial analysis in identifying the ecological determinant of habitat selection and niche differentiation within species. © 2011 Royal Australasian Ornithologists Union. Fil: Macchi, Leandro. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Laboratorio de Investigaciones Ecológicas de las Yungas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina Fil: Blendinger, Pedro Gerardo. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Laboratorio de Investigaciones Ecológicas de las Yungas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina Fil: Nuñez Montellano, Maria Gabriela. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Laboratorio de Investigaciones Ecológicas de las Yungas; Argentina. Universidad Nacional de Tucumán. Instituto de Ecología Regional. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Ecología Regional; Argentina