SummaryWe documented the breeding distribution and estimated abundance of Chubut Steamerducks Tachyres leucocephalus, a flightless waterbird endemic to a relatively small section of coastline in Patagonia, Argentina. The distribution of Chubut Steamerducks is restricted to approximately 700 km of coast. We counted 1,703 adult steamerducks at a subset of shorelines within their range and estimated 1,841 adults after correcting for visibility for shore-based surveys. To estimate adult densities in unsurveyed areas, we used two different methods of extrapolation, resulting in estimates of 1,587 and 1,832 adults. Combined with numbers from surveyed shorelines, the total breeding population size is estimated to be between 3,428 and 3,673 adults. In addition, we counted 1,899 juvenile steamerducks, which occur in irregular aggregations. The Interjurisdictional Marine Park in San Jorge Gulf contains about 46% of the entire population, which may provide some protection from disturbance and habitat destruction. However, oil pollution, other anthropogenic activities, and invasive species still pose potential threats to the population. ResumenHemos documentado el rango de distribución y estimado el tamaño poblacional del pato vapor cabeza blanca Tachyres leucocephalus, un ave acuática no voladora endémica de una estrecha sección costera de Patagonia, Argentina. La distribución del pato vapor cabeza blanca esta restringida a aproximadamente 700 km de costa. Hemos contado 1,703 individuos a lo largo de sectores costeros discretos dentro de su rango de distribución, y estimado un total de 1,841 adultos luego de aplicar los correspondientes factores de corrección. Para estimar la densidad de adultos en los sectores no relevados, hemos utilizado dos métodos diferentes de extrapolación, resultando en un número estimado de 1,587 y 1,832 adultos. De esta forma, al sumar individuos relevados y estimados, el tamaño poblacional oscilaría entre los 3,428 y 3,673 adultos reproductores. Asimismo, hemos contado un total de 1,899 juveniles de pato vapor cabeza blanca, los cuales ocurren en agregaciones regulares. El Parque Interjurisdiccional Marino Golfo San Jorge alberga alrededor del 46% de la población, lo cual podría representar una medida de protección frente a disturbios o destrucción del hábitat. Sin embargo, la contaminación por hidrocarburos y otras actividades de orígen antrópico junto con la introducción de especies, representan un potencial peligro para la población completa.

The present study focuses on the nematode community in the stomachs of three sympatric anatid bird species from the Central Patagonian coast, Argentina. The bird species include the Chubut steamer duck, Tachyeres leucocephalus, the crested duck, Lophonetta specularioides, and the black-necked swan, Cygnus melancoryphus. Up to 138 nematodes representing five species were recovered from 10 of the 13 ducks examined, with an overall prevalence of 77% and a mean intensity of 13.8. Nematodes isolated from the gizzard were Streptocara formosensis and Sciadiocara legendrei (Acuariidae) in T. leucocephalus, and Epomidiostomum vogelsangi (Amidostomatidae) in C. melancoryphus, whereas Echinuria uncinata (Acuariidae) and Tetrameres (Petrowimeres) fissispina (Tetrameriidae) were found in the proventiculus of L. specularioides. In particular, S. legendrei was registered for the first time in South America and T. leucocephalus, whereas T. fissispina represents a new record in Argentina and L. specularioides. Moreover, E. vogelsangi and E. uncinata were isolated in Patagonia for the first time. The birds studied herein are sympatric in their distribution, and two of them are syntopic breeders; however, they were infected with different parasite species. This situation could be partially due to their diet or their seasonal movements, or a combination of both. Despite the low number of hosts examined, this work enhances our knowledge about parasites from a frequently occurring group of birds on the Patagonian coast, a subject that has not been studied extensively in South America.

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Streptocara; formosensis (Nematoda: Acuariidae) is recorded for the first time from South America (Chubut Province, Argentina) and from the Chubut steamerduck, Tachyeres leucocephalus (Aves: Anatidae), enlarging its host and geographical distribution. To our knowledge, this is the first record of a parasite in this bird species.

Although several studies have analysed spatial or temporal variation in the size of eggs in waterfowl (Anseriformes), no study has analysed variation throughout the breeding range of a species. Chubut Steamer Ducks (Tachyeres leucocephalus) are flightless marine waterfowl endemic to a small section of coastline in Patagonia, Argentina. We partitioned and analysed sources of variation in the size of 989 Chubut Steamer Duck eggs, from 175 clutches, obtained during the breeding seasons of 1998 and 2004-08 at 31 islands throughout the range of the species. In relation to other Anseriformes, we found a low level of variation (coefficient of variation = 6.4%) at the species level. Most variation in the size of eggs were within clutches (59.0%) and among clutches (34.5%); variation between islands within geographical areas accounted for only 6.5% of variation, and there was no variation among geographical areas. Variation in size of eggs was not related to year, apparent clutch-size or latitude. The low variation at the species level and the lack of variation in size of eggs between geographical areas could be a consequence of the low genetic diversity and restricted distribution of the species. This study highlights the potential importance of variation in egg-size for understanding ecological processes linked to the natural history of avian species.