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Resumen.-Los transmisores satelitales y los aparatos con sistemas de posicionamiento geográfico frecuentemente agregan un peso substancial a las aves a las que se los sujetan. Los estudios sobre los efectos de estos instrumentos se han enfocado en medidas indirectas, mientras que la influencia directa del peso extra en el comportamiento pelágico es poco conocida. Empleamos localizadores geográficos de 2.5 g para investigar el efecto del peso extra sobre el comportamiento pelágico de Calonectris diomedea mediante la comparación de los rasgos de un único viaje de forrajeo entre un grupo de aves que portaba pesos de 30 g, un grupo que portaba pesos de 60 g y un grupo control. Los pesos fueron colocados sobre la espalda de las aves usando las técnicas típicas para sujetar los transmisores satelitales a las aves marinas. El peso extra incrementó la duración de los viajes de las aves y disminuyó su eficiencia de forrajeo y el peso corporal adquirido en el mar. Estos efectos indirectos pueden estar relacionados con los rasgos de forrajeo: las aves con pesos suplementarios mostraron un esfuerzo de búsqueda mayor que las aves control, viajaron distancias más grandes, cubrieron un área de forrajeo mayor e incrementaron el ámbito máximo de forrajeo. Más aún, el tiempo pasado sobre la superficie del mar durante la noche fue mayor para los grupos con peso suplementario que para los grupos control, lo que mostró que el peso suplementario también afectó los patrones de actividad. Nuestros resultados destacan la necesidad de cuantificar los efectos del equipo de monitoreo usado comúnmente para estudiar el comportamiento pelágico de las aves marinas. Sugerimos que los localizadores geográficos pueden ser usados para obtener datos de control de los movimientos de los viajes de forrajeo y de los patrones de actividad.-100 - Abstract.-Satellite transmitters and geographic-positioning-system devices often add substantial mass to birds to which they are attached. Studies on the effects of such instruments have focused on indirect measures, whereas the direct influence of extra mass on pelagic behavior is poorly known. We used 2.5-g geolocators to investigate the effect of extra mass on the pelagic behavior of Cory's Shearwaters (Calonectris diomedea) by comparing the traits of a single foraging trip among a group carrying 30-g weights, a group carrying 60-g weights, and a control group. The weights were attached to the birds' backs using typical techniques for attaching satellite transmitters to seabirds. The extra mass increased the duration of the birds' trips and decreased their foraging efficiency and mass gained at sea. These indirect effects may be related to foraging traits: weighted birds showed a greater search effort than control birds, traveled greater distances, covered a greater foraging area, and increased the maximum foraging range. Furthermore, the time spent on the sea surface at night was greater for weighted than for control groups, which showed that the extra mass also affected activity patterns. Our results under...

The intensity of mating competition and the potential benefits for female of mating with certain males can be influenced by several extrinsic factors, such that behavioral decisions can be highly context-dependent. Short-lived species with a single reproductive season are a unique model to study context-sensitive mating decisions. Through exhaustive sampling in the field and simultaneous choice tests in the laboratory, we evaluated operational sex ratio (OSR) and female mate choice at the beginning and end of the reproductive season in the annual killifish Austrolebias reicherti. We found seasonal change in both OSR and female mate choice. At the start of the reproductive season the OSR did not deviate from parity, and females preferred larger males. Later in the reproductive season, while the proportion of males in the ponds decreased, females became unselective with respect to male size. The particular biological cycle of annual killifish, where both life expectancy and mating opportunities decline sharply over a short timescale, could account for the seasonal change in female choice. Reduction in choosiness could arise from diminished reproductive prospects due to a decline in male availability. Moreover, as the end of the season approaches, any benefits of choosiness are presumably reduced: a female’s fitness will be higher if she mates with any male than if she forgoes reproduction and dies. Future work will disentangle the mechanisms underlying seasonal changes in mating preferences, notably direct responses to demographic factors, environmental cues, or intrinsic changes during development.