Avian brood parasites, including cuckoos and cowbirds, have multiple negative effects on their hosts. We analysed the effects of Shiny Cowbird Molothrus bonariensis parasitism on different components (e.g. egg losses, hatching success, chick survival and nest abandonment) of House Wren Troglodytes aedon reproductive success. We also conducted an experiment to discriminate between two mechanisms that may reduce hatching success in parasitized clutches: lower efficiency of incubation due to the increase in clutch volume and disruption of host incubation by the early hatching of Cowbirds. Egg puncturing by Shiny Cowbirds reduced host clutch size at hatching by 10–20%, and parasitized nests had a decrease in hatching success of 40–80%. Egg losses and hatching failures were positively associated with the intensity of parasitism. Brood reduction was greater in parasitized nests, but the growth rate of the chicks that fledged was similar to that in unparasitized nests. The combined effects of egg losses, hatching failures and brood reduction decreased the number of fledged chicks by 80%. In addition, egg puncturing increased the likelihood of nest abandonment by Wrens. Experimental data showed that hatching failures occurred when there was a combination of: (1) an increase in the volume of the clutch by the addition of the Cowbird egg without removal of host eggs, and (2) the addition of the Cowbird egg before the onset of incubation. This was relatively common in House Wren nests, as Cowbirds generally parasitize before the onset of incubation. Our results indicate that Shiny Cowbird parasitism imposes a major impact on House Wrens, as it affects all components of the Wren's reproductive success.

Go ´mez-Bahamo ´n et al. show that speciation is associated with changes in migratory behavior in fork-tailed flycatchers (Tyrannus savana). Divergence occurred through loss of migratory behavior of a single lineage. This mode of speciation likely occurred across New World flycatchers (Tyrannidae).

Abstract.-Little is known about the timing of migration, migration routes, and migratory connectivity of most of the >230 species of birds that breed at south temperate latitudes of South America and then migrate toward the tropics to overwinter. We used light-level geolocators to track the migration of 3 male and 3 female Fork-tailed Flycatchers (Tyrannus savana) captured on their breeding territories in Argentina. All birds initiated fall migration between late January and late February, and migrated 45 to 66 km day -1 in a northwesterly direction through central South America to either one or two wintering areas. Five individuals first spent several weeks (in April and May) in western Amazonia (mainly Peru, northwestern Brazil, and southern Colombia) before moving east to spend the rest of the non-breeding season in central Venezuela and northern Brazil. One individual occupied primarily one wintering area in eastern Colombia, northwestern Brazil, and southwestern Venezuela. Fall migration took approximately 7-12 weeks to complete and covered a distance of 2,888-4,105 km. We did not analyze spring migration data because of broad overlap with the austral spring equinox. These results are the first data on wintering locations, migration timing, and routes of individual migrant passerine birds that breed in South America. Given the general lack of similar data for practically all migratory birds that breed in South America, geolocator technology has the potential to revolutionize our understanding of how birds migrate-and the threats they face-on South America's rapidly changing landscape. La migración a Larga Distancia de Aves en América del Sur Revelado por GeolocalizadoresResumen.-Poco se sabe sobre la fenología de la migración, las rutas de migración, y la conectividad migratoria de la mayoría de los >230 especies de aves que se reproducen en latitudes templadas del sur de Sur América, y que luego migran hacia los trópicos para invernar. Utilizamos geolocalizadores para estudiar la migración de tres machos y tres hembras de Tyrannus savana capturados en sus territorios de cría en Argentina. Todas las aves iniciaron la migración de otoño entre finales de enero y finales de febrero, y migraron de 45 a 66 km dia -1 hacia el noroeste por el centro de América del Sur, hasta una o dos áreas de invernada. Cinco individuos primero pasaron varias semanas (en abril y mayo) en la Amazonia occidental (principalmente Perú, noroeste de Brasil, y sur de Colombia), antes de moverse hacia el este para pasar el resto de la temporada no reproductiva en el centro de Venezuela y el norte de Brasil. Un individuo estuvo principalmente en un área de invernada en el este de Colombia, noroeste de Brasil, y el suroeste de Venezuela. La migración de otoño duró aproximadamente 7-12 semanas, y cubrió una distancia de 2888-4105 km. No analizamos los datos de la migración de primavera a causa de una amplia superposición con el equinoccio de la primavera austral. Estos resultados representan los primeros datos sobre los lugares de invernad...

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Some brood parasites kill all their host's offspring shortly after hatching, whereas others are tolerant and are reared in mixed host-parasite broods. This difference may arise because nestling parasites face a ''provisions trade-off,'' whereby the presence of host nestlings can increase or decrease a parasite's food intake depending on whether host young cause parents to supply more extra food than they consume. We model this trade-off and show that the optimal nestmate number from a parasite's perspective depends on the interaction of 2 parameters describing a parasite's stimulative and competitive properties, relative to host young. Where these parameters differ from one host-parasite pair to the next, either nestmate killing or nestmate tolerance can be favored by natural selection for maximum intake. We show that this extends to variation between hosts of generalist parasites. In an experimental field study, we found that nestling shiny cowbirds (Molothrus bonariensis) reared by house wrens (Troglodytes aedon) had higher food intake and mass growth rate when accompanied by host young than when alone, whereas those reared by chalk-browed mockingbirds (Mimus saturninus) had higher food intake, mass growth, and survival when reared alone than with host young. In both hosts, total provisioning was higher when host nestlings were present, but only in house wrens did cowbirds secure a sufficient share of that extra provisioning to benefit from host nestlings' presence. Thus, a provisions trade-off might generate opposing selective forces on the evolution of nestmate killing not only between parasite species but also within parasite species using multiple hosts.

We investigated the association between brood parasitism by shiny cowbirds (Molothrus bonariensis), and behaviour and nest-site characteristics of chalk-browed mockingbirds (Mimus saturninus). This host builds nests on trees, it is aggressive against intruders and it is larger than shiny cowbirds. We conducted focal observations of mockingbird nests, and registered mockingbird activity and attentiveness around the nest. To characterize nest sites, we measured nest cover, nest height, and distance from the nest to the closest perch, and included host laying date and year as additional predictor variables. We also evaluated experimentally host agonistic behaviours directed towards a female cowbird and a control model, and the association between aggressive behaviour and parasitism. Nest attentiveness, nest cover and laying date were associated with parasitism. These results contradict the hostactivity hypothesis, because more attentive pairs were less parasitized, and the nest-exposure hypothesis, because more concealed nests were more parasitized. Experiments showed that unparasitized pairs were more aggressive against cowbird models than were parasitized ones. Our findings indicate that shiny cowbirds prefer to parasitize more concealed nests, where they could lay undetected by the host, and that mockingbird nest attentiveness and aggression towards cowbirds are effective first lines of defence against brood parasitism.

Many species of birds exhibit a latitudinal gradient in annual reproductive investment, laying more eggs and producing more nestlings at higher latitudes. However, few studies have evaluated the mechanisms that underlie such patterns and such differences in grassland birds specifically. We monitored nests of Fork-tailed Flycatchers (Tyrannus savana) over two breeding seasons at a tropical site in Bolivia (in 2010-11 and 2011-12) and three breeding seasons at a southern temperate site in Argentina (2010Argentina ( -11, 2011Argentina ( -12 and 2012, testing two hypotheses explaining variation in clutch-size among populations: the food-limitation hypothesis and the nest-predation hypothesis. Mean clutch-size and mean brood-size were significantly larger at the temperate study site than at the tropical site. Availability of arthropod food per individual bird was significantly higher at the temperate site. There was no relationship, positive or negative, between rates of nest predation and either clutch-or brood-size, and thus no support for the nest-predation hypothesis. We conclude that food availability explains much of the latitudinal variation in clutch-size in this species. We discuss avenues for future research on the mechanisms underlying geographical variation in the life histories of Neotropical birds.

In young birds, the intensity of the begging signal varies according to the chick's internal condition (i.e., hunger or nutritional state) but may be additionally modulated by the external conditions experienced in the nest (e.g., nestmate competition or feeding success). Flexibility of begging in response to nest environment should be especially beneficial for the chicks of generalist brood parasites, which encounter diverse brood conditions depending on the host nest in which they are reared. We investigated variability in the begging behavior of an extreme generalist brood parasite, the shiny cowbird (Molothrus bonariensis). We compared visual and vocal begging variables of cowbird young of 5 days of age reared in nests of 2 common hosts, one smaller (house wren, Troglodytes aedon) and one larger (chalk-browed mockingbird, Mimus saturninus) than the parasite. Cowbird chicks were standardized for short-term hunger prior to recording and were similar in mass/condition in each host, but nevertheless begged more intensely in mockingbird than house wren nests, as indicated by increased stretching of the neck, lower latency to beg, increased time spent begging per bout and greater call rate. These results are consistent with young cowbirds adjusting their begging intensity based on the intrabrood competition of a given host, independent of need, an adaptability that likely contributes to their success as parasites utilizing diverse host species.