Diego T. Tuero

Long-Distance Bird Migration within South America Revealed by Light-Level Geolocators

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 Americas rapidly changing landscape.

Host behaviour and nest-site characteristics affect the likelihood of brood parasitism by shiny cowbirds on chalk-browed mockingbirds

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 host-activity 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.

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.

Dense canopy cover over House Wren (Troglodytes aedon) nests increases latency of brood parasitism by Shiny Cowbirds (Molothrus bonariensis)

Abstract Obligate brood parasites must find host nests in which to lay their eggs. The search for a suitable host nest is predicted to be more difficult if the host nest is well-concealed by vegetation (nest-concealment hypothesis) and brood parasitism of better concealed nests should be less common than parasitism of less well-concealed nests. We experimentally tested this hypothesis by placing nest-boxes used by House Wrens (Troglodytes aedon) in woodland sites with sparse and dense canopy cover and measuring the frequency (proportion of nests parasitised), latency (days elapsed since laying of the first host egg and parasitism), and intensity (number of parasite eggs in parasitised nests) of parasitism by Shiny Cowbirds (Molothrus bonariensis). The frequency and intensity of parasitism did not differ between sites with sparse or dense canopy cover, but the latency of parasitism was shorter in sites with sparse cover than in sites with denser cover. Brood parasites that find host nests more quickly have higher reproductive success, whereas House Wrens have higher reproductive success when parasites lay their eggs later during the incubation period, owing to reduced mortality of host nestlings. Because woodland degradation is associated with sparser canopy cover, host species nesting in degraded woodlands may suffer more from parasitism than those nesting in protected woodlands.

Drivers of clutch-size in Fork-tailed Flycatchers (Tyrannus savana) at temperate and tropical latitudes in South America

Abstract 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 (2010–11, 2011–12 and 2012–13), 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.

Molting while breeding? Lessons from New World Tyrannus Flycatchers

Abstract Songbirds must annually undergo two energetically demanding but important activities: breeding and feather molt. Due to the high energetic investment that each demands, these two events are generally not carried out simultaneously. However, substantial variation in the level of annual reproductive investment among populations may result in variation in molt-breeding overlap between them. With the goal of understanding whether different songbird populations overlap molt and breeding, and, if so, to determine directions for research on the potential tradeoffs involved, we describe the relationship between clutch size, molt, and energetic condition within a genus of New World Flycatchers (Tyrannus). Of 219 Flycatchers sampled, only one individual molted flight feathers while breeding, suggesting that molting flight feathers and breeding simultaneously is too energetically expensive at any clutch size. However, some Flycatchers molted body feathers during the breeding season. When we tested for an effect of clutch size, sex and energetic condition on body molt intensity during the breeding season, only clutch size and sex had significant effects, with a negative effect of clutch size on body molt intensity in males but not in females. Based on these results, we develop a set of hypotheses to guide future studies on the potential tradeoffs between investment in reproduction and molt.ZusammenfassungMauser während der Brutzeit? Was machen die neuweltlichenTyrannusFliegenschnäpper? Singvögel müssen zweimal im Jahr energetisch aufwändige, aber wichtige Prozesse durchlaufen: Brutzeit und Mauser. Da beide Aktivitäten hohe energetische Investitionen erfordern, laufen sie üblicherweise nicht gleichzeitig ab. Aber größere Unterschiede zwischen einzelnen Populationen in den jährlichen Fortpflanzungsinvestitionen haben möglicherweise Überlappungen von Mauser mit Brutzeit zur Folge. Ziel unserer Untersuchung war es, besser zu verstehen, ob und wie sehr sich Mauser und Brutzeit bei unterschiedlichen Singvogel-Populationen überschneiden, und festzuhalten, in welche Richtung zukünftige Untersuchungen der möglichen Vor- und Nachteile (tradeoffs) gehen sollten. Hierfür beschreiben wir für eine Gattung von Fliegenschnäppern (Tyrannus) aus der Neuen Welt die Zusammenhänge zwischen Gelegegröße, Mauser und energetischem Zustand. Von den 219 untersuchten Fliegenschnäppern zeigte nur ein einziges Tier whärend der Brutzeit auch Mauser an den Schwungfedern, was nahelegt, dass unabhängig von der Gelegegröße die Mauser der Schwungfedern und gleichzeitiges Brüten energetisch zu teuer ist. Dessen ungeachtet mauserten einige Fliegenschnäpper während der Brutzeit ihr Körpergefieder. Als wir untersuchten, ob es mögliche Auswirkungen der Gelegegröße, der sexuellen Aktivitäten und des energetischen Zustands der Vögel auf die Intensität ihrer Mauser während der Brutzeit gibt, zeigten nur die Gelegegröße und sexuellen Aktivitäten einen signifikanten Effekt; es gab einen negativen Effekt der Gelegegröße auf die Intensität der Mauser des Körpergefieders bei Männchen, nicht aber bei Weibchen. Auf der Basis dieser Befunde stellen wir eine Reihe von Hypothesen auf als Anregung für zukünftige Untersuchungen des potentiellen tradeoffs zwischen der Investition in Fortpflanzung und Mauser.

Egg pecking and puncturing behaviors in shiny and screaming cowbirds: effects of eggshell strength and degree of clutch completion

Shiny cowbirds (Molothrus bonariensis) are generalist brood parasites that use hosts varying in body and egg size. On the contrary, screaming cowbirds (M. rufoaxillaris), which are larger than shiny cowbirds, are host specialist that use mainly one host of similar body and egg size. Both parasites peck and puncture eggs when visiting nests. Through puncturing eggs, cowbirds can reduce the competition for food their chicks face (reduction of competition hypothesis), but the same behavior could also be a mechanism to enforce host to renest when nests are found late in the nesting cycle (farming hypothesis). Eggshell strength increases the difficulty to puncture eggs and therefore may modulate egg-pecking behavior. To test these hypotheses, we studied the effect of the degree of clutch completion and egg size on egg-puncturing behavior. Moreover, we evaluated if morphological differences between cowbird species and eggshell strength affected egg-pecking behavior. We presented captive females a nest with complete (four eggs) or incomplete (one egg) clutches of house wren (small egg size, low eggshell strength), chalk-browed mockingbird (large egg size, intermediate eggshell strength), or shiny cowbird (medium egg size, high eggshell strength). The proportion of nests with punctured eggs was similar for complete and incomplete clutches. Cowbirds punctured more eggs in complete than in incomplete clutches, but in complete clutches, they did not destroy the entire clutch. There were no differences in the egg-pecking behavior between cowbird species, which pecked more frequently the eggs with the strongest eggshell. Our findings are consistent with the reduction of competition hypothesis.Significance statementBrood parasitic birds do not build nests and raise their chicks. Instead, they lay eggs in nests of other species (hosts), which carry out all parental care. Some brood parasites, like the cowbirds, peck and puncture eggs when they visit host nests. This behavior may help to reduce the competition for food that their chicks face in the nest (reduction of competition hypothesis) or may enforce hosts to renest (farming hypothesis). We experimentally studied egg-pecking and egg-puncturing behaviors in the host generalist shiny cowbird and the host specialist screaming cowbird. We found that the degree of clutch completion and egg size modulate egg-puncturing behavior and eggshell strength modulates egg-pecking behavior. Our results indicate that by puncturing eggs, cowbirds reduce nest competition.

Follow the rain? Environmental drivers of Tyrannus migration across the New World

ABSTRACT Predictable seasonal changes in resources are thought to drive the timing of annual animal migrations; however, we currently understand little about which environmental cues or resources are tracked by different migratory bird species across the planet. Understanding which environmental cues or resources birds track in multiple migratory systems is a prerequisite to developing generalizable conservation plans for migratory birds in a changing global environment. Within the New World, climatic differences experienced by Nearctic–Neotropical migratory (NNM; i.e. breed in North America and spend the nonbreeding period in the Neotropics) and Neotropical austral migratory (NAM; i.e. breed and spend the nonbreeding period wholly within South America) bird species suggest that their migratory strategies may be shaped by unique selective pressures. We used data gathered from individuals fitted with light-level geolocators to build species distribution models (SDMs) to test which environmental factors drive the migratory strategies of species in each system. To do so, we evaluated whether temperature, precipitation, and primary productivity (NDVI) were related to the seasonal distributions of an NNM (Eastern Kingbird [Tyrannus tyrannus]) and NAM species (Fork-tailed Flycatcher [T. savana]). Both Eastern Kingbird and Fork-tailed Flycatcher locations were positively correlated with high precipitation during their nonbreeding seasons. Eastern Kingbird locations were positively correlated with both NDVI and temperature during their breeding season and both pre- and post-breeding migrations. Fork-tailed Flycatcher locations were positively correlated with both temperature and precipitation during both migrations, but only temperature during the breeding season. The value of extending the application of geolocator data, such as in SDMs, is underscored by the finding that precipitation was such an important predictor of the nonbreeding distributions of both types of migrants, as it remains unclear how global climate change will affect wet–dry cycles in the tropics.

Ecological determinants of Tyrannus flycatcher nestling growth at north- and south-temperate latitudes

ABSTRACT An organisms life history strategy is made up of a suite of physiological, behavioral, and ecological traits, which vary at both the interspecific and intraspecific levels in accordance with selective pressures operating on individuals. For birds, 2 primary ecological factors have been proposed to explain intraspecific and interspecific variation in nestling growth: nest predation and food availability. Individual nestling growth rates have important consequences for overall fitness because growth speed could influence subsequent reproductive performance and survival. We studied the relationship between ecological factors (i.e. precipitation level and predation rate) and nestling growth patterns of 2 New World flycatcher species (Tyrannidae) of the genus Tyrannus (Fork-tailed Flycatcher [T. savana] and Scissor-tailed Flycatcher [T. forficatus]) breeding at south- and north-temperate latitudes. We tested the hypothesis that nestling growth rates are driven by nest predation rates and predicted that nestling growth rates would be higher in species experiencing higher nest predation rates. We also tested the hypothesis that nestling growth rates are related to precipitation levels (a proxy for food abundance) and predicted that nestling growth rates would be higher at sites with higher precipitation levels. Growth rate was not associated with predation rate, but it varied with precipitation level, with faster nestling growth rates during wet years for the Scissor-tailed Flycatcher living at north-temperate latitudes. Among species, similar growth rates were found during wet years. These results indicate that, at least as proximate causes, precipitation explains intraspecific and interspecific growth rate variation in Tyrannus species to a larger degree than predation. Additionally, the variation in growth rate we observed between wet and dry years indicates a high level of plasticity in growth rate in this group of insectivorous birds.

Parasite Adaptations During the Nestling and Fledgling Stages

Young of altricial brood parasites are fully dependent on their foster parents for a considerable period of time before being able to survive on their own. Therefore, they are expected to exhibit traits that allow them to avoid host defences and manipulate host’s parental behaviour to their favour. Many morphological, physiological and behavioural traits have been proposed as adaptations for brood parasitism based on their apparent selective advantage for parasitic chicks. In this chapter, we describe and discuss these putative adaptations to examine, in the light of available evidence, whether or not those traits have evolved specifically to increase parasites’ fitness. We show that whereas some adaptations are well-supported and indisputable, other traits require a closer scrutiny taking into account the parasite’s evolutionary history before concluding that they are true adaptations for brood parasitism. We propose some future directions for research on key adaptations to parasitism and coevolutionary interactions between parasites and their hosts during the nestling and fledgling stages.