ABSTRACT. Methods used to study parental behavior are largely idiosyncratic and are rarely tested to verify that they yield reliable data. In particular, the minimum number and duration of observations used to describe behavior accurately is rarely, if ever, justified. Using data on parental behavior of Eastern Kingbirds (Tyrannus tyrannus) collected by video in Oregon, we evaluated three questions regarding sampling methods: (1) Is within-pair behavior sufficiently consistent among days so that observations from a single session are representative of parental behavior? (2) Does a 1-h sampling window fairly represent parental behavior or is a longer period needed to accurately describe behavior? (3) Assuming that a 1-h period is sufficient, does the first hour immediately after disturbance to start recordings yield reliable data? We found that among-pair differences in parental behavior were consistent over different days and that a single hour of observation was representative of parental behavior over a longer (2-3 h) period of observation. However, our data also suggested that the first hour yielded less representative data than the remaining 2-3 h; nonetheless, consistent among-pair differences were still detectable. The shortcoming of data collected in the first hour appeared to be due to a lingering effect of human disturbance from setting up recording devices rather than a time-of-day effect. We suggest that researchers test the appropriateness of their methods, but our kingbird data suggest that observations over relatively brief periods can yield useful and reliable data. RESUMEN. Cuantificación del cuidado parental en aves: cual es el mínimo tamaño de muestra necesario?Los métodos usados para el estudio del cuidado parental son mayormente idiosincráticos y raramente son confrontados para verificar que producen datos confiables. En particular, el numero mínimo y la duración de las observaciones usadas para describir el comportamiento es raramente justificado. Usando datos de cuidado parental en Tyrannus tyrannus colectados en video en Oregon, evaluamos tres preguntas correspondientes a los métodos de muestreo: (1) ¿Es el comportamiento dentro de la pareja lo suficientemente consistente a lo largo de los días como para que observaciones de unaúnica sesión representen el cuidado parental? (2) ¿Puede una ventana de muestreo de una hora representar moderadamente bien el cuidado parental o se necesitan periodos mas largos para describir el comportamiento con precisión? (3) Asumiendo que un periodo de una hora es suficiente; ¿Puede la primera hora inmediatamente después del disturbio al empezar las grabaciones producir datos confiables? Encontramos que las diferencias entre las parejas en el cuidado parental es consistente a lo largo de diferentes días y que una hora de observación era representativo del cuidado parental a través de un periodo de observación mayor (2 a 3 horas). Sin embargo, nuestros datos también sugieren que la primera hora de observación produjo datos menos representativos que las siguiente...

The relative importance of predators and resources (i.e. food) for the dynamics of migratory bird populations is poorly known. Resource availability may be more likely in resource poor environments, but given that nest failure in most systems is due mainly to predation, predator effects may predominate. We document a rapid decline of an isolated eastern kingbird Tyrannus tyrannus population breeding in the Great Basin Desert of eastern Oregon, USA, and evaluate whether it was driven by limited food resources (water availability ~ food), nest predation, or first‐year or adult return rate (RRJ and RRA, respectively) that reflect nonbreeding season events. Most nests failed (~68% of nests) due mainly to nest predation (> 90% of failures); nestling starvation was rare. Bioyear precipitation (October–April), breeding season precipitation, and river flow all varied widely but none could account for annual variation in either nest success (NS) or fledging success of successful nest (FSSN). Neither RRJ nor RRA varied with year, any measure of water resources, or reproductive success. Annual population growth rate (ln[Nt+1/Nt]) was independent of year, all measures of water availability, RRJ and RRA, and NS of replacement nests, but was high following years of high NS of initial nests; FSSN was also higher in years of high NS. High rates of nest predation, due likely to the expansion of the local American crow Corvus brachyrhynchos population, appeared to drive the population decline. Empirical data and population simulations indicated that replacement nests contributed little to λ, while simulations revealed that, given site‐specific vital rates, the decline would have been even greater without immigration (λ = 0.812) than the observed (λ = 0.914). Long distance dispersal may thus be critical for persistence of kingbirds in the naturally highly fragmented habitat that it occupies in the Great Basin Desert.

We documented parental behaviour and paternity of eastern kingbirds (Tyrannus tyrannus) to test the predictions that paternal care would decline with increasing loss of paternity, increasing nesting density (a proxy for probability of paternity loss), male quality, and number of fertile females available in the population. Extra-pair young were found in 58% of 45 nests for which behaviour was recorded and a higher proportion of young were extra-pair as nesting density increased. Male feeding rate declined with increasing nesting density and male quality, but neither feeding rate nor a composite measure of paternal behaviour varied with number of fertile females or paternity. Although alternative explanations exist, one interpretation of the reduced paternal care at high nesting density was that it was a response to perceived threats of paternity loss. The ultimate basis for the lower paternal effort of higher quality males is unclear but we discuss several possible explanations.

For many bird species, but especially aerial insectivores, reproduction depends on weather. Climate change is likely to intensify effects, but with uncertain consequences. We report 22 years of data on Eastern Kingbird (Tyrannus tyrannus) reproduction for two populations located in different hygric environments undergoing climate change; mesic central New York, USA, (NY; 12 years) and xeric southeastern Oregon, USA, (OR: 10 years). Laying date became earlier with increasing temperature in the 30-day period preceding laying in identical fashion at both sites, and in years of early laying, clutch size was larger, length of laying season increased, and failed initial nesting attempts were more often replaced. High temperature in the 10-days preceding mean laying date was associated with shorter laying seasons, while a site by 10-day temperature interaction reflected an increase and decrease of clutch size with increasing 10-day temperature in NY and OR, respectively. Seasonal rate of clutch size decline was higher when the laying season was short but also slowed in xeric OR when rain was abundant in the 10-days prior to mean laying date. Nest predation drove annual variation in young fledged/nest, but the latter also increased and decreased with increasing maximum temperature during the nestling phase in mesic NY and xeric OR, respectively. Potential effects of climate change on kingbird populations are thus high given the dependence of reproduction on weather, and climate change likely contributed to declines of kingbirds in OR. Declines of kingbirds in NY appear unrelated to warming climates because higher temperatures advanced laying dates and yielded greater nest productivity. However, length of laying season declined across years at both sites, and thus early season gains may be negated by poor conditions late in the season that may be causing shorter laying seasons. Further work is needed to identify causes for the latter changes.Les conditions météorologiques et les changements climatiques entraînent la variation annuelle de la reproduction d'un insectivore aérien responsables des saisons de ponte plus courtes. Des travaux supplémentaires sont nécessaires à ce sujet pour qu'on puisse identifier les causes.