Migratory birds modulate niche tradeoffs in rhythm with seasons and life history.

Movement is a key means by which animals cope with variable environments. As they move, animals construct individual niches composed of the environmental conditions they experience. Niche axes may vary over time and covary with one another as animals make tradeoffs between competing needs. Seasonal migration is expected to produce substantial niche variation as animals move to keep pace with major life history phases and fluctuations in environmental conditions. Here, we apply a time-ordered principal component analysis to examine dynamic niche variance and covariance across the annual cycle for four species of migratory crane: common crane (Grus grus, n = 20), demoiselle crane (Anthropoides virgo, n = 66), black-necked crane (Grus nigricollis, n = 9), and white-naped crane (Grus vipio, n = 9). We consider four key niche components known to be important to aspects of crane natural history: enhanced vegetation index (resources availability), temperature (thermoregulation), crop proportion (preferred foraging habitat), and proximity to water (predator avoidance). All species showed a primary seasonal niche "rhythm" that dominated variance in niche components across the annual cycle. Secondary rhythms were linked to major species-specific life history phases (migration, breeding, and nonbreeding) as well as seasonal environmental patterns. Furthermore, we found that cranes' experiences of the environment emerge from time-dynamic tradeoffs among niche components. We suggest that our approach to estimating the environmental niche as a multidimensional and time-dynamical system of tradeoffs improves mechanistic understanding of organism-environment interactions.

Longer days enable higher diurnal activity for migratory birds.

Seasonal geophysical cycles strongly influence the activity of life on Earth because they affect environmental conditions like temperature, precipitation and day length. An increase in daylight availability during summer is especially enhanced when animals migrate along a latitudinal gradient. Yet, the question of how day length (i.e. daylight availability) influences the activity patterns of long-distance, latitudinal migrants is still unclear. Here, we ask whether migration provides benefits to long-distance migrants by enabling them to increase their diurnal movement activities due to an increase in daylight availability. To answer this question, we tested whether four vastly different species of long-distance migratory birds-two arctic migrants and two mid-latitude migrants-can capitalise on day length changes by adjusting their daily activity. We quantified the relationship between daily activity (measured using accelerometer data) and day length, and estimated each species' daily activity patterns. In addition, we evaluated the role of day length as an ultimate driver of bird migration. All four species exhibited longer activity periods during days with more daylight hours, showing a strong positive relationship between total daily activity and day length. The slope of this relationship varied between the different species, with activity increasing 1.5-fold on average when migrating from wintering to breeding grounds. Underlying mechanisms of these relationships reveal two distinct patterns of daily activity. Flying foragers showed increasing activity patterns, that is, their daytime activities rose uniformly up to solar noon and decreased until dusk, thereby exhibiting a season-specific activity slope. In contrast, ground foragers showed a constant activity pattern, whereby they immediately increased their activity to a certain level and maintained this level throughout the day. Our study reveals that long days allow birds to prolong their activity and increase their total daily activity. These findings highlight that daylight availability could be an additional ultimate cause of bird migration and act as a selective agent for the evolution of migration.

Los ciclos geofísicos estacionales influyen fuertemente la actividad de la vida en la Tierra ya que afectan diversas condiciones ambientales como la temperatura, la precipitación y la duración del día. El aumento de la disponibilidad de luz solar durante el verano favorece especialmente a las especies que migran a lo largo de un gradiente latitudinal. Sin embargo, el efecto de la duración del día (es decir, la disponibilidad de luz solar) en los patrones de actividad de las especies que migran latitudinalmente largas distancias, aún no es claro. Aquí nos preguntamos si un aumento de la disponibilidad de luz solar representa un beneficio para los animales que migran largas distancias, al prolongar el periodo de actividad diurna. Para responder a esta pregunta, investigamos si cuatro especies diferentes de aves migratorias de larga distancia, dos migrantes árticos y dos migrantes de latitudes medias, pueden ajustar su actividad diaria y aprovechar los cambios en la duración del día. Cuantificamos la relación entre la actividad diaria (medida con datos de acelerómetro) y la duración del día, y estimamos los patrones de actividad de cada especie. Además, evaluamos el papel de la duración del día como mecanismo último subyacente a la migración de las aves. Las cuatro especies mostraron períodos de actividad más largos durante los días con más horas de luz, mostrando una fuerte relación positiva entre la actividad diaria total y la duración del día. Esta relación varió entre las especies evaluadas. La actividad aumentó en promedio 1.5 veces durante la migración desde las áreas de invernada a las áreas de reproducción. Los mecanismos subyacentes a estas relaciones revelan dos patrones de actividad diaria. Las especies que forrajean en vuelo mostraron un aumento en su patrón de actividad. En este caso, la actividad diurna aumentó uniformemente hasta el mediodía y disminuyó hasta el atardecer, mostrando una pendiente de actividad específica para la estación. De otro lado, las especies que forrajean en tierra mostraron un patrón de actividad constante. Según este patrón, la actividad diurna aumenta hasta un determinado nivel, a partir del cual se mantiene durante el resto del día. Nuestro estudio revela que el aumento en la longitud del día le permite a las aves prolongar su actividad e incrementar su actividad diaria total. Estos resultados señalan que la disponibilidad de luz diurna podría ser otro mecanismo último subyacente a la migración de las aves y puede actuar como un factor de selección en la evolución de la migración.

Bio-logging, new technologies to study conservation physiology on the move: a case study on annual survival of Himalayan vultures.

Bio-logging, the on-animal deployment of miniaturised electronic data recorders, allows for the study of location, body position, and physiology of individuals throughout their ontogeny. For terrestrial animals, 1 Hz GPS-position, 3D-body acceleration, and ambient temperature provide standard data to link to the physiology of life histories. Environmental context is added at ever finer scales using remote sensing earth observation data. Here we showcase the use of such bio-logging approaches in a conservation physiology study on endangered Himalayan vultures (Gyps himalayensis). We determine environmental, behavioural, and physiological causes of survival in immature birds that roam from wintering sites in India, Bhutan, and Nepal towards summer areas in Tibet and Mongolia. Five of 18 immature griffons died during one year. Individuals that died had failed to migrate sufficiently far northward (>1500 km) in spring. Individuals likely died if they flew against headwinds from the north or were less able to find thermal updrafts. Surviving individuals migrated to cold and dry areas with low population density. We highlight flight experience, long distance movements, and remote places with low human population as factors critical for the survival of Himalayan vultures. High-resolution bio-logging studies can advance conservation management by pinpointing where and why migratory animals have problems and die.

Behavioural adaptations to flight into thin air.

Soaring raptors can fly at high altitudes of up to 9000 m. The behavioural adjustments to high-altitude flights are largely unknown. We studied thermalling flights of Himalayan vultures (Gyps himalayensis) from 50 to 6500 m above sea level, a twofold range of air densities. To create the necessary lift to support the same weight and maintain soaring flight in thin air birds might modify lift coefficient by biophysical changes, such as wing posture and increasing the power expenditure. Alternatively, they can change their flight characteristics. We show that vultures use the latter and increase circle radius by 35% and airspeed by 21% over their flight altitude range. These simple behavioural adjustments enable vultures to move seamlessly during their annual migrations over the Himalaya without increasing energy output for flight at high elevations.

Erhaia Davis & Kuo (Gastropoda, Rissooidea, Amnicolidae) also in Bhutan.

The occurrence of at least one species of Erhaia in Bhutan, viz. Erhaia wangchukisp. n., is confirmed by DNA sequencing. A second unnamed species from Bhutan, that might be congeneric, is known from only a single shell. According to the molecular analysis, E. wangchuki is most closely related to a still undescribed Erhaia species from China. These two species together with E. jianouensis and Akiyoshia kobayashii, both also from China, form a well supported clade. Awaiting additional molecular data, the apparent inconsistency regarding Erhaia versus Akiyoshia is not dealt with here. The extant true sister species of E. wangchuki could be among the four SE Himalayan species from Bhutan and Nepal that are classified with Erhaia on the basis of conchological data only.