The role of wingbeat frequency and amplitude in flight power.

Body-mounted accelerometers provide a new prospect for estimating power use in flying birds, as the signal varies with the two major kinematic determinants of aerodynamic power: wingbeat frequency and amplitude. Yet wingbeat frequency is sometimes used as a proxy for power output in isolation. There is, therefore, a need to understand which kinematic parameter birds vary and whether this is predicted by flight mode (e.g. accelerating, ascending/descending flight), speed or morphology. We investigate this using high-frequency acceleration data from (i) 14 species flying in the wild, (ii) two species flying in controlled conditions in a wind tunnel and (iii) a review of experimental and field studies. While wingbeat frequency and amplitude were positively correlated, R2 values were generally low, supporting the idea that parameters can vary independently. Indeed, birds were more likely to modulate wingbeat amplitude for more energy-demanding flight modes, including climbing and take-off. Nonetheless, the striking variability, even within species and flight types, highlights the complexity of describing the kinematic relationships, which appear sensitive to both the biological and physical context. Notwithstanding this, acceleration metrics that incorporate both kinematic parameters should be more robust proxies for power than wingbeat frequency alone.

Young frigatebirds learn how to compensate for wind drift.

Compensating for wind drift can improve goalward flight efficiency in animal taxa, especially among those that rely on thermal soaring to travel large distances. Little is known, however, about how animals acquire this ability. The great frigatebird (Fregata minor) exemplifies the challenges of wind drift compensation because it lives a highly pelagic lifestyle, travelling very long distances over the open ocean but without the ability to land on water. Using GPS tracks from fledgling frigatebirds, we followed young frigatebirds from the moment of fledging to investigate whether wind drift compensation was learnt and, if so, what sensory inputs underpinned it. We found that the effect of wind drift reduced significantly with both experience and access to visual landmark cues. Further, we found that the effect of experience on wind drift compensation was more pronounced when birds were out of sight of land. Our results suggest that improvement in wind drift compensation is not solely the product of either physical maturation or general improvements in flight control. Instead, we believe it is likely that they reflect how frigatebirds learn to process sensory information so as to reduce wind drift and maintain a constant course during goalward movement.

Development of flight and foraging behaviour in a juvenile seabird with extreme soaring capacities.

The early life of animals is a period of high mortality, when foraging capacities are assumed to be improved progressively. In birds, this critical period involves the improvement of the flight. How do young birds gain these capacities has rarely been studied in natural conditions especially in seabirds that spend most of their life at sea. We used detailed GPS and body acceleration data on 37 great frigatebirds (Fregata minor), to test the hypothesis that juveniles starting their first flights have lower flying capacities than adults, but that these capacities will improve during a long learning period, before independence from parents, specific to this seabird that can spend months on the wing at sea. We found that most flight components improved over time to tend towards those of adults, especially the travel speed, range, duration and maximum altitude of trips. However, unexpectedly, juveniles had higher ascent rates, soaring and gliding capacities above the sea than adults. Moreover, energy expenditure of juveniles was similar to adults during low cost travelling movements and during active foraging, but juveniles spent more time foraging actively than adults. Our results suggest that flight tactics based on long-distance effortless movements specific to this family are acquired during a long period, but soaring and gliding capacities are already inherited by juveniles and possibly favoured by morphological adaptations specific to juveniles. These adaptations might explain the extreme dispersive capacities of juveniles.

Cyclone avoidance behaviour by foraging seabirds.

In the context of climate change, how extreme climatic events, such as cyclones, will affect the foraging abilities of marine vertebrates is still poorly known. During the course of a study on the foraging behaviour of two tropical seabirds, red-footed boobies and great frigatebirds, several cyclones have affected their breeding grounds and foraging zones, allowing us to study their response to extreme wind conditions. We examined whether adults and young naïve birds were able to predict the arrival of a cyclone and behave accordingly to reduce mortality risks and optimise foraging. We show that when a cyclone approached, juveniles and adults of the two species differed in their decisions to leave the colony for the sea. When the winds reached gale force, the juveniles of both species and adult frigatebirds remained at the colony, whereas adult boobies continued their foraging routine. The mortality of the individuals remaining on land remained limited. When encountering at-sea gale conditions, adult birds were able to avoid the centre of the low pressure systems and moved westward to bypass the route of the cyclones and circumvent the moving cyclone. Frigatebirds climb to high altitudes when close to the eye of the cyclone to bypass it at high speeds. These movements likely reduce the mortality risk at sea but can temporarily cause birds to move outside their normal range at sea or over land masses. We discuss the potential consequences of an increase in cyclonic conditions on seabird populations.

Ontogeny of foraging behaviour in juvenile red-footed boobies (Sula sula).

The early life stages represent a crucial period that can strongly influence population dynamics. We studied the development of foraging behaviour in the red-footed booby, a tropical seabird with an extensive post-fledging care period (3 to 6 months). Adults and juveniles were observed from shore and tracked at sea using GPS loggers over 3 consecutive 12-day periods. Juveniles initially made a majority of flights inland, likely to practice flying, and formed groups of up to 10 juveniles before making short trips at sea. They left the island later and returned earlier than the adults, allowing them to be fed on the nest. Over time, juveniles left the colony alone more frequently and increased the range of their trips while remaining significantly closer to the colony than the adults. They spent more time intensively foraging (slow and sinuous trajectory) than adults, which could reflect attempts to capture prey. Juveniles foraged independently of their parents but associated frequently with congeners, particularly during area-restricted search (ARS) behaviour. The extensive post-fledging care period observed may be explained by the need to develop proper foraging skills adapted to tropical waters, where resources are particularly scarce and unpredictable.

Frigate birds track atmospheric conditions over months-long transoceanic flights.

Understanding how animals respond to atmospheric conditions across space is critical for understanding the evolution of flight strategies and long-distance migrations. We studied the three-dimensional movements and energetics of great frigate birds (Fregata minor) and showed that they can stay aloft for months during transoceanic flights. To do this, birds track the edge of the doldrums to take advantage of favorable winds and strong convection. Locally, they use a roller-coaster flight, relying on thermals and wind to soar within a 50- to 600-meter altitude band under cumulus clouds and then glide over kilometers at low energy costs. To deal with the local scarcity of clouds and gain longer gliding distances, birds regularly soar inside cumulus clouds to use their strong updraft, and they can reach altitudes of 4000 meters, where freezing conditions occur.

Vultures of the seas: hyperacidic stomachs in wandering albatrosses as an adaptation to dispersed food resources, including fishery wastes.

Animals are primarily limited by their capacity to acquire food, yet digestive performance also conditions energy acquisition, and ultimately fitness. Optimal foraging theory predicts that organisms feeding on patchy resources should maximize their food loads within each patch, and should digest these loads quickly to minimize travelling costs between food patches. We tested the prediction of high digestive performance in wandering albatrosses, which can ingest prey of up to 3 kg, and feed on highly dispersed food resources across the southern ocean. GPS-tracking of 40 wandering albatrosses from the Crozet archipelago during the incubation phase confirmed foraging movements of between 475-4705 km, which give birds access to a variety of prey, including fishery wastes. Moreover, using miniaturized, autonomous data recorders placed in the stomach of three birds, we performed the first-ever measurements of gastric pH and temperature in procellariformes. These revealed surprisingly low pH levels (average 1.50±0.13), markedly lower than in other seabirds, and comparable to those of vultures feeding on carrion. Such low stomach pH gives wandering albatrosses a strategic advantage since it allows them a rapid chemical breakdown of ingested food and therefore a rapid digestion. This is useful for feeding on patchy, natural prey, but also on fishery wastes, which might be an important additional food resource for wandering albatrosses.