Biology subjects

Shepard, E. L. C.

Publications and source records attributed to Shepard, E. L. C..

5 recordsLinked to original sources

bioRxiv2024

Changing air density affects flight costs in low-flying birds

Regional patterns in wind underpin the low-cost migratory flyways of billions of birds and insects1-3, but how large-scale changes in temperature affect flight is unknown. Flight costs should increase with rising temperatures, because lift decreases as density decreases, whereas weight remains unchanged. The effects of air density on flight costs are well-established in the context of high-altitude movements and migration4-7. Here, we examine the impact of air density on low-flying birds, in relation to seasonal, regional and global changes in temperature. Using multi-sensor loggers, we find that air density was the most important predictor of wingbeat frequency in red-tailed tropicbirds (Phaethon rubricauda) breeding year-round in Mauritius. Lower air densities in the Austral summer were associated with a small but significant increase in mean wingbeat frequency, which translated to an estimated 1-2% increase in flight costs. The variation in flight costs increased by an order of magnitude when considered in space, rather than time, with flight costs varying by [≥] 10 % across the tropicbirds range. Changes in air density can therefore be an important determinant of flight costs even when birds are operating close to sea-level. Indeed, mapping air density at sea-level revealed that global temperature gradients cause effective altitude to vary by >2 km when considered as seasonal averages. This "invisible topography" at sea-level could have influenced the biogeography of flight morphologies and life-history traits.

zoology↗

bioRxiv2023

Stealth hunters: landing force predicts hunting success in Barn Owls

Predator-prey arms races have led to the evolution of finely-tuned disguise strategies. While the theoretical benefits of predator camouflage are well established, no study has yet been able to quantify its consequences for hunting success in natural conditions. We used high-resolution movement data to quantify how barn owls (Tyto alba) conceal their approach when using a sit-and-wait strategy. We hypothesized that hunting barn owls would modulate their landing force, potentially reducing noise levels in the vicinity of prey. Analysing 87,957 landings by 163 individuals equipped with GPS tags and accelerometers, we show that barn owls reduce their landing force as they approach their prey, and that landing force predicts the success of the following hunting attempt. Landing force also varied with the substrate, being lowest on man-made poles in field boundaries. The physical environment therefore affects the capacity for sound camouflage, providing an unexpected link between predator-prey interactions and land-use. Finally, hunting strike forces in barn owls were the highest recorded in any bird, relative to body mass, highlighting the range of selective pressures that act on landings and the capacity of these predators to modulate their landing force. Overall, our results provide the first measurements of landing force in a wild setting, revealing a new form of motion-induced sound camouflage and its link to hunting success.

animal behavior and cognition↗

bioRxiv2022

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.

biophysics↗

bioRxiv2022

Pelagic seabirds reduce risk by flying into the eye of the storm

Cyclones can cause mass mortality of seabirds, sometimes wrecking thousands of individuals. The few studies to track pelagic seabirds during cyclones show they tend to circumnavigate the strongest winds. We tracked adult shearwaters in the Sea of Japan over 11 years and find that the response to cyclones varied according to the wind speed and direction. In strong winds, birds that were sandwiched between the storm and mainland Japan flew away from land and towards the eye of the storm, flying within [≤] 30 km of the eye and tracking it for up to 8 hours. This exposed shearwaters to some of the highest wind speeds near the eye wall ([≤] 21 m s-1), but enabled them to avoid strong onshore winds in the storms wake. Extreme winds may therefore become a threat when an inability to compensate for drift could lead to forced landings and collisions. Birds may need to know where land is in order to avoid it. This provides additional selective pressure for a map sense and could explain why juvenile shearwaters, which lack a map sense, instead navigating using a compass heading, are susceptible to being wrecked. We suggest that the ability to respond to storms is influenced by both flight and navigational capacities. This may become increasingly pertinent due to changes in extreme weather patterns. Significance StatementCyclones can cause billions of dollars of damage and loss of human life. They can also cause mass mortality and strandings in seabirds. We used GPS tracking data from streaked shearwaters breeding in the worlds most active cyclone basin to understand how seabirds respond to these systems. Birds varied their response according to the wind speed and direction, generally flying towards the eye of the cyclone in strong winds. This surprising strategy enables dynamic soaring birds to control their exposure to risky wind vectors that could drift them onshore. Nonetheless, birds may need to know where land is in order to avoid it. Juveniles lack this "map sense", making them susceptible to wrecking in some scenarios.

ecology↗

bioRxiv2019

The challenges of estimating the distribution of flight heights from telemetry or altimetry data

BackgroundGlobal positioning systems (GPS) and altimeters are increasingly used to monitor vertical space use by aerial species, a key aspect of their niche that we need to know to understand their ecology and conservation needs, and to manage our own use of the airspace. However, there are various sources of error in flight height data (\"height\" above ground, as opposed to \"altitude\" above a reference like the sea level): vertical error from the devices themselves, error in the ground elevation below the tracked animals, and error in the horizontal position of the animals and thus the predicted ground elevation below them.\n\nMethodsWe used controlled field trials, simulations, and the reanalysis of raptor case studies with state-space models to illustrate the effect of improper error management.\n\nResultsErrors of a magnitude of 20 meters appear in benign conditions (expected to be larger in more challenging context). These errors distort the shape of the distribution of flight heights, inflate the variance in flight height, bias behavioural state assignments, correlations with environmental covariates, and airspace management recommendations. Improper data filters such as removing all negative recorded flight height records introduce several biases in the remaining dataset, and preclude the opportunity to leverage unambiguous errors to help with model fitting. Analyses that ignore the variance around the mean flight height, e.g., those based on linear models of flight height, and those that ignore the variance inflation caused by telemetry errors, lead to incorrect inferences.\n\nConclusionThe state-space modelling framework, now in widespread use by ecologists and increasingly often automatically implemented within on-board GPS data processing algorithms, makes it possible to fit flight models directly to raw flight height records, with minimal data pre-selection, and to analyse the full distribution of flight heights, not just the mean. In addition to basic research about aerial niches, behaviour quantification, and environmental interactions, we highlight the applied relevance of our recommendations for airspace management and the conservation of aerial wildlife.

ecology↗