The true costs of bird flight: From the laboratory to the field

Project Information

FLIGHT

Grant agreement ID: 715874

DOI

10.3030/715874

Project closed

EC signature date 22 March 2017

Start date 1 April 2017

End date 31 December 2023

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 996 042,50

EU contribution

€ 1 996 042,50

Coordinated by

SWANSEA UNIVERSITY
United Kingdom

Periodic Reporting for period 4 - FLIGHT (The true costs of bird flight: From the laboratory to the field)

Reporting period: 2021-10-01 to 2023-12-31

Summary of the context and overall objectives of the project

Air is an extraordinary medium: It can move at high speeds, is often turbulent, and the strength and directionality of the flow can vary from seconds to seasons, as well as through evolutionary time. This variability has a profound effect on birds and other flying animals, as the air can work with or against them. A central challenge in understanding, and ultimately predicting, how airflows affect avian ecology is in ascertaining how airflows affect flight costs. FLIGHT will use breakthrough methodologies to make new baseline measurements of the costs of flight from unrestrained birds flying in a wind tunnel. FLIGHT will then derive a proxy for energy use in flight using data from animal-borne loggers, using onboard measurements of acceleration. This will enable flight costs to be quantified with high resolution in the wild. Loggers will be deployed on wild birds to resolve flight costs in relation to fine scale movement paths and the wind and turbulence that birds experience. Data will be collected from homing pigeons, Columba livia, as an example of an archetypal flapping flier, as well as animals that switch between flapping and soaring flight. This will provide insight into the way the physical environment impacts flight costs for animals that respond to air currents in a variety of ways and enable the team to model how flight–related energy expenditure might vary geographically and seasonally. The results will be brought together to develop a framework predicting how airflows affect avian ecology. This is increasingly important as the aerial environment is also changing, and recent studies have shown that shifting wind regimes are impacting the energy required to fly, which can in turn affect reproductive success.

FLIGHT used an interdisciplinary approach to provide new insight into how airflows affect flight costs and capacities. The successful development of wake respirometry is providing high-resolution estimates of the chemical energy required to fly without the constraint of a mask. This was coupled with high-frequency measurements of the wings and body during flight. Extensive tracking data from birds in the wild revealed the profound influence of wind and thermal availability on the costs of flight for animals with diverse flight strategies. This was combined with modelling to demonstrate how global patterns in wind and air density affect flight costs, flight strategy and flight morphology.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A new, low-turbulence wind tunnel was designed and commissioned for the study of bird flight at Swansea University. The tunnel has a large flight area and is mounted on a tiltable frame, enabling the study of level, climbing and gliding flight. We used the wind tunnel to develop a novel method of quantifying energy expenditure, positioning a high-resolution carbon dioxide (CO2) sensor downwind from study birds. We first demonstrated that this system could resolve breath-by-breath changes in relative CO2 levels for stationary birds on a perch (including in birds as small as 12 g). Extending this method to estimate the total CO2 production in flight required involved a step-change. Our solution was to fix a pipe fixed behind the bird to “capture” the wake, where expired CO2 is concentrated. This air is then mixed and passed over the CO2 sensor. This wake respirometry approach can resolve the CO2 produced in individual breaths for birds flying without masks and provide new estimates of the energetic costs of flight.

We used animal-attached tags to quantify the body acceleration of birds in flight, and then used first principles to convert this to estimates of flight power. We also used Motion Capture cameras to quantify flight power for pigeons flying in the wind tunnel, applying infra-red markers to their feathers and reconstructing their wing and body motion.

We deployed animal-attached tags on seven species in the wild to record high frequency data on their motion, heading and altitude. The study species represented a range of flight strategies; from pigeons, as archetypical flapping fliers, to birds that switch between flapping and gliding (tropicbirds), through to extreme, obligate soaring species (condors). These data (combined with data previously collected from > 40 other species) revealed the extent that birds use gliding flight, how that varies with aerial conditions and the biological context. We also investigated how wind affects landing.

In the case of homing pigeons, tagging data were coupled with fine-scale measurements of turbulence, recorded by flying an ultralight along the flight path. This provided new insight into their kinematic and behavioural response to turbulence and revealed that tagging data can be used to predict changes in turbulence levels.

Taken together, the project findings (18 papers to date) provide novel insight into how airflows affect flight costs, risks and trajectories, and affect global patterns in flight morphology.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Over 50 years ago Tucker pioneered the use of mask respirometry to estimate the rate of energy use in flight, training birds to fly in a wind tunnel while wearing a mask. This became the state of the art for the decades to come. However, the effects of the mask itself are substantial, with the extra mass and drag increasing flight costs (by an estimated 3-30 %), and potentially altering flight style. Moving away from masks requires the development of an entirely new way of measuring metabolic energy expenditure. Our development of wake respirometry represents a breakthrough as it enables flight costs to be quantified for unrestrained birds, while still providing breath-by-breath resolution.

Our multi-species study on the cost of transport changes the dogma about the costs of flight. In 1972 Schmidt Nielsen published his seminal study on the cost of transport, showing that flight was more costly that swimming but less costly than terrestrial locomotion, when considered per unit distance travelled. Using tagging data from over 46 species, we provide the first meta-analysis of the costs of transport for animals flying in the wild. Our results demonstrate that flight costs are highly variable, being as costly as running and as cheap as swimming, depending on the wind and thermal conditions.

Models of global scale variations in flight costs provide a breakthrough in understanding how air density affects flight costs. The effect of temperature on air density and flight performance is well-established in the aviation industry, but the effects of air density on animal flight have only been considered in the context of high-altitude movements. Our results predict that global gradients in temperature and air density drive substantial variation in flight costs at sea level. Indeed, mapping air density at sea-level revealed that temperature gradients cause effective altitude to vary by >2 km. In a follow-on study we demonstrate how this this “invisible topography” at sea-level can predict large scale trends in morphology.

FLIGHT also provided novel insight into how airflows affect landing. Landing is the riskiest part of flight. This study was the first to investigate landing in the wild and demonstrate the critical importance of wind conditions on landing success. It also proposed a novel mechanistic link between breeding habitat selection and landing capacity.

A pigeon flying in the Swansea University wind tunnel