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

Periodic Reporting for period 3 - FLIGHT (The true costs of bird flight: From the laboratory to the field)

Période du rapport: 2020-04-01 au 2021-09-30

Air is an extraordinary medium: It can move at high speeds across ocean basins, it 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, what this means for decisions taken by animals that fly, as well as avian ecology more broadly, is in ascertaining how airflows affect flight costs. FLIGHT will use breakthrough methodologies to (1) make new baseline measurements of the costs of flight from unrestrained birds flying in a wind tunnel, and (2) derive a proxy for energy use in flight using data from animal-borne loggers. Loggers will then be (3) deployed on wild birds to resolve flight costs in relation to fine scale movement paths and airflow selection. Data will be collected from homing pigeons, Columba livia, as an example of an archetypal flapping flier, as well as gulls, 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 (4) 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.

The laboratory-based elements of the project focus on the development of new methods to quantify the energetic costs of flight, which can be estimated both from the chemical energy input and the mechanical power output. We have advanced the use of “wake respirometry” as a novel way of measuring the metabolic rate of flying birds by demonstrating that our system has sufficient sensitivity to record the CO2 produced by pigeons in flight in a wind tunnel. In tandem with this, we have collected data to assess the characteristics that predict which individual birds fly well in this alien environment, an important issue for future work. We have also collected high-frequency data on wingbeat kinematics from birds in a wind tunnel that will enable us to estimate the mechanical power output during flight using data from animal-borne loggers. A new, state-of-the art tunnel has been designed and is in production to enable equivalent data to be collected from larger birds, and birds that soar such as gulls.

Relating flight effort to airflow characteristics requires data from birds flying in the wild, as the flight costs depend on how individuals respond to the flow. We have collected unique datasets from species that differ in morphology and flight strategy, from pigeons, as archetypical obligate powered fliers, to gulls and two other species of facultative soaring bird, right through to extreme, obligate soaring species; the Andean condor, Vultur gryphus, and the greater frigatebird, Fregata minor. These data have revealed the extent to which facultative and obligate soaring birds use passive flight, with condors flapping for only 1% of the time they are airborne. We have also produced manuscripts demonstrating the critical importance of the take-off and landing phases, and how the energy expended in the latter can be influenced by airflows in some species. Manuscripts arising from the work have, so far, necessarily focused on individual systems. In the next phase we will consider generalities in how airflows impact flight behavior across species, and, for the main model systems, use laboratory validations to produce performance envelopes that define the energetic options available to birds according to the physical environment they encounter.

Initial trials in the wind tunnel have demonstrated that it is possible to detect changes in CO2 using sensors behind/ in the wake of resting and flying birds (objective 1). The equipment is sufficiently sensitive to resolve even breath-by-breath changes in CO2 levels from birds at rest. There are substantial challenges associated with the conversion of this signal to estimates of whole-animal CO2 production and energy expenditure. For this, the team has developed designs for alternative experimental set-ups, which will be tested in the next phase of the project when the Swansea wind tunnel is operational.

We have used high frequency data on animal movement, heading and altitude from animal-attached tags, and in-house software to reconstruct fine-scale flight paths by dead-reckoning between GPS fixes, providing unprecedented insight into the movement paths and flight strategies of birds in the wild (objectives 3 and 4). Analyses of soaring behaviour and flight effort have been undertaken for condor data and are ongoing for the pigeon and (multiple species of) seabird data. Further analyses will assess the environmental factors that affect route choice (over metres to kilometres) and flight effort in a range of species representing obligate and facultative soaring birds, as well as obligate flapping fliers. We are also testing a new speed sensor to be fitted to our tags. If successful, this will provide novel onboard measurements of airspeed – a parameter that birds vary with respect to the wind, and which is also a key determinant of flight costs.

In the case of homing pigeons, the project team have coupled data on bird movements (from onboard dataloggers) with high-frequency measurements of the air characteristics experienced by birds along their flight paths. Meteorological data were collected using equipment attached to an ultralight, which was flown close to the birds (but sufficiently far for the ultralight not to influence the birds flight). The resulting meteorological data are complex and require substantial processing to account for variation in the speed and orientation of the ultralight. If this is achievable, the data will be used to quantify the wind vector and turbulence, in order to provide insight into how both parameters influence pigeon flight in the wild.

As well as examining the factors that affect the effort required for birds to fly from A to B, the project has also provided a novel analysis of how airflows affect the costs and capacities of birds to land. This was undertaken by combining observations of seabirds landing at their breeding cliffs, with computational fluid dynamics models of how the air flows around these inaccessible areas. The results demonstrated that even moderate winds can effectively prevent birds from landing, presumably due to a reduction in their flight control. However, the sensitivity of birds to the wind conditions varied with species according to their manoeuvrability and the size of their landing platform. Further work will explore whether and how airflow characteristics affect which cliffs seabirds colonise.

Pigeon undergoing a training flight in the Max Planck wind tunnel

Periodic Reporting for period 3 - FLIGHT (The true costs of bird flight: From the laboratory to the field)

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