The implications of costly airflows for space-use and movement decisions in birds

Abstract

The behavioural ecology of flight has largely considered how birds respond to mean flow conditions, but it is the gusty or extreme airflows that are likely to be particularly challenging. This thesis addresses this, examining the strategies birds use to negotiate turbulence over land, and exceedingly strong winds at sea. I first develop a method for sensing turbulence at fine scales using data collected onboard the animals themselves, taking homing pigeons (Columba livia) as model flapping fliers. Fine scale variation in the flight altitude and body displacement emerged as effective proxies of turbulence. I then assess the impact of freestream turbulence on flapping fliers and find that pigeons adapted their wingbeat kinematics (frequency and amplitude) to increase their flight stability in response to turbulence, but did so without a clear increase in flight effort. In my final two chapters I examine the responses of two seabird species to strong winds, first at sea, and then on land. Specifically, I investigate how streaked shearwaters (Calonectris leucomelas) respond to tropical cyclones, and how common guillemots (Uria aalge) select their breeding cliffs in relation to airflow conditions. I find that shearwaters fly towards the eye of the storm. This tendency increases with cyclone intensity and may enable birds to avoid strong onshore winds and reduce the associated risks of forced landings and/ or injury. Finally, computational fluid dynamics models reveal that guillemots select breeding cliffs that are sheltered from wind and storm conditions, rather than from the mean wind alone, or heat stress. This model of habitat selection could also predict habitat use across islands. Overall, this highlights the varied and sometimes surprising capacities of birds to cope with extreme and variable airflows and operate in areas that are, as yet, inaccessible to aircraft.