Catching the invisible - aerodynamic tracks and kinematics of bat and bird flight

Abstract: This thesis is about the flight of vertebrates and the studies of different modes of flight. Flight has always fascinated mankind and this fascination gave inspiration for our attempts at flying. Yet there are many different modes of flight among vertebrates and this thesis focuses mainly on the flapping flight of bats and intermittent flight of birds, as well as compares some of the factors concerning the differences between the flight of bats and birds, and the underlying aerodynamics. In the first three papers we have described the characteristics of flight for small, nectivorous bats, G. soricina, flying in a wind tunnel over a wide range of speeds. Paper I describes the wake structure of flying bats, revealing novel structures not previously seen in neither birds nor bats, such as e.g. individual vortex loop for each wing, and a rather complex wake pattern. In paper II we analyzed the wake of G. soricina further and compared the near and far wake measurements in the wind tunnel. We could show that, the strength of start and stop vortices does not differ significantly between the near and far wake, but there is a risk of missing details of the wake morphology by only looking at the far wake. Paper III concerns the kinematics of flying G. soricina bats and ties the wingbeat kinematics together with the observed wake, collected simultaneously. In Paper IV we compared quantitative measures of the circulation of the wake of the bat G.soricina with the wakes of three bird species. Our results showed differences in how the circulation was regulated and how the wake pattern changed in birds and bats. Paper V concerns bounding flight of birds and presents the results of a wind tunnel study of bounding budgerigars and Java sparrows, showing body lift production at all investigated flight speeds in both species. It also points to several differences between the two species that may be caused by the differences in the wing morphology. Body drag was also estimated during bounds and although it was large at lower speeds due to the high body angle during bounds, it was close to 0.2 at higher speed. To fully understand ecology and behavior of animals we have to understand the underlying mechanisms and factors involved. The technological advances have opened up a new possibilities to study flight in much more detail than previously possible. However there are still just a few detailed studies available and this thesis can hopefully ad to our knowledge about flight. Future studies will also need to couple the aerodynamics with morphological data in comparative analyses. The rapid development of methods available for aerodynamic studies opens new possibilities for future projects and every new study performed awakens new questions.