One of the differences between birds and bats is that birds have a tail that work as a separate wing surface, while bats have no tail or a tail that is an integrated part of the wings. Findings from our studies of bat wakes have showed strong wing root vortices of opposite sign to the wing tip vortices, indicative of a lower lift production over the bod. Our earliest data from birds did not suggest a lower lift production over the body, which indicated that the birds might use their tail to extract some of the energy in the wake and improve performance.
We conducted the first PIV measurements in the transverse plane behind a free-flying bird during forward flight to test if birds follow the same wake pattern as the bats. We found novel wake structures previously not shown in birds, including wing weak root vortices of opposite as well as the same sign as the wing tip vortices. The difference in strength of the wing tip and wing root vortices was larger than in bats and suggests substantial lift production at the body. Also, the induced downwash, the flow generated by the animal, suggest some lift production at the body i.e. that the body plays a role in weight support during active flight in blackcaps.
To determine if the higher lift generated by the body/tail was indeed a result of the tail interacting with the root vortices, we studied birds both with and without a tail. We were unable to detect any differences in the wake pattern between birds with and without a tail and thus conclude that the birds do not use the tail to exploit vortices shed at the wing root during the downstroke, at least not during steady level flight. Instead the difference in body morphology between birds and bats is the likely cause for the difference in wake pattern. The tail of birds has also been hypothesized to reduce the drag of the bird, by acting as a splitter plate, but our results do not support this hypothesis. Instead our bird with a tail had a slightly higher drag than the one without tail. The function of the tail in birds during level steady flight thus remains unclear.
Last modified 16 Oct 2012