Files in this Data Supplement:

• Supplemental Figure S1 -

  Fig. S1. Vorticity along a transect cutting horizontally through a tip vortex during mid-downstroke. The vortex is the same as in Fig. 3B. The vorticity resembles a normal distribution, which motivates the use of a threshold and adding back the vorticity below the threshold assuming a normal distribution (see Materials and methods) (see also Spedding et al., 2003). The green line shows the threshold value used.

• Supplemental Figure S2 -

  Fig. S2. Isosurface plots of the streamwise vorticity (ω_x) in the wake of the bird without a tail at 6 m s⁻¹ flight speed, viewed obliquely from above (A), from behind y, z (B), straight from above Ut, y (C) and from the side Ut, z (D). The vorticity field does not include the entire span of the bird, so the data are mirrored at the centre plane of the body (determined as the centre between the left and right wing root vortices) to facilitate interpretation of the wake. No blending or smoothing was done and the mirroring did not visibly affect the wake topology. Iso values are normalized relative to Fig. 4 by the relative disc loading and are set at −69.3 s⁻¹ (red) and +69.3 s⁻¹ (blue).

• Supplemental Figure S3 -

  Fig. S3. Isosurface plots of the out-of-plane velocity relative to the flight speed in the wake of the bird without a tail at 6 m s⁻¹ flight speed viewed obliquely from above (A), from behind y, z (B), straight from above Ut, y (C) and from the side Ut, z (D). The velocity field did not include the entire span of the bird, so the data were mirrored at the centre plane of the body (determined as the centre between the left and right wing root vortices) to facilitate interpretation of the wake. No blending or smoothing was done and the mirroring did not visibly affect the wake topology. Iso values were normalized relative to Fig. 5 by the relative disc loading and were set at U−0.385 m s⁻¹ (red) and U+0.385 m s⁻¹ (blue).

• Supplemental Figure S4 -

  Fig. S4. Isosurface plots of the vertical velocity in the wake of the bird without a tail at 6 m s⁻¹ flight speed viewed obliquely from above (A), from behind y, z (B), straight from above Ut, y (C) and from the side Ut, z (D). The velocity field did not include the entire span of the bird, so the data were mirrored at the centre plane of the body (determined as the centre between the left and right wing root vortices) to facilitate interpretation of the wake. No blending or smoothing was done and the mirroring did not visibly affect the wake topology. Iso values were normalized relative to Fig. 6 by the relative disc loading and were set at −0.5 m s⁻¹ (red) and +0.5 m s⁻¹ (blue).