In vivo kinematic data collection and analyses

To determine how cadaveric range of motion relates to in vivo flight, we recorded wing kinematics from two species: pigeons (C. livia; n = 1; mass, 613 g) and zebra finches (T. guttata; n = 4; masses, 16 to 17 g). We obtained all birds from breeders and housed them in cages with ad libitum access to seed mix and water under 12-hour/12-hour light/dark cycle. All husbandry and data collection procedures were approved by the Animal Care Committee of the University of British Columbia.

To capture changes in wing shape during flight, up to five points were marked on the dorsal surface of one wing on each bird. We placed a 4-mm-diameter, removable, highly reflective marker at each point to allow for data capture at 240 frames/s via a five-camera tracking system (OptiTrack; NaturalPoint Inc.). Marker placement followed that in our cadaveric study (fig. S2) except the following: (i) Because of difficulties in visualizing the shoulder during flight, point 1 was placed midway along the length of the humerus instead of at the humeral head. (ii) Because of difficulties in marker visualization, point 5 was not used during zebra finch trials, and accordingly, wing twist was not assessed for this species. Each marker weighed 0.0287 g, resulting in 0.1435 g added mass for the pigeon and 0.1148 g added mass for zebra finches. These added masses amount to ~0.03 and ~0.72% of body mass, respectively.

We allowed the pigeon to fly in an arena with dimension of 118 inches by 73 inches by 55 inches, whereas zebra finches flew in a smaller cage with dimensions of 40 inches by 15.5 inches by 14 inches. During all flight trials, we provided seed ad libitum. For each bird, all data collection occurred within 15 to 30 min of wing marking and release within the flight arena or cage.

Range of motion in extension and bending (both species) and twisting (pigeon only) were then calculated for each species via similar methods to those in our cadaveric study. We obtained data only from frames in which all markers were visible. While analyzing zebra finch flights, we pooled data from all four individuals. We then determined whether any of the angular data recorded in vivo for each species fell outside the corresponding range of motion α-hulls we established with cadavers via the inashape3d() function in the alphashape3d package (51).

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