Hummingbirds were captured in various locations around Coto Brus, Costa Rica in collaboration with the Stanford Center for Conservation Biology (20). After banding, selected hummingbirds (up to six per day) were transported to the Las Cruces Biological Station where they were trained to feed from a syringe containing sugar water (3:1 water/sugar). Once sufficiently trained, hummingbirds were placed in a 50 cm × 50 cm × 50 cm flight chamber with a perch and feeder, as shown in Fig. 2. The perch and feeder were instrumented with force sensors to measure animal weights before and after each flight. Acrylic side walls enclosed the flight chamber, while the top and bottom consisted of carbon fiber aerodynamic force plates (498 and 496 g, respectively; KVE Composites Group) attached to three force sensors each. The custom-made force sensors consisted of an aluminum flexural spring with a known stiffness. Capacitive sensors [MicroSense model 8800 electronics module with model 2805 probes (resolution, ~0.8 nm), National Instruments USB-6210 DAQ, Lenovo ThinkPad T440s, and MATLAB R2015b] sampled spring displacements at 10 kHz, which were then converted into forces using the respective spring stiffness. The instantaneous sum of forces on the top and bottom force plates is equal to the instantaneous vertical aerodynamic force generated by the hovering animals (14, 35, 36). Hummingbird flights were recorded at 2000 frames per second (fps) using Phantom Miro M310 and LC310 cameras. The color camera (LC310) captured the right-side hummingbird view, while the grayscale camera (M310) captured both an angled-up view from behind and in front of the bird with the help of a mirror (Fig. 2). Each flight recording consisted of a hummingbird taking off, drinking from the feeder, and then landing. Forces were sampled during the entire duration, while high-speed cameras recorded up to ~4 s (8310 frames) of hovering at the feeder. After three successful flight recordings, birds were transported back to their location of capture and released. For three nights, bats were captured and flown in the same flight chamber. We were not able to train these wild bats to feed from the feeder within the time constraints of a catch-and-release field study. Coincidentally, this made the comparison between the nectar bats and fruit bat fairer because fruit bats are not known to be trainable to feed on the wing. Accordingly, recordings were made while bats hovered around the flight chamber (advance ratio of 0.069 ± 0.036) between successful and unsuccessful attempts to perch on the feeder, perch, or screw heads on the side walls (without attempting to escape). Bat flights were recorded at 1000 fps (~8 s of flight) using two Phantom Miro M310 cameras and infrared lights (CMVision Wide Angle IRD50). After three successful recordings, bats were released. For five hummingbirds and two bat individuals, the number of recordings was less than three (see table S1). All procedures were approved by the Stanford Administrative Panel on Laboratory Animal Care and carried out under permits from Sistema Nacional de Áreas de Conservación (SINAC) and Ministerio de Ambiente y Energía (MINAE) of Costa Rica.

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