Hummingbird vertical force processing
This protocol is extracted from research article:
Biomechanics of hover performance in Neotropical hummingbirds versus bats
Sci Adv, Sep 26, 2018; DOI: 10.1126/sciadv.aat2980

Aerodynamic forces were averaged across hundreds (see table S1) of wingbeats to obtain representative normalized vertical force traces of each individual (see movie S1). First, step responses from the perch and feeder during takeoff and landing were used to determine hummingbird weights. Next, aerodynamic forces were filtered offline at 180 Hz (about six times their wingbeat frequency) using an eighth-order digital low-pass Butterworth filter to isolate animal frequencies from structural frequencies of the setup (35). Temperature drift caused flexural springs to slowly expand and contract during recordings. To account for this, a linear drift model was applied to process the hummingbird recordings, which assumes that the bird generates a vertical force equal to their weight on average (mean drift per wingbeat was 0.98% of bird weight). Normalized vertical force profiles were calculated by dividing the force trace from each wingbeat by the bird’s weight during that flight (resulting in 100% weight support on average due to linear drift model). All normalized vertical force profiles for each individual were interpolated to 1000 points, starting at the beginning of the downstroke (0%) and ending at the end of the upstroke (100%). These interpolated traces were then averaged to find a representative normalized vertical force for each hummingbird, as shown in fig. S4. Follow-up experiments with Anna’s hummingbirds helped determine the effect of sensor drift and air leakage through the 5-mm gaps between the plates and side walls (fig. S7). Linear drift corrections to our capacitive sensors (fig. S7A) match the force traces from a setup using ATI Nano43 sensors with negligible drift (fig. S7B). Placing a thin strip of Saran Wrap along the gaps prevented air leakage (fig. S7C). The air leakage seemed to filter the force amplitude of the downstroke and valleys but did not result in differences between the five flights of an individual within each treatment, and the upstroke amplitude was basically unaffected (fig. S7D). This demonstrates consistency and our ability to fairly make comparisons within the Costa Rica field experiment with the air gap and the capacitive sensor drift correction. If future comparisons with our study require higher accuracy than achieved in this study, then we recommend considering the minor filter effect due to the air gaps in our vertical force recordings.

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