DOI: 10.5061/dryad.tmpg4f573
title: 3D coordinates of the foraging flights of wild black skimmers

description: Birds commonly exploit environmental features such as columns of rising
 air and vertical windspeed gradients to lower the cost of flight. These
 environmental subsidies may be especially important for birds that forage
 via continuous flight, as seen in black skimmers. These birds forage
 through a unique behavior, called skimming, where they fly above the water
 surface with their mandible lowered into the water, catching fish on
 contact. Thus, their foraging flight incurs the costs of moving through
 both air and water. Prior studies of black skimmer flight behavior have
 focused on reductions in flight cost due to ground effect but ignored
 potential beneficial interactions with the surrounding air. We
 hypothesized a halfpipe skimming strategy for skimmers to reduce the
 foraging cost by taking advantage of the wind gradient, where the skimmers
 perform a wind gradient energy extraction maneuver at the end of a
 skimming bout through a foraging patch. Using video recordings, wind
 speed, and wind direction measurements we recorded 70 bird tracks over
 four days at two field sites on the North Carolina coast. We found that
 while ascending the skimmers flew more upwind and then flew more downwind
 when descending, a pattern consistent with harvesting energy from the wind
 gradient. The strength of the wind gradient and the flight behavior of the
 skimmers indicate that the halfpipe skimming strategy could reduce
 foraging costs by up to 2.5%.
description: The dataset was collected using videos from three digital Canon
 EOS 6D digital SLR cameras equipped with 50-millimeter lenses. Videos were
 recorded at 29.97 frames per second. They were synchronized by visual
 events identified separately in each camera. They were then calibrated for
 3D position reconstruction following the protocol from Corcoran and
 Hedrick (2019) via bundle adjustment using previously determined pinhole
 model lens parameters and shared 2D information visible in at least two of
 the three cameras. The scene scale was established from the distances
 between the three cameras, and calibration parameters were converted to
 direct linear translation (DLT). The calibration was aligned to place the
 water surface at <em>Z</em>=0 with positive
 <em>Z</em> pointing upward and the X and Y directions forming
 the horizontal plane, with the X axis aligned to wind direction such that
 bird movements in the increasing horizontal X axis were downwind (in the
 direction of the wind) and decreasing was upwind (against the wind). Bird
 track coordinates were acquired with DLTdv (Hedrick, 2008), and bundle
 adjustment calibrations were performed using the MATLAB computer vision
 toolbox. Each bird track was then smoothed using a 4-pole digital
 Butterworth low pass filter with a cutoff frequency of 0.5 Hz.
description: # Black skimmers foraging flights This dataset contains the XYZ
 coordinates of black skimmers foraging flight tracks taken in 5 different
 recording sessions. The flights were recorded using three video cameras,
 videos were synchronized the scene was calibrated and aligned using the
 DLTdv (Hedrick, 2008) application on MatLab. The tracks were smoothed
 using a 4-pole digital Butterworth low pass filter with a cutoff frequency
 of 0.5 Hz. We found that while ascending the skimmers flew more upwind and
 then flew more downwind when descending, a pattern consistent with
 harvesting energy from the wind gradient.  ## Description of the data and
 file structure For each CSV file the columns represent the XYZ coordinates
 for each tracked point, so that the first three columns will contain XYZ
 (in that order) coordinates for point one, columns 4-6 will be the second
 point, and so forth. For all files 'NaN' is a missing data point
 on a track. Missing data results from one or more cameras not capturing
 the moving or static object being tracked. Here is what you will find for
 each file and some extra information: #### **North Pond** Recording date:
 September 15, 2020 The columns in this file represent the XYZ coordinates
 for each tracked point: * Birds: Columns 1-42, 61-63 * Water alignment
 points: Columns 55-57 * Coastline points: Columns 85-87 * No skimming
 flights: Columns 43-54, 58-60, 64-84    Wind speed at 2 m: 4 - 6 m/s Wind
 direction NE #### **Shell Island I** Recording date: October 21, 2021 The
 columns in this file represent the XYZ coordinates for each tracked point:
 * Calibration points: Columns 1-3  * Water alignment points: Columns 4-6 *
 Wind vane: Columns 7-9 * Coastline points: Columns 10-12      * Birds:
 Columns 13-54 * Wind vane v2 end of file (not used): Columns 55-57 Wind
 speed by height: * Ground (0.035 m): 3.3-4.4 m/s * 1 m:  6.0-7.1 m/s * 2
 m: 5.8-6.2 m/s * 2.5 m: 5.4-5.7 m/s #### **Shell Island II** Recording
 date: October 21, 2021 The columns in this file represent the XYZ
 coordinates for each tracked point: * Calibration points: Columns 1-3 *
 Tracking stability of camera 1: Columns 4-6 * Wind vane: Columns 7-9 *
 Water alignment points: Columns 10-12      * Coastline points: Columns
 13-15 * Birds: Columns 16-81 Wind speed by height: * Ground (0.035 m):
 1.2-1.5 m/s * 1 m:  2.4-2.8 m/s * 2 m: 2.8-3.01 m/s * 2.5 m: 2.5-2.8 m/s
 #### **Shell Island III** Recording date: October 26, 2021 The columns in
 this file represent the XYZ coordinates for each tracked point: *
 Calibration points: Columns 1-3  * Water alignment points: Columns 10-12 *
 Wind vane: Columns 13-15 * Birds: Columns 4-9, 16-42 Wind speed by height:
 * Ground (0.035 m): 2.2-2.6 m/s * 1 m:  3.4-3.6 m/s * 2 m: 5.5-5.7 m/s *
 2.5 m: 4.6-4.8 m/s #### **Shell Island IV** Recording date: April 8, 2022
 The columns in this file represent the XYZ coordinates for each tracked
 point: * Calibration points: Columns 1-3  * Water alignment points:
 Columns 4-6 * Wind vane: Columns 7-9 * Birds: Columns 10-33 * Coastline
 points: Columns 34-36    Wind speed by height: * Ground (0.035 m): 5.11,
 5.6, 5.56, 4.76, 4.66, 8.59 m/s * 0.5 m: 7.40, 8.46, 9.30, 8.06, 7.40 m/s
 * 1 m: 7.86, 6.92, 6.29, 7.11, 6.63 m/s * 1.5 m: 9.33, 8.39, 8.48, 7.10,
 6.36 m/s * 2 m: 5.55, 6.34, 6.60, 5.77, 8.15 m/s * 2.7 m: 4.99, 6.02,
 5.67, 6.88, 8.41, 7.1 m/s