DOI: 10.25338/b8z34q
title: Wind and route choice affect performance of bees flying above versus within a cluttered obstacle field

description: Bees flying through natural landscapes encounter physical challenges, such
 as wind and cluttered vegetation.  The influence of these factors
 on the flight performance of bees remains unknown.  We analyzed
 548 videos of wild-caught honeybees (Apis mellifera) flying through an
 enclosure containing a field of vertical obstacles that bees could fly
 within (through open corridors, without maneuvering) or above. 
 We examined how obstacle field height, wind presence and direction
 (headwinds or tailwinds) affected altitude, ground speed, and side-to-side
 casting (lateral excursions) of bees.   When obstacle
 fields were short, bees flew at altitudes near the midpoint between the
 tunnel floor and ceiling.  When obstacle fields approached or
 exceeded this midpoint, bees typically, but not always, increased their
 altitudes to fly over the obstacles.  Bees that flew above the
 obstacle fields exhibited 40% faster ground speeds and 36% larger lateral
 excursions than bees that flew within the obstacle fields, likely due to
 the visual feedback from obstacles and narrow space available within the
 obstacle field.  Wind had a strong effect on ground speed and
 lateral excursions, but not altitude.  Bees flew 12-19% faster in
 tailwinds than in the other wind conditions, but their lateral excursions
 were 19% larger in any wind, regardless of its direction, than in still
 air.  Our results show that bees flying through complex
 environments display flexible flight behaviors (e.g., flying above versus
 within obstacles), which affect flight performance.  Similar
 choices in natural landscapes could have broad implications for foraging
 efficiency, pollination, and mortality in wild bees. 
description: <i>Experimental set-up</i> Experiments were conducted in a
 laboratory flight tunnel (20.0 x 19.1 x 115.0 cm; width x height x length)
 with a field of obstacles (hereafter referred to as the ‘obstacle field’)
 in the middle of the tunnel.  The obstacle field consisted of vertical
 columns (diameter = 7 mm) arranged in three parallel rows of five
 obstacles each, running along the length of the tunnel.  Obstacles were
 made of dark green, cylindrical blocks (LEGO, Billund, Denmark) that
 contrasted with the black and white speckled pattern of the tunnel’s
 walls, and flight data indicated that bees were able to detect and avoid
 these obstacles.  All obstacles within an obstacle field were of the same
 height, and the obstacles extended only partway to the tunnel’s ceiling,
 allowing bees to fly either within or above the obstacle field.  The total
 height of the tunnel was 191 mm and the obstacle field heights tested were
 11, 40, 69, 98, or 127 mm (Figure 1b), so a minimum of 64 mm (~1/3 of the
 total vertical height) between the top of the obstacle field and the
 ceiling remained free of obstructions.  We consider the 11-mm obstacle
 field as a control for the presence of obstacles because this obstacle
 field was too short for bees to fly within.  There were approximately 20
 mm between the outer rows of the obstacle field and the walls in each
 arrangement.  Fans (AC Infinity, City of Industry, CA, USA) on each end of
 the tunnel produced a mild wind of the same mean flow speed (0.54 m
 s<sup>-1</sup>, measured with a Velocicalc Air Velocity Meter
 Model 9535, TSI, Shoreview, Minnesota, USA) above the obstacle field and
 within the corridors of the obstacle field (i.e., the space between the
 rows of obstacles); within the rows of obstacles (i.e., immediately
 downstream of an individual obstacle), the wind speed dropped to 0.36 m
 s<sup>-1</sup>. Freely flying honeybees
 <i>Apis mellifera </i>(n = 58) were collected on the campus of
 the University of California, Davis.  Single bees were flown in the tunnel
 with an obstacle field of one of the five possible heights, and the
 obstacle field height used for each bee was determined by a random number
 generator.  Lights (26 Watts, full spectrum; Hagen, Mansfield, MA, USA)
 were alternately turned on and off at each end of the tunnel to motivate
 the bees to fly back and forth past the obstacle field (towards a light). 
 We filmed between 5 and 13 flights per bee (mean = 9), with approximately
 half the flights in still air and half the flights with wind, for a total
 of 548 recorded flights.  We randomly assigned each bee to begin their
 flights with either wind or still air.  Bees flew in headwinds (flying
 into the wind) or tailwinds (flying with the wind), depending on the
 direction they flew relative to the air flow on a given transit.  We
 define the two flight directions in our tunnel as ‘up-tunnel’ and
 ‘down-tunnel’ – bees experienced headwinds when flying in the up-tunnel
 direction with wind and tailwinds when flying in the down-tunnel direction
 with wind. Flights
 were filmed with two synchronized Phantom v611 high-speed video cameras
 (Vision Research, Inc., Wayne, NJ, USA) sampling at 500 frames
 s<sup>-1</sup>, each positioned 30° from the vertical on
 opposing sides of the obstacle field and viewing down the length of the
 tunnel.  Cameras were calibrated using a standard checkerboard calibration
 method and built-in MATLAB functions.  This method captures lens
 distortion and projective geometry (using the intrinsic parameters), as
 well as the global positions and orientations of the cameras relative to
 the flight tunnel (via the extrinsic parameters).  
 <i>Kinematic analysis</i>
             We used a detection and tracking pipeline to
 automatically track the centroid of bees in each camera view as they
 transited the obstacle field.  From each frame, we subtracted the
 background and found one or more candidate positions of the bee using
 MATLAB’s built-in blob detection functions.  We associated these
 detections into a single trajectory over time using a Kalman filter and
 Munkres' assignment algorithm.  We then used DLTdv6 to check and
 manually correct the automatically tracked positions of bees.  We also
 labeled the positions of obstacles in the field using DLTdv6.  Using the
 camera calibration, we converted the two-dimensional locations of the
 objects in each view into three-dimensional coordinates of the bees and
 obstacles.  We analyzed bees’ trajectories from when they entered to when
 they exited the obstacle field, and we smoothed the trajectories with
 quintic spline curves.  From each flight’s position data, we calculated
 the median altitude of the bee across its entire flight and the range of
 altitudes of the bee across its entire flight.  To assess flight
 performance, we calculated two metrics: ground speed – the median of the
 bee’s speed relative to the ground, based on its movement in the
 horizontal plane (i.e., lateral and fore-aft motion), and lateral
 excursion, quantified by variation (i.e., interquartile range) in the
 bee’s lateral position over the entire flight.