Zebra finches use brightness cues for gap selection in flight

Flying animals that are adapted to life in cluttered environments require effective and robust guidance mechanisms to avoid collisions. Birds and insects use optic flow cues to avoid obstacles when flying, but these are only generated by self-motion and are likely to be used in conjunction with other cues such as visual contrast between foreground and background objects. Orchid bees use a strategy based on relative brightness to find clear paths through dense environments. To investigate the possibility that birds use a similar strategy, we presented zebra finches Taeniopygia guttata with symmetric or asymmetric brightness distributions behind the aperture of a tunnel and recorded their flight through it. The background brightness conditions influenced both the birds decision to enter the tunnel and their flight direction upon exit. Zebra finches were more likely to initiate flight through the tunnel if they could see a bright background through its aperture and were more likely to fly to the bright side on exiting the tunnel. We found no evidence of a centring response during gap negotiation; instead, the zebra finches entered the tunnel by turning tightly around its near edge. These results hint at a possible pre-planning of the trajectories before the onset of flight.