Brightness cues affect gap negotiation behaviour in zebra finches

Flying animals have had to evolve robust and effective guidance strategies for dealing with habitat clutter. Birds and insects use optic flow cues to sense and avoid obstacles, but orchid bees have also been shown to use brightness cues during gap negotiation. Brightness cues might therefore be of general importance in structuring visually guided flight. To test the hypothesis that brightness cues affect gap negotiation behaviours in birds, we presented captive zebra finches Taeniopygia guttata with a symmetric or asymmetric background brightness distribution on the other side of a tunnel. The background brightness conditions influenced both the birds’ decision to enter the tunnel aperture, 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 it; they were also more likely to fly to the bright side upon exiting. We found no evidence of the centring response that would be expected if optic flow cues were balanced bilaterally during gap negotiation. Instead, the birds entered the tunnel by targeting a clearance of approximately one wing length from its near edge. Brightness cues therefore affect how zebra finches structure their flight when negotiating gaps in enclosed environments.

飞行类动物为应对栖息地中的杂乱环境，演化出了稳健且高效的导航策略。鸟类与昆虫借助光流（optic flow）线索感知并规避障碍物，而此前研究表明兰花蜂在穿越间隙时还会利用亮度线索。由此可见，亮度线索可能对视觉引导飞行的行为模式构建具有普遍重要性。为验证亮度线索会影响鸟类间隙穿越行为这一假说，我们向圈养的斑胸草雀（Taeniopygia guttata）展示了隧道另一侧呈对称或非对称分布的背景亮度环境。背景亮度条件同时影响了鸟类进入隧道入口的决策，以及它们飞出隧道后的飞行方向。斑胸草雀若能通过隧道看到明亮背景，则更倾向于发起穿隧飞行；且在飞出隧道后，它们也更大概率会飞往明亮一侧。我们未发现间隙穿越过程中光流线索双侧平衡时所预期出现的居中反应。取而代之的是，鸟类会以距离隧道近缘约一个翅长的间隙为目标进入隧道。由此可见，亮度线索会影响斑胸草雀在封闭环境中穿越间隙时的飞行模式构建。