Oilbirds produce echolocation signals beyond their best hearing range and adjust signal design to natural light conditions

Oilbirds are active at night, foraging for fruits using keen olfaction and extremely light-sensitive eyes, and echolocate as they leave and return to their cavernous roosts. We recorded the echolocation behaviour of wild oilbirds using a multi-microphone array as they entered and exited their roosts under different natural light conditions. During echolocation, the birds produced click bursts (CBs) lasting less than 10 ms and consisting of a variable number (2–8) of clicks at 2–3 ms intervals. The CBs have a bandwidth of 7–23 kHz at −6 dB from signal peak frequency. We report on two unique characteristics of this avian echolocation system. First, oilbirds reduce both the energy and number of clicks in their CBs under conditions of clear, moonlit skies, compared with dark, moonless nights. Second, we document a frequency mismatch between the reported best frequency of oilbird hearing (approx. 2 kHz) and the bandwidth of their echolocation CBs. This unusual signal-to-sensory system mismat...

油鸱（Oilbirds）为夜行性鸟类，依靠敏锐的嗅觉与极度敏感的视觉觅食果实，在进出洞穴栖息地时会进行回声定位。本研究借助多麦克风阵列，记录了野生油鸱在不同自然光条件下进出其栖息洞穴时的回声定位行为。在回声定位过程中，油鸱会发出持续时长不足10毫秒的咔哒声爆发（click bursts, CBs），这类爆发由2~8个咔哒声组成，声间隔为2~3毫秒。该类咔哒声爆发在信号峰值频率-6dB处的带宽为7~23kHz。本研究报道了该鸟类回声定位系统的两项独特特征：其一，相较于无月黑夜，在晴朗月夜环境下，油鸱会降低其咔哒声爆发的能量与所含咔哒声的数量；其二，我们发现已报道的油鸱最佳听觉频率（约2kHz）与其回声定位咔哒声爆发的带宽存在频率不匹配现象。这种罕见的信号-感知系统不匹配……