Data from: 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 mismatch probably reflects avian constraints on high-frequency hearing but may still allow oilbirds fine-scale, close-range detail resolution at the upper extreme (approx. 10 kHz) of their presumed hearing range. Alternatively, oilbirds, by an as-yet unknown mechanism, are able to hear frequencies higher than currently appreciated.

油鸱（Oilbird）为夜行性鸟类，凭借敏锐的嗅觉与对光线极度敏感的双眼觅食果实，且在进出栖息洞穴时进行回声定位。本研究采用多麦克风阵列，记录了野生油鸱在不同自然光条件下进出栖息洞穴时的回声定位行为。
在回声定位过程中，油鸱会发出点击脉冲串（click bursts，CBs），单串脉冲的持续时长不足10毫秒，由2~8个间隔为2~3毫秒的点击声组成，点击声的数量不定。此类脉冲串在信号峰值频率−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，油鸱在晴朗月夜环境下，其发出的点击脉冲串的能量与点击声数量均会有所降低；其二，本研究发现，已报道的油鸱听觉最佳频率（约2kHz）与其回声定位点击脉冲串的带宽之间存在频率不匹配现象。
这种罕见的信号-感知系统不匹配，大概率反映了鸟类在高频听觉方面的生理限制，但油鸱仍可借助其推测听觉范围上限（约10kHz）处的信号，实现精细的近距离细节分辨。此外，油鸱可能通过某种尚未阐明的机制，感知到远超当前学界认知的高频声波。