Why some bird songs sound like water dripping
New research reveals that the ‘liquid’ quality of certain bird songs is created by unusual vocal control that allows pressure to build before sound is released.
Watery sounds in birdsong have long intrigued scientists
Some bird songs carry an unmistakable ‘liquid’ or ‘watery’ quality, reminiscent of droplets falling into water. These gurgling sounds are especially familiar from the introductory notes of the Brown-headed Cowbird, and they stand apart from more typical whistles or trills. A new study shows that this effect is not produced by echoes in the vocal tract, but by precise and unusual control of the sound source itself.
The researchers focused on how birds generate these sounds using the syrinx - the specialised vocal organ at the base of the trachea. Unlike human voices, most songbirds have two independent sound sources, one on each side of the syrinx. The study shows that watery timbre emerges when these two sound sources are controlled in a highly coordinated and unconventional way.
Pressure first, sound later
At the heart of the watery effect is a striking asymmetry in the sound’s amplitude. Each note begins with an extremely rapid rise in loudness, followed by a slower decay. This pattern closely mirrors the sound made when an air bubble collapses after a droplet hits a water surface.
Physiological recordings revealed how birds achieve this. During the production of watery notes, the syrinx is actively closed while expiratory pressure continues to rise. Airflow is briefly blocked, allowing pressure to build to levels above the normal threshold for phonation. Only then is the syrinx opened, triggering immediate, high-amplitude oscillations of the vocal folds. The result is a sharp ‘attack’ that gives the sound its liquid character.
Alternating sound sources add to the illusion
In Brown-headed Cowbirds, successive notes in these introductory clusters are produced by rapidly alternating between the left and right sides of the syrinx. Because each side naturally produces slightly different frequencies, the sequence resembles droplets of varying size striking water in quick succession. This alternation enhances the impression of bubbling or dripping sounds rather than a single continuous tone.
The study’s modelling work confirmed that both ingredients are essential - the asymmetric amplitude envelope and the rapid switching between the two sound sources. Remove either element, and the watery quality disappears.
Starlings copy not just the sound, but the mechanics
To test whether this motor control is truly linked to the watery timbre, the researchers examined European Starlings that had learned to imitate cowbird songs. Remarkably, the starlings did not merely copy the acoustic result. They reproduced the same underlying motor patterns, including prolonged syringeal closure during pressure build-up.
When starlings imitated cowbird note clusters, they showed the same silent, high-pressure phase before sound onset, and their imitations displayed the same rapid attack and slow decay. In contrast, neither cowbird whistles nor normal starling warbling song involved this unusual pressure-and-closure pattern.
Why whistles sound different
The findings also explain why cowbird whistles lack watery timbre. Whistles are produced without the prolonged closure of the syrinx during pressure build-up. Instead, sound begins as pressure gradually rises, producing smoother onsets and more symmetrical amplitude profiles. Without the abrupt release of stored pressure, the liquid-like quality never emerges.
What this means for birdsong and imitation
This work highlights that timbre - often treated as a vague or subjective quality - can be traced directly to specific biomechanical and physiological processes. It also shows that vocal imitation in birds can extend beyond copying pitch and rhythm, reaching down to the level of fine motor control and respiratory coordination.
More broadly, the study suggests that some aspects of birdsong evolution may depend as much on how sounds are produced as on how they are perceived. The watery notes of cowbirds are not just decorative flourishes - they are the product of demanding vocal control that integrates anatomy, muscle action, and airflow in a highly specialised way.
December 2025
Get Breaking Birdnews First
Get all the latest breaking bird news as it happens, download BirdAlertPRO for a 30-day free trial. No payment details required and get exclusive first-time subscriber offers.
Share this story
