How cowbirds make a watery song ^{CEFR B2}

New research explains in detail how brown-headed cowbirds (Molothrus ater) create a distinctive "watery" timbre that sounds like droplets hitting water: a quick burst followed by a fading ripple. The team was led by University of Utah biologist Franz Goller and included scientists with backgrounds in neuroscience and physics. Goller, a professor emeritus of biology, has studied bird vocalizations for many years with support from the National Institutes of Health. The work appears in Current Biology.

Birds produce sound with the syrinx, a vocal organ that has two separate sound sources, one on the left and one on the right. Goller and colleagues found that cowbirds generate the gurgling, bubbling quality by rapidly switching between the two sides. The left pair of vocal folds is much larger than the right, and parts of the notes with the watery timbre are alternations of left and right contributions. The researchers linked the effect to precise breathing control: cowbirds briefly hold their breath while building strong air pressure in the syrinx, then release that pressure suddenly. This produces a splash-like burst with a rapid attack and slow decay, which mimics a droplet hitting a surface. A second, more audible sound occurs about 50 milliseconds later when an air bubble implodes; different-sized droplets create different frequencies.

The team also tested learning by raising captive starling hatchlings alongside cowbirds. The starlings learned to reproduce the cowbird vocalizations and used the same unusual breathing and valve control. Coauthor Brenton Cooper is a neuroscientist who studies speech and language, and Gabriel B. Mindlin is a professor of physics at the University of Buenos Aires. The researchers conclude that timbre can arise from complex interactions between physiology and motor control, and that cowbirds use coordinated breathing and two sound sources much like sophisticated sound engineers.