Don’t worry: this isn’t Lewis Carroll’s maddeningly unanswered riddle “Why is a raven like a writing desk?” (Although, if you’re interested, it turns out that has been answered.) There is an answer to this one—two answers, in fact.
Answer 1: They both make sounds by vibrating strings.
Well, strings, feathers—they’re all the same, right? This Club-winged Manakin produces its courtship song by vibrating its wing feathers: they strike each other about 107 times per second.
Anna’s Hummingbirds also make sounds by vibrating their feathers, but they go for a chirp! rather than a buzz.
Answer 2: They both (maybe) amplify sounds using a stiff sound board to vibrate air.
When birds make sounds vocally, one part of their anatomy that comes into play is the windpipe, or trachea. Most birds have what you might call a “normal” trachea: it goes from the mouth to the lungs, and doesn’t get up to any funny business in between. Some birds, however, have elongated tracheas. These tracheas also go from the mouth to the lungs, but in between, they coil up like a snail shell in the bird’s chest. This coiling allows the bird to fit a very long trachea inside its little body. A Crinkle-collared Manucode specimen “had the trachea so greatly enlarged that the native skinner took fright, thinking it was a giant worm” (Clench 1978).
Why bother to have a giant coiling trachea? The hypothesis with the most support currently is that it lets you pretend to be a much bigger bird. Think about it: the bigger the bird, the longer its trachea, right? So if you’re a small bird, but you manage to have a long trachea by coiling it up, you’ll sound like a big bird (Fitch 1999). Think of brass instruments: the longer the instrument, the deeper the sound. A tuba has a lower-pitched sound than a trumpet. Like the birds’ elongated tracheas, a tuba is coiled up to fit a lot of length into a small space.
But that’s a tuba, not a violin. What about the violin?
Gaunt et al. (1987) studied tracheal elongation in cranes. They initially thought that the birds were similar to wind instruments, but a few experiments—including having the birds breathe helium and then vocalize, which is a pretty great experiment—failed to support this. Instead, they propose that the birds are more like a violin: the elongated trachea coiled against the sternum (breastbone) transmits the vocal vibrations to the sternum, which in turn vibrates the air in the bird’s body. (Birds have a system of air sacs as well as lungs in their body: they are absolutely full of air.) The stiff sternum and the air sacs are essentially behaving the same way as the wooden, air-filled body of a violin, amplifying the vibratory sounds of the strings.
This is a neat idea that Fitch (1999) argues is probably not true. While a violin’s sound clearly comes from the body of the violin, and the violin has those f-shaped openings to allow sound to escape from the body of the violin, cranes’ sounds come from their bills, not their bodies, and it isn’t clear how the air sacs—wrapped as they are in muscle and skin—would transmit their vibrations to the outside air. Still, as far as I know, no one has shown definitive evidence either way.
So maybe birds with elongated tracheas are violins, or maybe they’re tubas. Either way, they’re pretty neat instruments.
Clench MH. 1978. Tracheal elongation in Birds-of-Paradise. Condor 80(4):423-430.
Fitch WT. 1999. Acoustic exaggeration of size in birds via tracheal elongation: comparative and theoretical analyses. Journal of Zoology 248:31-48.
Gaunt AS, et al. 1987. The effects of tracheal coiling on the vocalizations of cranes (Aves; Gruidae). Journal of Comparative Physiology A 161:43-58.