The syrinx is the vocal organ of birds, located at the bifurcation of the trachea into the two bronchi. It produces sound by vibrating membranes located at the top of the trachea. Most birds have a syrinx and use it to produce vocalizations. However, there are some groups of birds that lack a syrinx entirely.
Ratites
The ratites are a group of flightless birds that includes ostriches, emus, rheas, cassowaries, and kiwis. These birds do not have a syrinx and are unable to vocalize. Instead, they rely on non-vocal forms of communication like hisses, booms, and grunts. The evolutionary loss of the syrinx in ratites is likely related to their flightlessness, as vocalization became less critical for species that did not need to call long distances.
Some key facts about ratites and their lack of a syrinx:
- Ostriches produce a booming, guttural sound by inhaling and forcing air through their nostrils.
- Emus can hiss, boom, and grunt. The booming is produced by an inflatable neck sac.
- Rheas produce low booms that can be heard up to 2 km away. They also hiss and grunt.
- Cassowaries create boom and hiss sounds aided by resonating casques on their heads.
- Kiwis are the only ratites that can vocalize to an extent, making soft wheezing calls.
Screamers
Screamers are a group of unusual waterbirds found in South America that are named for their piercing vocalizations. There are three species: the horned screamer, the southern screamer, and the northern screamer. Surprisingly, these birds produce their screams without a syrinx. Instead, they use vibrations of their nasal cavity walls to create sound.
Key facts about screamers:
- The screamers’ calls can be heard up to 1.5 km away.
- As well as screaming calls, they can produce bill-snapping sounds.
- Screamers have an unusually wide gape that likely assists with their vocal mechanism.
- The evolutionary loss of a syrinx in screamers may be related to changes in their palate and tongue morphology compared to other birds.
Storks
Storks are large wading birds known for their spectacular migrations and characteristic bill-clattering displays. Most stork species are nearly silent, but white storks and a few others make weak vocalizations like hisses and bill snaps. So how do storks communicate? It turns out that storks are missing much of the necessary syringeal anatomy to phonate like other birds.
Highlights of stork vocalizations:
- White storks can hiss and bill snap, but lack a true song.
- The black stork has a weak, hoarse call only given at the nest.
- The open-billed stork is nearly mute, giving only occasional croaks.
- Mycteria storks like the yellow-billed lack vocal muscles needed for syringeal sound production.
- Storks may use bill clattering for non-vocal communication instead of songs and calls.
New World Vultures
New World vultures include condors, turkeys, and their relatives in the family Cathartidae. Compared to other birds, these mostly scavenging birds have unusually small syrinxes or lack them completely. As a result, they have limited vocal abilities compared to predatory birds like eagles or hawks.
Vocalization notes for New World vultures:
- Condors can only grunt and hiss, lacking melodious calls.
- Turkey vultures make raspy hisses but have no true song.
- Lesser yellow-headed vultures can only give weak sibilant wheezes.
- The syrinx may be reduced to allow for a larger trachea optimal for soaring flight.
- With no need to defend hunting territories, vultures require limited vocalizations.
Tinamous
Tinamous are partridge-like ground birds found in Central and South America. They have a highly unusual syringeal anatomy compared to other birds. Many species completely lack a song, while others can only produce simple croaks, whistles, and weak calls.
Tinamou vocal peculiarties:
- Most tinamous have a rudimentary syrinx with 2-3 sound-producing membranes instead of 4-5.
- The Chaco tinamou has just one pair of syringeal muscles instead of four pairs in songbirds.
- Tinamous lack a songbox brain region, correlated with simpler vocalizations.
- Their poor flight ability means limited need for vocal communication and territorial defense.
- Some species like the screaming tinamou can produce louder, more complex calls.
Anhimids
The Anhimidae family contains two unusual waterbirds: the horned screamer and the southern screamer. Formerly classified as screamers, these South American species have been given their own family. They share the screamers’ syringeal peculiarties and inability to sing.
Details on anhimids:
- Like screamers, they create loud screams without a true syrinx through nasal cavity vibration.
- Both species communicate using screaming calls, bill clattering, and other noises.
- Their syrinx contains just 2 membranes compared to 4-8 in most birds.
- Some systematists consider their vocal anatomy so unique that the family may warrant its own order, making them more distinct from other birds.
- DNA evidence shows anhimids likely diverged separately from true screamers like the horned screamer.
Palaeognath Birds
The palaeognaths are a superorder that contains the ratites discussed above as well as the tinamous. These ancient birds are characterized by a distinctive palate anatomy. Most palaeognaths have regressed syrinxes missing some typical features:
- Tinamous, kiwis, and emus have just one pair of intrinsic syringeal muscles instead of several pairs.
- Ostriches and screamers completely lack intrinsic muscles.
- Cassowaries have retained more syringeal features than other ratites, allowing some vocalizations.
- These losses are linked to changes in the song motor pathway in palaeognath brains.
- Their innate vocal limitations suggest a similar syrinx and vocal behavior in dinosaur ancestors.
Species with Weak or Simple Vocalizations
There are many other bird species that lack complex vocal abilities, even if they have some syringeal anatomy. These include:
- Bustards – Can boom but lack melodious songs and calls.
- Pelicans – Only make simple grunts and bill claps due to basic syrinx.
- Tropicbirds – Vocalize through bill clattering rather than with syrinx.
- Turacos – Make puffing, croaking sounds with an unspecialized syrinx.
- Seriemas – Emit a limited range of harsh clicking and whistles.
- Hamerkop – Have a hyoid apparatus adapted for bill snapping rather than song.
While these birds can produce some vocalizations, their communication is limited compared to songbirds and parrots with more complex syrinxes.
Conclusion
Only a handful of birds completely lack a syrinx, including ratites, screamers, and some storks. However, many additional species have regressed or unspecialized syrinxes that limit their vocalizations to simple croaks, booms, hisses, and bill-clacking sounds. The reasons for syringeal regression are complex, but may relate to changes in flight ability, diet, habitat, or brain anatomy. While most birds possess beautiful songs thanks to specialized vocal organs, these groups show it’s possible for birds to communicate in other ways even without a complex syrinx.
