The Pocket Book of Bird Anatomy by Marianne Taylor

Author:Marianne Taylor
Language: eng
Format: epub
ISBN: 9781472976918
Publisher: Bloomsbury Publishing
Published: 2020-08-15T00:00:00+00:00

Airflow within the avian respiratory system.

VOCAL APPARATUS

Birds are extraordinarily accomplished when it comes to vocal sound. Many produce beautiful natural song that has inspired writers and musicians for centuries, while others are capable of flawless mimicry of all manner of sounds, from human speech to industrial machinery.

Even among the less accomplished vocalists, each species has its own particular breath-generated sound, whether trumpeting, piping, hooting, belching, groaning, warbling, grunting, whistling, or wailing, allowing the birds to recognise one another and communicate, and birdwatchers to identify each species they hear. The bird’s sound-generating organ is the syrinx, the avian voicebox. It is an organ unique to birds, and its structure, along with the unique avian breathing cycle, is the reason birds are so vocally accomplished.

Part of the respiratory tract, the syrinx is (in almost all birds) made up of the very base of the trachea, and also the uppermost parts of the two primary bronchi. Its outer wall is formed from cartilage, like the main part of the trachea, and it is surrounded by the syringeal muscles, which can change its shape by contracting. This affects the pitch of sounds that the bird makes.

At the junction where the bronchi branch apart, their walls change from cartilaginous to membranous for a short stretch. These folded tympaniform membranes generate sound when vibrated by exhaled air, as does the pessulus, a slim bar of cartilage at the centre of the junction. In a small number of birds (including owls and nightjars), the syrinx only occupies the trachea and not the bronchi, while in a few others (antbirds and their relatives, and the Oilbird) there are two separate syrinxes in each bronchus, not reaching the trachea.

MULTIPLE SOUNDS

Because the syrinx usually includes the top parts of both bronchi, it is possible for two different sounds to be made simultaneously, one by each branch. Sound can also be made continuously through repeated breathing cycles, permitting the non-stop songs of birds such as nightjars and certain warblers. Oilbirds, navigating in the darkness of their roosting caves, constantly produce sounds in their two separate syrinxes that sound like single clicks to our ears but are composed of a wide range of frequencies. By listening for the echoes from their calls, they can sense the whereabouts of other birds and the cave walls, making them one of only a handful of bird species that can echolocate. However, echolocation as sophisticated as that seen in bats has never evolved in birds. In most dark, forested environments, bats dominate as nocturnal aerial insect-hunters, but the nightjars fare better in open habitats where they can use sight, rather than sound, to find and track their prey.

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