Why does the hoatzin or ‘stink bird’ stink?

BBC Wildlife contributor James Parry answers your wild question. 

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The hoatzin of the Amazon Basin is a folivore, or leaf-eater. This so-called ‘stink bird’ reeks of fresh cow manure or sweet-smelling hay, because of its unusual diet. The bird has a special digestive system to process the huge quantity of foliage it needs to provide enough energy.

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The hoatzin’s gullet and crop – a food storage pouch near the throat – serve as fermentation chambers. Inside are specialist bacteria that begin the process of breaking down the plant’s tissue. The bird ‘chews’ leaves before swallowing, and ridges inside its crop help to break up the leaf bulk further so it can be processed more easily.

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A hoatzin digests its food incredibly slowly. A meal takes up to 45 hours to pass through their systems. This is why these birds loaf around for up to 80 per cent of the time – they are effectively chewing the cud.