Why do procellariiform birds produce a strong musky smell?

Procellariiform birds, also known as tubinares, possess a characteristic strong, musky odor due to the excretion of stomach oil.

This oil is a waxy secretion produced by specialized glands in the proventriculus, the first chamber of the bird's stomach. The smell is a byproduct of this substance, which serves several biological functions:1

• Defense: When threatened or alarmed, these birds instinctively eject the foul-smelling oil from their mouths as a defensive weapon against intruders.21
• Metabolic Regulation: The discharge helps the bird excrete surplus fat, excess vitamins, and salt accumulated from their marine diet, which could otherwise disrupt their metabolism.1
• Maintenance: The oil may assist in waterproofing the bird's feathers as they preen their plumage with their oil-stained bills.1

In some species, such as the fulmar, this behavior is highly developed, with chicks capable of squirting the oil shortly after hatching.1