Stomach oil

It was once thought that stomach oil was a secretion of the proventriculus, but it is now known to be a residue of the diet created by digestion of the prey items such as krill, squid, copepods and fish.

It is thought to serve several functions for Procellariiformes, primarily as an energy store; its calorific value is around 40 MJ/kg (9.6 kcal per gram), which is only slightly lower than the value for diesel oil.

For this reason a great deal more energy can be stored in oil form as opposed to undigested prey.

Surface nesting petrels and albatross can eject this oil out of their mouths (not nostrils, as has sometimes been suggested) towards attacking predators or conspecific rivals.

Against threatening mammals (including humans) it is not outright dangerous, but due to its extremely offensive smell it is usually highly repulsive and liable to spoil a predator's hunting success for quite some time.