Most breeding birds share responsibility for incubation of their eggs and change over at the nest regularly to allow each partner to feed. When the young hatch, the parents travel back and forth to their nest many times a day with food.

Albatrosses, however, are completely different - they spend many days at feeding at sea, often thousands of miles away from the nest, before returning to their mate or chick. So how do they manage this and how on earth do they store all the food they have eaten?

The digestive system of albatrosses and petrels has an upper stomach, or proventriculus, into which all of the food passes for initial digestion. Squid, fish, krill and other items on the albatross menu are broken down into a mixture of water, fats, proteins and everything else. The oil layer floats to the top, while water-soluble proteins and other compounds settle to the bottom.

The lower water layer is allowed out of the upper stomach into the lower gut that then continues to digest and absorb it in the normal way, making room for more food. When the bird is finally full, it returns to its island home.

The oil is a ready source of energy and small amounts of it can be allowed to trickle through to be fully digested by the parent. It can also easily be regurgitated as food for the youngster. This means that the chick can be fed, while the adult has enough energy for itself, not needing to feed again for several days, allowing its partner to fly off and feed before changing places again.

But there is another use for this oil... defence! Petrels and albatrosses can regurgitate foul-smelling oil from their stomachs as a means of protection. Adult albatrosses rarely use this method, as they can ward off enemies with their beaks and wings. Young chicks, however, if alarmed will readily eject large quantities of this half-digested food, given to them by their parents.

The oil is a rich, pungent and sticky fluid, an effective defence, as it will stick onto the fur or feathers of a predator and is difficult to remove, as well as being evil-smelling. It will coat the fur or feathers causing a loss of insulation and waterproofing properties that, in a cold and wet environment, can prove to be lethal.