|Most birds make many trips per day away from
their nest to find food.
Albatrosses and their petrel relatives do the reverse: they spend days or even weeks at sea without returning when the mate is minding the egg, and many days at the nest without leaving when they are incubating the egg and the mate is off looking for food.
Similarly, the nest-bound chick is stuck, without meals, when its parents are off on a long feeding trip. For most bird species, a chick would starve to death during these long stays alone at the nest. This doesn't happen to albatrosses because they have a special food storing tool: their digestive tracts can act as a separatory funnel.
A separatory funnel is used to separate liquids that have different densities. The lighter liquid floats on top of the heavier liquid, and the heavier liquid can be let out of the funnel by opening a hole at the bottom of the funnel. Below you can see a separatory funnel at work. First we pour a mixture of vegetable oil (colored yellow) and water (colored red) together into the funnel. In the beginning the oil and water are all mixed together, but after a while the oil all floats to the top of the water because the oil is lighter than water. The oil and water have now separated into two "phases". If you want to separate the oil from the water permanently, you can simply open a hole at the bottom of the funnel until the water drains out. You then have a funnel of oil, not a mixture of oil and water.
The first part of the digestive equipment of albatrosses and their petrel relatives can do this. You can think of the squid and fish prey of albatrosses as a solidified mixture of fats, water, proteins, and small amounts of other stuff. An albatross grabs a prey item and swallows it, initiating digestion in the proventriculus, the first chamber of the digestive system.
When the item has been broken down, the proventriculus is full of oily liquid fats (these don't dissolve in water) and stuff that will dissolve in water. The oily fats are less dense than the water part, and these two parts separate into two layers. A hole opens in the bottom of the proventriculus, leading to the rest of the digestive tract, and the water layer drains off and is absorbed as a meal by the bird.
The bird keeps the fat layer in the proventriculus, which now has space since the water part is gone, so the bird catches more food. It goes through the process of digestion, separation, and draining until it has a large amount of fats. Then it heads for home.
Here is an x-ray photograph of this process happening. The bird's head is to the left, and the tail is to the right. The football-shaped thing is the proventriculus, where the digestion and separation happens. The water layer is black and low in the proventriculus. The oil layer is lighter, and rides on top of the water. The water layer can be drawn off from an opening low in the proventriculus, leaving the oil layer behind, just like a separatory funnel does. What do you think those dark spots are, heading for the tail, travelling down the intestine? Yup... bird poop.
The reason for all of this concentration of oil is that at the nest, this gob of fat pays off. Fat contains a lot of energy, which all creatures need. Every day the bird allows some of the fat to pass from storage in the proventriculus into the rest of the digestive system, where it is used to power body processes. But what does the bird do about water? Didn't it separate the oil from the water some time ago? Now that water is long gone, so how does the bird get drinks when it is spending long periods at its nest? That is the beauty of fat. Some would say that fat is beautiful.
When fat is metabolized, or processed to release its energy, water is produced as a product. In fact, for every gram of fat that is metabolized, the bird gets not only energy but also 1.07 grams of water! So, it has energy and water supplies that make it possible to stay at the nest site for days or weeks.
This page last updated on February 01, 1999 09:56 AM
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