Many people involved in regular travel for business or pleasure, pride themselves on being able to pack only essential items for the journey. Birds involved in their annual migration have this honed to a fine art through tens of thousands of years of evolution.
Birds only carry essential items on migration and one of these items is fuel. To fly large distances a bird body needs fuel and the most efficient body fuel is fat. This is laid down in various deposits for example under the skin. This can result in a substantial weight gain. The fat content of the body can rise from around five percent to over fifty percent. To achieve this, birds eat more before migration and also some change their diet. For example, before migration many insect eating warblers eat more fruit to give more energy more quickly. To compensate for the weight gain, birds reduce the weight of parts of the body not needed for the flight. For example the gizzard, liver, kidney and gut of the familiar Swallow is reduced by about twenty-five percent before they leave on their migration. In the course of a migration birds can lose up to half their body mass. While some birds complete their migration journey in one stage, others break the journey into a series of stages with stops for rest and topping up fuel reserves en route.
Let's look at just one example in a little more detail, the Bar-tailed Godwit. This is a wader that is seen on Arran's shores in the autumn and the spring as it migrates between one of its wintering areas in West Africa and its sub-Arctic breeding grounds. One of its sub-Arctic breeding areas is Alaska. Bar-tailed Godwit breeding in Alaska have been satellite tracked flying non-stop from Alaska to their wintering area in New Zealand. That is a journey of over 10,000km. It takes about eight days. How does a bird manage to fly for that length of time? Bodies of Bar-tailed Godwit, which had been accidently killed in Alaska when they hit a radio tower as they set off on their migration, have been compared to Bar-tailed Godwit in New Zealand who had completed their migration from Alaska. This comparison showed some astonishing differences. In the Alaskan godwits fifty-five percent of the body weight was fat, fuel for the journey. This is one of the highest fat contents recorded in any bird. The Alaskan Godwits had larger breast muscles and a larger heart. These are the exercise organs or engine to power the flight. By contrast the Alaskan godwits had very small gizzard, liver, kidneys and gut. The Bar-tailed Godwits seemed to have largely dispensed with parts of their metabolic machinery that were not essential during flight, presumably converting these to other tissue essential for the flight. About a week after arriving in New Zealand, the digestive organs of the Bar-tailed Godwit had been rebuilt and the heart and breast muscles had reduced in size. Now, I think that that is remarkable.