ii. Development

Many migrants, knowing their destiny, are ‘training’ for their journey from the day they hatch and it’s often a rush-job. Once hatched, juvenile migrants may have only a few weeks to complete their development and prepare for their first migration - phases of life that both require significant physical changes. Young Marsh warblers have just three weeks. In order to accomplish this, many migrants develop faster than resident birds. Some even show accelerated development in the egg.

Quickened development is particularly evident in birds that hatch late in the breeding season. The later you hatch, the less time there is.

iii. Annual preparations

Each year, birds can change their behaviour and entire physiology to prepare for the migration journey. This special suite of characteristics, only displayed before migration, is known as a bird's migratory disposition. A bird must achieve this condition before it departs in order to migrate successfully.

1. The right diet

Birds need to gain fat to fuel the migration journey. They do this by seeking out energy-rich foods such as berries and fruits. Birds also increase the amount of food they eat (known as hyperphagia) from around 10% of their body weight per day to 40%, with an average of 25-30% in most species. This is a vast increase: for a human to eat 30% of their body weight in a day, they would probably have to consume two large turkeys! Birds quickly convert this extra food into fat, which is stored in particular tissues, such as the liver and breast muscle. White-crowned sparrows double the amount of fat stored in these tissues before migration.

Did you know? Farmed ducks and geese have lost the migratory instinct but still increase the amount of fat stored in their liver when they are fed intensively. In some areas farmed birds are force fed in order to fatten the liver to up to twelve times its original weight. The livers are then used to make foie gras pâté, considered a delicacy by many people.