Juvenile birds take around two years to reach adult size and weight.
Juvenile birds do not have the white patches on either side of the neck.
The vast majority, he said, are juvenile birds making their first migration.
The juvenile birds are fairly similar in appearance to the adult female.
Juvenile birds are brown with dark barring and usually lack white on the crown.
The colour of juvenile birds, which are accompanied by their parents, is brown and white as seen in the picture.
About 45 juvenile birds were introduced to the island.
Juvenile birds are much like females but the throat is mottled.
Rarely, juvenile birds may have only trace amounts of white on the tail.
Juvenile birds have a brown band across the wings.