The Arctic tern is seen on land during the breeding season, which begins in May and lasts through to July; their main breeding location is in northern latitudes such as the UK.
The size of each clutch varies between one and three, which incubate for up to 24 days, during which time both sexes share incubation duties. The eggs are grey with speckles of brown and, after the eggs have completed incubation, they then hatch after around three weeks.
An Arctic tern egg.
The typical Atlantic migratory path of an Arctic tern.
Animals require other clues, which bring us back to the notion of “venturing off”. How does the Arctic tern know when to leave? One suggestion is that migration is related to the circadian rhythm. For most birds, migration is a daylight activity, whereby night is for resting. During the night, specific sets of hormones, such as melatonin, are released, giving the birds signals to sleep. When darkness approaches, and longer days wane, this decreased daylight, and ultimately raised levels of melatonin, provide a clue to the bird that it is time to migrate.