Body size and timing of fledging of Atlantic puffins in the Faeroes and Northwest Norway

We studied the way, timing and diurnal rhythm of young Atlantic Puffins (Fratercula arctica) leaving colonies in NW Norway and Faeroes. The colonies differed in nesting habitat and predatory pressure. Both adult and young Puffins from Bleiksøy were significantly larger and heavier than those from Nólsoy. Young birds left the Bleiksøy colony about ten days later in the season, but considerably earlier during the night (22.00-04.00 h; peak: 00.00-01.00 h) than those on Nólsoy (24.00-0600 h; peak: 02.00-03.00 h). The fledging period was more synchronized on Bleiksøy, both within the season and on particular nights. Body mass and size of young puffins leaving the colony tended to decrease through the fledging period. Well-developed young birds from sloping parts of the Bleik colony flew down to the sea, while those from the lower-lying, flatter parts of the colony and those in poor condition walked to the sea. Fledglings which left on the wing reached the sea about one hour earlier and could fly beyond the inshore waters before daybreak, thus avoiding the risk of gull predation. They were more numerous during the first six days of the 10-day fledging period and were larger than those, which left on foot. The latter lost much time and energy overcoming dense vegetation, boulders and the tidal zone on their way to the open sea. As a result, they were still near the shore at dawn, in the zone intensively penetrated by predators. We suggest that predatory pressure of the big gulls, which are numerous on Bleiksøy but almost absent from Nólsoy, could have a significant impact on timing of fledging of their prey.