Voedselvluchten van Noordse Pijlstormvogels Puffinus puffinus bij ZW-Ierland in de nazomer

Seawatches carried out at Cape Clear Island (SW-Ireland) in late summer invariably show huge numbers of Manx Shearwaters flying east in the morning and returning west in the evening. These birds belong to several large colonies (23,000-25,000 pairs) on islets off the coast of Kerry and the numbers of birds passing Cape Clear (especially in the evenings) indicate that quite a considerable proportion of them flies eastwards in search of foraging areas. Since young Manx Shearwaters are fed only once every 1.6-2 nights by either parent, the average parent bird is likely to be east of Cape Clear between the period of median passage east on one day and the period of median passage west on the next. This means that, at a flying speed of 45 km per hour, most birds leave the vicinity of the colonies round 6:30 h GMT, pass by Cape Clear round 8:30 h GMT and then still have 38 hours to fly on, catch food and return to pass Cape Clear round 22:00 h GMT the next day and reach the colonies 1.5-2 hours later. Of these 38 hours, some 30 are daylight hours, which may be spent foraging. If the birds really feed primarily off southern Ireland east of Cork (fig. 1), they would spend a further 6 hours travelling and find themselves with 24 hours left to feed and catch food for the young for two days. Birds that only stay out for one day, would then dispose of 8 hours for foraging for a one day ration of food for themselves and their chick. Thus, it is shown that by staying out longer, Manx Shearwaters save on travelling time, which enables them to spend more time fishing.