SURVIVAL OF MANX SHEARWATERS PUFFINUS PUFFINUS

The breeding success of Manx Shearwaters at Skokholm Island, Pembrokeshire, was followed in study burrows, and data on survival of the young were augmented by ringing large numbers as they were about to leave the island and recapturing them in later years.
The weight of the young at fledging and the date at which they leave affect their chances of survival; overall, as many as 30% of the young birds may survive to reach breeding age, which is thought to be normally about 5–6 years. Adult mortality varies between wide limits of about 5 and 20%, but juvenile survival appears to be of about the right order to balance adult losses.
The significance of the one-egg clutch is discussed. At the normal time of laying the female may not be able to obtain sufficient food to form a second egg. A two-egg clutch, laid at a later date, would not produce more surviving young since these would fledge too late in the season to have a good chance of survival.
The advantage of leaving the island at a heavy weight is discussed, and it is concluded that the fat stores laid down give heavy young a greater chance than light young of migrating a long distance without food. It is possible that the young may even reach their winter quarters (off Brazil) without having to feed on passage if they depart with enough stored fat.