Immediate and delayed effects of a gale in late spring on the breeding of the Shag Phalacrocorax aristotelis.

Abstract

At the beginning of May 1982, a westerly gale destroyed or damaged 49% of 761 Shag Phalacrocorax aristotelis nests on the exposed west side of the Isle of May, southeast Scotland; 518 nests on the sheltered east side acted as controls against which to assess subsequent effects of the gale on Shag reproduction and recruitment.
On sites affected by the gale, 90% of pairs rebuilt their nest and laid a second clutch, on average 18 days after the loss of the first one. First-time breeders nested closer to the sea, suffered greater damage to their nests, and deserted their damaged nests more readily after the gale than did experienced ones; a quarter of potel1tial recruits to the west side were deterred from breeding in that year.
By the end of the season, the number of chicks fledged per pair was 31% lower on the west side than on the cast. The difference was caused partly by nest desertion, partly by greater hatching failure of full clutches in apparently undamaged nests and especially by lower fledging success of pairs that renested. Irrespective of breeding experience, delayed early breeders produced more chicks than late breeders nesting at the same time, showing that late breeders were of poorer intrinsic quality than early breeders. This was one reason for a seasonal decline in Shag productivity, although environmental factors also played a role.
During April-July, gales like that of May 1982 occur on average once every 5 years on the Isle of May. Despite the disruption that they may cause to nesting Shags, the impact on the long-term growth of the colony is small.