POST FLEDGING SURVIVAL TO BREEDING AGE OF SHAGS PHALACROCORAX-ARISTOTELIS IN RELATION TO YEAR, DATE OF FLEDGING AND BROOD SIZE

Long term ringing of chicks and retrapping of adult Shags at a colony in south-east Scotland have provided information on survival to breedi ng age for 25 cohorts. The mean survival for 18 cohorts at risk of bei ng recaptured for 10 years was 0.307 +/- 0.2. Generalized linear model ling demonstrated that survival varied according to year, fledging dat e and brood size. The effect of each variable was apparently independe nt of those of the other two. Young hatched early in the season surviv ed better than those hatched later which, when linked to the seasonal decline in breeding success, results in early laying individuals contr ibuting 10 times as many recruits to subsequent generations as do bird s laying two months later. Chicks in broods of three had a higher esti mated survival than those in broods of two and both appeared to surviv e very much better than those in broods of one. Differences in the sur vival of cohorts could not be explained by annual differences in the t iming of breeding or mean brood size. Rather, differential survival of cohorts appeared to be due to the timing of mortality during the firs t winter.