Effects of Hatching Date on Post-Fledging Survival in Herring Gulls

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Abstract
From 1966-1968, 14,395 herring gulls of known hatching date were ringed on the Isle of May, Scotland [United Kingdom], from a colony whose breeding season was extended in 1967-68 by large scale egg removal followed by re-laying. Post-fledging survival in relation to date of hatching was analyzed by examining 1st winter recoveries and by studying large numbers of ringed birds that were culled between 1972-1975 when they returned to the island to breed. There was no major difference in the proportions of young recovered up to Dec. 31 in the year of hatching, although fewer recoveries of very late-hatched young were reported. This result was not verified by the much larger sample of recoveries provided by the culls. The proportions of young that survived to be culled showed that in the 1966 year class only young hatching after June 29 survived less well than the remainder. This group represented only 3% of the total sample. In the following year class, there was no relationship between hatching date and post-fledging survival, even among abnormally late hatching chicks from repeat clutches. Post-fledging mortality of 1968 young appeared to be twice as high as in the previous 2 yr classes, and the proportion recovered in the culls was inversely correlated with date of hatching. This indicated a progressively increased post-fledging mortality with lateness of hatching. In 1968, but not in the previous 2 yr, juveniles from broods of 3 fledged chicks survived less well than those from smaller broods. For those hatching later in the season, even juveniles from broods of 2 appeared to be at a disadvantage compared with those from broods of 1. Various reasons for the relationship between date of hatching and post-fledging survival are discussed.