Growth of Guillemot Uria aalge Chicks in Relation to Time of Hatching

On the island of Stora Karls6, the Baltic Sea, the fledging weight of Guillemot chicks has been found to decrease gradually after the peak of fledging. To find the cause of the decline, we collected data on egg size, chick growth, feeding rate, and fledging age of chicks in pairs breeding at normal time and pairs whose breeding was experimentally delayed by egg removal. Replacement eggs were ca. 6% smaller than first eggs, but no difference in hatching weight was found between normal and late chicks. Nor were any differences found in fledging age and feeding rate between the two categories. Late chicks grew significantly more slowly than normal chicks, and at age 18-21 d, weighed on average only 222 g compared with 256 g for normal chicks. We suggest that a seasonal deterioration in food quality in combination with a decreased availibility of the prey could be responsible for the lower growth rate in late chicks.