1. In the 4 years 1454–1957 the breeding population of Shags on Lundy appeared to he static. Most of the nest-sites were used each year. Mortality was deduced from the re-use of old nest-sites and the return of colour-marked birds.

2. Most of the clutches were started during a period of 3 to 4 weeks in each year but dates varied by as much as 1 6 days in different years.

3. Different types of ncst-site and nest-material are used on different parts of the island. Larger nests were more successful than smaller ones.

4. The mean-clutch size for the five years 1954–1958 was 307, varying annually by us much as 0–4 egg. Clutch-size decreases with the advance of the season.

5. Incubation starts with the second egg in c/3 but a day or more later in c/4.

6. Hatching success 1954–1957 varied only between 69 and 73%. Infertility was the mam cause of hatching failure.

7. Eggs were weighed and measured in 1957 and 1958. In each year the larger clutches contained larger eggs than the smaller clutches. The mean weight of eggs was greater in 1957 (when clutches were bigger) than in 1958. This was due to a higher specific gravity.

8. Fledging success was 90–95%, except in 1956 when it was 67%. The heavier losses in this year appeared to be due to food shortage. Most chick losses occur in the first ten days.

9. The growth of chicks was studied in 1957. Male chicks can frequently be differentiated when 30 days old by their heavier weights. The fourth chick in a brood is normally below average weight during the first half of the fledging period.

10. Some marked juveniles remain on the island up to 50 days after leaving the nest. During this time they show partial dependence on their parents.

11. Later clutches are less successful than early clutches. Nests situated on narrow cliff ledges were less successful than nests at other sites.