Population dynamics and breeding success of the Shag, Phalacrocorax aristotelis, on the Farne Islands, Northumberland.

Abstract

1. A population study of the shag (Phalacrocorax aristotelis (L.)) started on the Farne Islands in 1961 and continued until 1971. Many breeding individuals had originally been ringed as chicks and the majority of the population consisted of individually marked birds of known age.
2. Mean annual adult survival was normally 0.83 ± 0.007, but 0.24 and 0.38 respectively during the `red-tides' of 1968 and 1975. Survival after the first year (0.51) and second year (0.75) was not affected to an important extent by age.
3. All the nest-sites were ranked according to their physical characteristics and suitability for breeding. Only 4% of the sites which were used during the study were considered to be entirely satisfactory.
4. A multiple regression analysis was carried out on the variation in breeding success of 857 pairs (40% of the total), for which twelve variables were measured. Breeding success was much better in the 3 years after the `red-tide' of 1968 but the regression coefficients of the independent variables were similar.
5. It was possible to account for fifty-five percent of the individual variation in the seasonal start of laying. The most important factors here were age of the partners with the contribution by the male and female equal. The oldest birds on the best sites started to lay 5 weeks earlier than the youngest on the poor sites, but the mean date of laying of an age class varied by as much as 5 weeks from year to year depending on the annual differences in the dispersal of adults during the late winter.
6. Clutch size on the Farne Islands was 3.01 ± 0.04 with hatching success of eggs incubated for the full term at 0.87 ± 0.02. Variations in clutch size were responsible for only 4% of variation in the number of young hatched. Most nesting losses were due to the weather, especially rough seas which particularly affected the poor quality sites.
7. Variation in the number of young fledged by a pair was accounted for by the quality of their chosen nest-site and to a lesser extent by previous breeding experience. No other factors were found to be important and those specifically excluded were direct effects of age after the first attempt to breed, nest density, pair status and site or mate fidelity.
8. There was a shortage of good nest-sites on the Farne Islands and mean nest-site quality declined significantly as the population increased.
9. Between 1963 and 1968, the shag population increased and the shortage of nest-sites progressively reduced the production of fledged young per pair. This effect was especially marked among the first time breeders which arrived late each season and were forced to occupy relatively poor sites.
10.  After an exceptionally high mortality of adults in 1968, caused by a `red-tide', nest-sites were not in short supply and productivity per pair greatly increased, especially among the first time breeders which moved to better sites.
11. The breeding efficiency of first time breeders was compared to established breeders. After excluding the effects of nest-site they bred 0.70 as well as established breeders.
12. The proportion of potential breeders which emigrated to the Farne Islands from the Forth was similar to that of emigration from the Farne Islands; 0.098 of potential female recruits.
13. Immigration, especially of young females, has been the dominant factor determining the size of the population on the Farne Islands during these studies and it allowed the population there to recover rapidly from the exceptional mortality caused by the `red-tides'. Of 704 females which bred for the first time between 1962 and 1971, 420 (60%) were immigrants.
14. A computer model showed that nest-site quality has been an important factor restraining the growth of the population. In future, nest-site availability is likely to limit breeding numbers to about 900 pairs.
15. The prolonged nesting season and the relation between nest-site quality and age of the occupying pair is considered to be the result of competition for sites which are tangible assets, not conventional trophies.
16. Social constraints on nesting meant that some poor sites in colonies were occupied before good ones in non-occupied areas but the progressive occupation of sites during the growth of the Farne Islands' population has been efficient for the individual pairs and for the population as a whole. The established birds with the highest breeding potential occupied the best sites.