This minimum production, mx (number of females produced per breeding female), can be computed from survival rates
mx = (1-P)/1x = 0.13/0.57 = 0.23
where P is annual adult survival and 1x is survival of fledged chicks up to first breeding. Data on survival are taken from Coulson and White (1959) and from Norwegian banding recoveries.
The kittiwake has, however, established a number of new colonies, and although the local increase in some of these is spectacular, the long-term change during the last 15 years is only about 1% increase per year in northern Norwegian colonies (E. Brun, unpublished data). In southern Norway, the population has increased much more rapidly (Brun 1971c).
Common murre (Uria aalge)
The common murre (Fig. 2) has shown a considerable decrease. The most spectacular decrease is at Sør-Fugløy, where a colony of 10,000 pairs in 1940 was reduced to 4,000 pairs in 1961, to 1,100 pairs in 1966, and to only about 10 breeding pairs in 1974 (Table 5). Most of the census work was done in 1964 and 1974. The general trend in population change, as expressed by the yearly decrease or increase, has been extrapolated forward to 1974 or back to 1964 for those colonies where censuses were missing for either of these years, to enable a better comparison (Table 6). The overall decrease in Norwegian colonies of the common murre is, thus, near 5% per year; the few cases with a positive trend are based either on very small figures or on extrapolation from old, inadequate censuses.
Thick-billed Murre (Uria lomvia)
The thick-billed murre (Fig. 3) was first proved to breed in Norwegian colonies in 1964; it was then found at three localities and has since been found breeding at eight localities (Table 7). It is now fairly certain that the thick-billed murre is not a newcomer but has remained unnoticed among the common murre for generations, possibly since the original immigration of the Uria species after the last glacial period. Data are not sufficient to show whether this small population of thick-billed murres is decreasing at the same rate as the common murre.
| Locality | Year of census | Number of pairs | Percent of population | |
|---|---|---|---|---|
| 1. | Kjør | 1975 | 80 | <0.1 |
| 2. | Heglane | 1970 | 4 | <0.1 |
| 3. | Ferkingstadøyene | 1970 | 5 | <0.1 |
| 4. | Utsira | 1970 | 2 | <0.1 |
| 5. | Utvær | 1970 | 200 | <0.1 |
| 6. | Ryggsteinen | 1970 | 2 | <0.1 |
| 7. | Veststeinen | 1970 | 1,500 | 0.1 |
| 8. | Einevarden | 1970 | 1,500 | 0.1 |
| 9. | Svinøy | 1970 | 100 | <0.1 |
| 10. | Runde | 1974 | 30,000 | 2.4 |
| 11. | Saløy | 1970 | 2 | 0.1 |
| 12. | Sklinna | 1974 | 2,000 | 0.2 |
| 13. | Lovunden | 1968 | 60,000 | 4.8 |
| 14. | Fugløy i Gildeskål | 1968 | 800 | 0.1 |
| 15. | Røst | 1964 | 700,000 | 55.7 |
| 16. | Værøy | 1974 | 70,000 | 5.6 |
| 17. | Nykvåg | 1967 | 40,000 | 3.2 |
| 18. | Frugga | 1975 | 5,000 | 0.4 |
| 19. | Anda | 1970 | 10,000 | 0.8 |
| 20. | Bleik | 1968 | 40,000 | 3.2 |
| 21. | Sør-Fugløy | 1968 | 40,000 | 3.2 |
| 22. | Nord-Fugløy | 1967 | 218,000 | 17.3 |
| 23. | Loppa | 1968 | 180 | <0.1 |
| 24. | Hjelmsøy | 1964 | 20,000 | 1.6 |
| 25. | Gjesværstappen | 1973 | 18,000 | 1.4 |
| 26. | Kongsøy | 1966 | 30 | <0.1 |
| 27. | Syltefjord | 1966 | 100 | <0.1 |
| 28. | Hornøy | 1967 | 160 | <0.1 |
| 29. | Reinøy | 1967 | 40 | <0.1 |
| Total | 1,257,705 | 100.0 |