The ecology and breeding success of the Long-tailed skua (Stercorarius longicaudus) and the Brown skua (Catheracta lonnbergi) is studied in a longterm programme. The difference in allometric growth between the different species and populations of skuas is interesting when it is related to the ecology and distribution history of these species and populations. Skuas have a dynamic distribution history and an opportunistic way of living. Populations of the same species in different localities have often a different ecology. Therefore, studies on different populations of the same species are carried out.