Seabird Conservation Requirements

The small clutch size, low breeding success, higher juvenile than adult mortality and prolonged period of immaturity of seabirds makes them particularly vulnerable to any factors that increase the mortality rate of adults. Since these characteristics mean that there is limited scope for density-dependent compensations in natality or recruitment, an increase in adult mortality rate is likely to lead to population decline. Although age at first breeding may be reduced when population size falls (see 2.6), and breeding success may increase, these can only change by small amounts, so could only balance very slight increases in adult mortality rates. By contrast, a reduction in natality may have a very small effect on population size over a long period because the annual survival rates of adult seabirds are often extremely high. If all the 50 000 or so pairs of Fulmars at St Kilda, in the Outer Hebrides, failed to fledge another chick for the next 20 years and no immigration took place from other colonies, the number of pairs of Fulmars at St Kilda could remain the same for the first ten years of reproductive failure because chicks fledged in earlier years would continue to recruit for at least ten years (probably for nearer 20). Then numbers would decline by only about 3% per annum (the adult mortality rate), so that after 20 years of total reproductive failure and assuming that St Kilda was a closed population, there would still be about 37 000 pairs of Fulmars remaining.