This chapter considers how an individual's chance of dying is influenced by its age and sex. After a preliminary discussion about age-specific death rates, we will review the various ways of constructing life tables, which tabulate the information on age-specific death rates in an orderly way, and finally we will compare some of the life tables of different species of mammals and birds.

Age-specific death rates

We can develop our understanding of age-specific death rates by considering the work of Peter and Rosemary Grant on the large cactus (ground) finch (Geospiza conirostris), in the Galápagos archipelago. During the period 1978–83 they marked 1244 nestlings and followed their subsequent survival year by year. The nestlings could not be sexed, and they made the reasonable assumption that half were male and half were female. Only 27 of the 622 female nestlings survived for one year, 20 for two years, 13 for three years, and so on, until all the females were dead by seven years of age (see Table 14.1 for full data set). If we plot the number of survivors versus age (Fig. 14.1) we obtain the shape of the survivorship curve.

The heavy early mortality obscures the shape of the curve beyond the first year of age. We can deal with this problem by plotting the number of survivors on a logarithmic scale (Fig. 14.2).