To describe the life-history strategy of a species, you have to investigate the variability of characteristics accounting directly for the probability of both survival and successful reproduction. Such characteristics are, for instance, body size and weight after hatching, growth length and growth rate as nestling, age and weight at the first reproduction, number and body size of offspring, reproduction frequency and length of life span. – One main focus of the PhD project is directed to sex-specific differences in i) reproductive success, ii) parental investment in offspring and iii) growth and development as nestling. To determine sex-specific differences during the course of the swifts’ life, there is a need to sex each bird genetically by means of a tissue sample (blood or a buccal swab) in the lab (because sexes could not be distinguished by body characteristics like plumage colour, body size or weight in the field). Besides, taking tissue samples enables determination of the proportion of extra-pair young and of the sex ratio among adult and juvenile birds.