Nestlings in four nests of Black Woodpeckers were more or less frequently measured (maximum wing chord. Appendix 1) and weighed in late afternoon or in the evening (Appendix 2). In only a single nest were nestlings measured at hatching time (= day 0), and data from this nest were used to calibrate ages of other nestlings. Wing length was a better measure of age than body mass, because the latter reached its apex already at approximately day 14. Males were on average heavier than females of the same age (Figure 2, Appendix 2). Sexing by the red patch on the hindcrown was possible from the 14th day onwards (with males having a more extensive patch), although some males may have just as much red as females (in such cases, weight may be sex-specific).