Short-term (within-brood) effects of an experimental handicap on the trade-off between reproduction and self-maintenance in skylarks. A) – E) Adult body mass and immune parameters. Values are expressed as the difference between the baseline measure taken when their nestlings were small, and the final measure taken when their nestlings were about to fledge. F) – H) Nestling body mass and immune measures from control and experimental parents; the latter were assigned to treatment groups 0-7 days earlier. I) Average length of animal prey in droppings of nestling skylarks. J) Proportion of the main prey type (beetles, order Coleoptera) in the diet of nestlings. Bars depict mean and standard error. Numbers represent sample size of individual birds. For nestlings the number of nests is given in parentheses. Stars denote statistically significant differences. If both years are plotted the interaction between year and treatment was significant. Statistical analyses can be found in Results and Additional file 1: Table S1.