Figure 9.15 Female benefits of extra-pair copulations. In bluethroats, the strength of the immune system can be measured in terms of a wing-swelling response when the wing is injected with a chemical derived from plants. Larger wing-swelling responses indicate stronger immune systems. Error bars are standard deviations.
Data from A. Johnsen, V. Andersen, C. Sunding, and J. T. Lifjeld, Female bluethroats enhance offspring immunocompetence through extra-pair copulations, Nature 406 (2000): 296–299.