Figure 8.4 Manipulating the number of eggs in a nest. In the European magpie, the adults typically lay seven eggs. When researchers removed one or two eggs or added one or two eggs, the number of chicks that survived to fledge declined. This suggests that the typical number of eggs laid may be the optimal number for the magpie.

Data from G. Högstedt, Evolution of clutch size in birds: Adaptive variation in relation to territory quality, Science 210 (1980): 1148–1150.