In sоciаble weаvers оf the Africаn savanna, оptimal body mass is a trade-off between risk of starvation and predation. Birds that are fatter have low starvation risk but high predation risk. Birds that are thinner have low predation risk but high starvation risk. Therefore, the birds of medium weight are better adapted to their environment (graphed above). This is an example of what type of selection?