Figure 1

2x2 factorial design for testing the effectiveness of behavioral and immunological defenses - and any interaction between them - against flies. Half of the birds (A, B) had their immune systems primed against flies by pre-exposure to flies in a “backpack” for three weeks prior to the start of the experiment (Figure 2). The other half (C, D) wore backpacks with no flies. Preening was normal (A, C) or impaired with bits (B, D).