The direction and strength of selection for prey colouration by predators vary in space and time and depend on the composition of the predator community. We tested the hypothesis that bird selection pressure on prey colouration changes through the season due to changes in the proportion of naïve juvenile individuals in the bird community, because naïve and educated birds differ in their responses to prey colours. Bird predation on caterpillar-shaped plasticine models in two boreal forest sites increased sevenfold from early summer to mid-summer, and the time of this increase coincides with the fledging of juvenile birds. In early summer, cryptic (black and green) models were attacked at fivefold higher rates compared with conspicuous (red and yellow) models. By contrast, starting from fledging time, cryptic and conspicuous models were attacked at similar rates, hinting at a lower selectivity by naïve juvenile birds compared with educated adult birds. Cryptic models exposed in a group together with conspicuous models were attacked by birds at a threefold lower rate than cryptic models exposed singly, thus supporting the aposematic commensalism hypothesis. However, this effect was not observed in mid- and late summer, presumably due to the lack of avoidance of conspicuous prey by the juvenile birds. We conclude that selection pressure on prey colouration weakens considerably when naïve birds dominate in the community, because the survival advantages of aposematic colouration are temporarily lost for both the conspicuous and their neighbouring cryptic prey.

捕食者对猎物颜色的选择方向和强度随空间和时间的不同而变化，并取决于捕食者群落的组成。我们测试了这样的假设：鸟类对猎物颜色的选择压力随着季节的变化而变化，这是由于鸟类群落中幼年个体比例的变化，因为幼鸟和受过教育的鸟类对猎物颜色的反应不同。从初夏到仲夏，两个北方森林地区的鸟类对毛毛虫形状的橡皮泥模型的捕食增加了七倍，而且这一增加的时间与幼鸟的羽化时间一致。在初夏，神秘的（黑色和绿色）模型受到攻击的比率是显眼的（红色和黄色）模型的五倍。相比之下，从羽翼未丰的时期开始，神秘的和显眼的模型受到相似的攻击率，这暗示与受过教育的成年鸟相比，天真的幼鸟的选择性较低。与单独暴露的神秘模型相比，与显眼模型一起暴露的隐秘模型受到鸟类攻击的比率要低三倍，从而支持了共生主义假说。然而，这种效应在夏末和夏末并未观察到，可能是由于幼鸟没有避开显眼的猎物。我们得出的结论是，当幼稚的鸟类在群落中占主导地位时，对猎物颜色的选择压力会大大减弱，因为对于显眼的猎物和邻近的隐蔽猎物来说，颜色警戒的生存优势都会暂时丧失。