Data from: Egg shape mimicry in parasitic cuckoos

Parasitic cuckoos lay their eggs in nests of host species. Rejection of cuckoo eggs by hosts has led to the evolution of egg mimicry by cuckoos, whereby their eggs mimic the colour and pattern of their host eggs to avoid egg recognition and rejection. There is also evidence of mimicry in egg size in some cuckoo-host systems, but currently it is unknown whether cuckoos can also mimic the egg shape of their hosts. In this study we test whether there is evidence of mimicry in egg form (shape and size) in three species of Australian cuckoos: the fan-tailed cuckoo Cacomantis flabelliformis which exploits dome nesting hosts, the brush cuckoo Cacomantis variolosus, which exploits both dome and cup nesting hosts, and the pallid cuckoo Cuculus pallidus, which exploits cup nesting hosts. We found evidence of size mimicry, and for the first time evidence of egg shape mimicry in two Australian cuckoo species (pallid cuckoo and brush cuckoo). Moreover, cuckoo-host similarity was higher for hosts with open nests than for hosts with closed nests. This finding fits well with theory, since it has been suggested that hosts with closed nests have more difficulty recognising parasitic eggs than open nests, have lower rejection rates, and thus exert lower selection for mimicry in cuckoos. This is the first evidence of mimicry in egg shape in a cuckoo-host system, suggesting that mimicry at different levels (size, shape, colour pattern) is evolving in concert. We also confirm the existence of egg size mimicry in cuckoo-host systems.

寄生性杜鹃（Parasitic cuckoos）会将卵产于寄主物种的巢穴内。寄主对杜鹃卵的排斥行为，驱动了杜鹃卵拟态（egg mimicry）的演化——即杜鹃通过拟态寄主卵的色泽与斑纹，规避寄主的卵识别与排斥行为。部分杜鹃-寄主系统中已发现卵大小（egg size）拟态的相关证据，但目前学界尚不清楚杜鹃是否能够拟态寄主的卵形状（egg shape）。本研究针对三种澳大利亚杜鹃，检验其卵形（形状与大小）是否存在拟态证据：分别为依赖穹顶状巢穴寄主的扇尾杜鹃（Cacomantis flabelliformis）、可同时利用穹顶状与杯状巢穴寄主的灌丛杜鹃（Cacomantis variolosus），以及仅依赖杯状巢穴寄主的淡色杜鹃（Cuculus pallidus）。本研究发现了卵大小拟态的相关证据，并首次在两种澳大利亚杜鹃（淡色杜鹃与灌丛杜鹃）中观测到卵形状拟态的证据。此外，相较于闭合巢穴寄主，开放巢穴寄主对应的杜鹃与寄主的卵形态相似度更高。该结果与理论预测高度吻合：已有研究指出，闭合巢穴的寄主相较于开放巢穴寄主更难识别寄生卵，其卵排斥率更低，因此对杜鹃拟态的选择压力也更弱。本研究首次在杜鹃-寄主系统中发现卵形状拟态的证据，表明不同维度的拟态（大小、形状、颜色斑纹）正协同演化。本研究同时证实了杜鹃-寄主系统中卵大小拟态的存在。