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）会将卵产于宿主物种（host species）的巢穴中。宿主对杜鹃卵的排斥行为推动杜鹃演化出卵拟态（egg mimicry）策略：杜鹃卵通过模拟宿主卵的色彩与斑纹，躲避宿主的识别与排斥。部分杜鹃-宿主系统中已存在卵大小拟态的相关研究证据，但目前学界尚未明确杜鹃是否能够模拟宿主的卵形状。本研究针对3种澳大利亚杜鹃的卵形态（形状与大小）拟态证据展开检验，研究对象分别为依赖穹顶状筑巢宿主的扇尾杜鹃（Cacomantis flabelliformis）、同时利用穹顶状与杯状筑巢宿主的八声杜鹃（Cacomantis variolosus），以及依赖杯状筑巢宿主的淡色杜鹃（Cuculus pallidus）。研究结果显示存在卵大小拟态的相关证据，且首次在2种澳大利亚杜鹃（淡色杜鹃与八声杜鹃）中发现卵形状拟态的相关证据。此外，相较于封闭巢宿主，开放巢宿主对应的杜鹃-宿主卵形态相似度更高。这一发现与理论预测高度契合：已有研究指出，封闭巢宿主识别寄生卵的难度更高，卵排斥率更低，因此对杜鹃拟态的选择压力也相对更弱。本研究首次在杜鹃-宿主系统中证实卵形状拟态的存在，表明不同维度的拟态（大小、形状、色彩斑纹）正协同演化，同时本研究也进一步确认了杜鹃-宿主系统中卵大小拟态的存在。