Brood parasitism and host-parasite relationships: Cuckoos adapt to reduce the time of hatching ahead of host nestlings by increasing egg thickness

The phenomenon of cuckoos' brood parasitism is well known and can be investigated using applied mathematical techniques. Among adaptive features of this phenomenon are certain egg parameters that ensure their shortened incubation period (<i>I</i>) and thus the successful survival of their offspring. In particular, the volume of a cuckoo egg is not less than, or exceeds, that of the host species, which should, in theory, increase <i>I</i>. Also, cuckoo eggs have thicker shell than that of nest hosts. Here, we analyzed the available geometric dimensions of eggs in 447 species and found an inverse correlation (-0.585, <i>p</i> < 0.05) between <i>I</i> and the shell thickness-to-egg surface area ratio (<i>T</i>/<i>S</i>). A mathematical relationship was derived to calculate <i>I</i> depending on <i>T/S</i>. This premise was confirmed by comparative calculations using egg images of two parasitic species, common (<i>Cuculus canorus</i>) and plaintive cuckoo (<i>Cacomantis merulinus</i>) and their hosts: great reed warbler (<i>Acrocephalus arundinaceus</i>), European robin (<i>Erithacus rubecula</i>), rufescent prinia (<i>Prinia rufescens</i>), and common tailorbird (<i>Orthotomus sutorius</i>). An average calculated <i>I</i> value for cuckoo eggs was one day less than that for host eggs. Our findings unravel additional details of how cuckoos adapt to brood parasitism and specific host-parasite relationships.