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. 2022 Apr 15;12:6320. doi: 10.1038/s41598-022-09872-9

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© The Author(s) 2022

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Eggshell thickness as a function of egg length in avian species. Eggshell thickness was measured at two points on the egg: (A) the meridian of the egg, which is the circumference around the widest part of the egg (N = 73) and (B) the apex of the egg, which is the most conical end opposite the air sac (N = 59). Linear regression was conducted on species that do not employ a brood parasitic reproductive strategy (circles). Brood parasitic cuckoos were plotted separately (filled black markers). Cuckoo species that parasitise acceptor hosts without egg ejection (black triangles) and cuckoo species that exploit egg-ejecting hosts (black diamonds) fell within the 95% confidence limit (grey shaded area) predicted for non-parasitic species. Figures of residuals when using phylogenetic correction are presented in supplementary material (Figs. S5 and S6).