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) 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. 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, p < 0.05) between I and the shell thickness-to-egg surface area ratio (T/S). A mathematical relationship was derived to calculate I depending on T/S. This premise was confirmed by comparative calculations using egg images of two parasitic species, common (Cuculus canorus) and plaintive cuckoo (Cacomantis merulinus) and their hosts: great reed warbler (Acrocephalus arundinaceus), European robin (Erithacus rubecula), rufescent prinia (Prinia rufescens), and common tailorbird (Orthotomus sutorius). An average calculated 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. Highlights • We assumed that cuckoo eggs have shorter incubation time (I) due to thicker shells. • To test this, we analyzed published data for 454 eggs from 447 bird species. • We found that I can be determined by shell thickness-to surface area ratio (T/S). • Mother cuckoo seems to be adapted to increasing the T/S ratio by increasing T. • We derived empirical formulae for calculating I based on S/V and T/S ratios

Accurate calculation of the content volume, density and original weight of museum curated eggs

Worldwide museums hold collections of eggshells representing material for descriptive studies. However, an obstacle to this is the lack of information about the original contents and weight of the entire egg (W). This study aimed to fill this gap though development of a methodological mechanism for calculating the volume of the egg interior (Vi), its density (Di) and W. To determine Vi, it is sufficient to measure four geometric dimensions of the egg and shell thickness. The Di value depends on the surface area-to-volume ratio (S/V) and can be calculated from an empirical relationship. For its derivation, data on 454 eggs from 447 avian species, 95 families and 13 orders were used. Imputing data on the contents and shell weight (Ws), we proposed a theoretical relationship for calculating W. We found a negative correlation between Di and S/V (which reflects the egg metabolism level) and suggest that a female in most species maintains the duration of egg incubation at a constant level that has practically an unchanged value for the respective species. A mathematical algorithm for calculating the Di value depending on the S/V ratio provides the missing link in calculating W of a whole egg from archived collection material