Interclutch variability in egg characteristics in two species of rail: Is maternal identity encoded in eggshell patterns?

Abstract

Maternal signatures are present in the eggs of some birds, but quantifying interclutch variability within populations remains challenging. Maternal assignment of eggs with distinctive appearances could be used to non-invasively identify renesting females, including hens returning among years, as well as to identify cases of conspecific brood parasitism. We explored whether King Rail (Rallus elegans) eggs with shared maternity could be matched based on eggshell pattern. We used NaturePatternMatch (NPM) software to match egg images taken in the field in conjunction with spatial and temporal data on nests. Since we had only a small number of marked breeders, we analyzed similar clutch images from a study of Eurasian Common Moorhens (Gallinula chloropus chloropus) with color-banded breeders for which parentage at many nests had been verified genetically to validate the method. We ran 66 King Rail clutches (n = 338 eggs) and 58 Common Moorhen clutches (n = 364 eggs) through NPM. We performed non-metric multidimensional scaling and permutational analysis of variance using the best egg match output from NPM. We also explored whether eggs could be grouped by clutch using a combination of egg dimensions and pattern data derived from NPM using linear discriminant analyses. We then scrutinized specific matches returned by NPM for King Rail eggs to determine whether multiple matches between the same clutches might reveal maternity among nests and inform our understanding of female laying behavior. To do this, we ran separate NPM analyses for clutches photographed over several years from two spatially distant parts of the site. With these narrower datasets, we were able to identify four instances where hens likely returned to breed among years, four likely cases of conspecific brood parasitism, and a within-season re-nesting attempt. Thus, the matching output was helpful in identifying congruent egg patterns among clutches when used in conjunction with spatial and temporal data, revealing previously unrecognized site fidelity, within-season movements, and reproductive interference by breeding females. Egg pattern data in combination with nest mapping can be used to inform our understanding of female reproductive effort, success, and longevity in King Rails. These methods may also be applied to other secretive birds and species of conservation concern.

Eggshell Maculation is Not a Signal of Maternal Condition in Two Species of Rail

“…Parents of both species share incubation duties, but there may be selection on Common Moorhens to have more highly pigmented eggs for camouflage prior to the start of incubation, or alternatively, for increased individual pattern variability. Both populations exhibited conspecific brood parasitism, but the rate was higher in Common Moorhens, as would be expected at higher breeding density (McRae 1998;Johnson and McRae 2022). We cannot determine the degree to which ecological and social stressors may have contributed to the investment by hens in eggshell pigments, but a comparative analysis of a larger number of species could help shed light on what factors underlie differences between species in the mean degree of eggshell pigmentation.…”

Multiple parasitism in an evictor brood parasite: patterns revealed by long-term monitoring, continuous video recording, and genetic analyses

“…Recent studies showed that maternal identity can be encoded in egg phenotype (Gómez et al 2021;Šulc et al 2021;Johnson and McRae 2022). Therefore, we expanded our dataset of genetically assigned offspring from 2016 and 2017 by eggs whose phenotypes were measured but could not be genetically assigned to cuckoo females because no DNA was sampled due to early host ejection or predation.…”