Analysis of Egg Variation and Foreign Egg Rejection in Rüppell’s Weaver (Ploceus galbula)

David C Lahti

doi:10.3389/fevo.2021.734126

Abstract

Egg appearance is notable for its variation and as a source of recognition cues in bird species that are subject to egg-mimicking brood parasitism. Here I analyze the egg appearance of an East African weaverbird species that has variable eggs and is a host of brood parasitism by an egg-mimicking cuckoo, in order to (1) compare population variation to variation within a clutch as a measure of the distinctiveness of eggs; (2) assess modularity versus correlation among egg appearance traits as an indication of the complexity of egg signatures; and (3) address whether the eggs are discretely polymorphic or continuously variable in appearance. I also compare three methods of assessing egg coloration: reduction of spectral data to orthogonal components, targeted spectral shape variables, and avian visual modeling. Then I report the results of egg replacement experiments that assess the relationship between egg rejection behavior and the difference in appearance between own and foreign eggs. Rüppell’s weaver (Ploceus galbula) eggs are variable in appearance between individuals and consistent within a clutch, but vary widely in the distinctiveness of particular traits. Most aspects of color and spotting are decoupled from each other, including coloration likely to derive from different pigments. Egg ground color is bimodal, with a broad continuous class of off-white/UV eggs and another broad class of blue-green eggs. Variation in all other traits is unimodal and usually normal in distribution. Females reject foreign eggs on the basis of the difference in brightness of the ground color and spotting of foreign eggs relative to their own, and the difference in degree to which spots are aggregated at the broad end of the egg. This aggregation is among the most distinctive features of their eggs, but the brightness of the ground color and spotting brightness are not; the birds’ use of brightness rather than the more distinctive chromatic variation to recognize eggs might reflect the salience of achromatic contrast in a dim enclosed nest.

Highlights

Dramatic and stable trait variation between individuals of the same species, sex, and age living in the same environment has always been compelling to evolutionary biologists, as it requires a more complex analysis than a singular prediction from optimality (Poulton, 1884; Dobzhansky, 1951; Ford, 1965)
To the extent that this phenomenon is occurring, host egg variation has evolved to facilitate self-recognition, since egg appearance traits are an extended phenotype of the mother; in practicality it can be considered offspring recognition since the parental trait is physically attached to the offspring and functions in distinguishing them from others
The eggs of the Rüppell’s weaver (Ploceus galbula) in the Awash Valley of Ethiopia vary from off-white to light blue-green, sometimes with a faint cast of reddish brown

Summary

Introduction

Dramatic and stable trait variation between individuals of the same species, sex, and age living in the same environment has always been compelling to evolutionary biologists, as it requires a more complex analysis than a singular prediction from optimality (Poulton, 1884; Dobzhansky, 1951; Ford, 1965). To the extent that this phenomenon is occurring, host egg variation has evolved to facilitate self-recognition, since egg appearance traits are an extended phenotype of the mother; in practicality it can be considered offspring recognition since the parental trait is physically attached to the offspring and functions in distinguishing them from others This particular mechanism of egg appearance variation within a species— increasing distinctiveness as a counteradaptation to brood parasitism—has been invoked with evidence as an explanation for increased egg variation in a number of species, such as the great reed warbler (Acrocephalus arundinaceus) (Moskát et al, 2002), blackcap (Sylvia atricapilla) (Honza et al, 2004), village weaver (Ploceus cucullatus) (Lahti, 2005), white-plumed honeyeater (Landstrom et al, 2010), tawny-flanked prinia (Prinia subflava) (Spottiswoode and Stevens, 2012), red-billed leiothrix (Leiothrix lutea) (Yang et al, 2014), and Paradoxornis parrotbills (Yang et al, 2015). I introduce a new brood parasite host system, where the host is known to have variable eggs between individuals

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Results

Conclusion