High within-clutch repeatability of eggshell phenotype in Barn Swallows despite less maculated last-laid eggs

AmericanOrnithology.org Volume 139, 2022, pp. 1–13 https://doi.org/10.1093/ornithology/ukac024 RESEARCH ARTICLE High within-clutch repeatability of eggshell phenotype in Barn Swallows despite less maculated last-laid eggs Ava-Rose F. Beech,1,2 Mattheus C. Santos,2 Emily B. Smith,3 Ben W. Berejka,2 Yujie Liu,2 Toshi Tsunekage,2 and Iris I. Levin1,2,*, Environmental Studies Program, Kenyon College, Gambier, Ohio, USA Department of Biology, Kenyon College, Gambier, Ohio, USA 3 Department of Biology, Agnes Scott College, Decatur, Georgia, USA *Corresponding author: 1 2 ABSTRACT Ecological and life-history variation and both interspecific and intraspecific brood parasitism contribute to diversity in egg phenotype within the same species. In this study, Barn Swallows (Hirundo rustica erythrogaster) laid eggs with high intraclutch repeatability in egg size, shape, and maculation. Despite this high intraclutch repeatability, last-laid eggs had consistently less of the eggshell covered in spots and fewer spots than earlier-laid eggs in the clutch. We examined sources of interclutch and intraclutch variation using both direct measurements and custom software (SpotEgg, NaturePatternMatch) that provide detailed information on egg characteristics, especially maculation measures. In addition to our main findings, maculation on different sides of the egg was highly repeatable; however, only shape, proportion of the eggshell maculated, and average spot size were repeatable between first and replacement clutches. Low intraclutch variation in maculation could allow females to recognize their clutch and this may be adaptive for colonial nesting species, such as the Barn Swallow. Characterizing intraspecific variation in egg size, shape, and maculation is the first step in understanding whether intraclutch variation is low enough—and interclutch variation high enough—such that eggs could serve as identity signals. Keywords: egg shape, egg size, eggshell maculation, Hirundo rustica, intraclutch variation, lay order, repeatability LAY SUMMARY • Characterizing variation in avian egg size, shape, and speckling can help us understand the mechanistic basis of and functional outcomes for the patterns we see. • Photographs of Barn Swallow (Hirundo rustica erythrogaster) eggs showed low within-nest and high between-nest variation in size, shape, and speckling. • The most repeatable measure of egg appearance was egg shape, while the least repeatable was average spot size. • First and replacement clutch eggs resembled each other only in shape, proportion of the eggshell speckled, and average spot size. • The lay order of the eggs explained some of the within-nest variation in egg appearance; last-laid eggs were less spotted than earlier-laid eggs. Alta repetibilidad dentro de la nidada del fenotipo de la cáscara del huevo en Hirundo rustica erythrogaster, a pesar de que los huevos del final de la puesta son menos maculados RESUMEN La variación ecológica y de la historia de vida, y el parasitismo de nidada tanto inter-específico como intra-específico, contribuyen a la diversidad en el fenotipo de los huevos dentro de la misma especie. En este estudio, individuos de Hirundo rustica erythrogaster pusieron huevos con alta repetibilidad dentro de la nidada en cuanto al tamaño, la forma y el maculado del huevo. A pesar de esta alta repetibilidad dentro de la nidada, los huevos del final de la puesta tuvieron consistentemente menos partes de la cáscara cubiertas de manchas y menos manchas que los huevos puestos al inicio de la puesta. Examinamos las fuentes de variación dentro de la nidada y entre nidadas utilizando mediciones directas y software personalizado (SpotEgg, NaturePatternMatch), que brindan información detallada sobre las características del huevo, especialmente las medidas de maculado. Además de nuestros hallazgos principales, el maculado en diferentes costados del huevo fue altamente repetible; sin embargo, solo la forma, la proporción de la cáscara del huevo maculada y el tamaño promedio de la mancha fueron repetibles entre la primera nidada y las de reemplazo. La baja variación dentro Copyright © American Ornithological Society 2022. All rights reserved. For permissions, e-mail: . Submission Date: March 14, 2022; Editorial Acceptance Date: May 17, 2022; Published June 3, 2022 2 Barn Swallow egg size, shape, and maculation A.-R. F. Beech et al. de la nidada en el maculado podría permitir que las hembras reconozcan su nidada y esto puede ser una adaptación para las especies que anidan en colonias, como H. r. erythrogaster. La caracterización de la variación intra-específica en el tamaño, la forma y el maculado de los huevos es el primer paso para comprender si la variación dentro de la nidada es lo suficientemente baja—y la variación entre nidadas lo suficientemente alta—como para que los huevos puedan servir como señales de identidad. Palabras clave: forma del huevo, Hirundo rustica, maculado de la cáscara, orden de la puesta, repetibilidad, tamaño del huevo, variación dentro de la nidada INTRODUCTION Ornithology 139:1–13 © 2022 American Ornithological Society Avian eggshells have a wide range of potential functions beyond the essential role of protecting developing embryos. Eggshell coloration and maculation in particular have been implicated in crypsis (e.g., Sánchez et al. 2004), thermoregulation (e.g., Westmoreland et al. 2007), sexual selection (e.g., Soler et al. 2005), and egg mimicry and antiparasitic recognition (e.g., Hanley et al. 2016). Despite a long history of studying the adaptive function of eggshells, there is a growing list of non-mutually exclusive explanations for interspecific and intraspecific patterns of egg shape, size, and maculation, with increasing evidence that many patterns are species- or site-specific. Recent work has highlighted the potential for aspects of eggshell phenotype to serve as identity signals (Birkhead et al. 2021, Quach et al. 2021, Šulc et al. 2021), which could be advantageous to colonially breeding birds or those affected by brood parasitism. To function as identity signals, traits are predicted to be highly variable, not necessarily condition-dependent or associated with fitness differences, and predicted to be highly repeatable within an individual (Dale et al. 2001, Quach et al. 2021). Characterizing repeatability of multiple measures of eggshell phenotype as well as understanding potential sources of phenotypic variation is a necessary step in determining whether eggshells could function as identity signals. Egg size, shape, and maculation characteristics can vary substantially between females of the same species, while also having consistently high intraclutch repeatability values, particularly in size and shape metrics (Christians 2002; Table 1). Numerous studies have documented repeatable measures of egg phenotype (Table 1), but fewer studies report repeatability values across multiple measures of egg size (area, (...truncated)