Abstract
Female birds can potentially utilise various mechanisms, such as intraclutch egg size variation (ICESV), to modulate offspring quality. If this represents an ‘adaptive’ strategy allowing females to adjust reproductive effort after laying of the first egg, as information on resources becomes more predictable later in the laying sequence, then we would predict that ICESV would vary with ecological context, e.g. in ‘good’ versus ‘bad’ years. Here, in European Starlings (Sturnus vulgaris), we tested specific predictions that (1) females would lay relatively small last laid eggs in ‘poor’ years (with early laying and below-average breeding productivity); and (2) that ICESV would be lower in good years (late laying, above-average productivity) so that late-hatched chicks would not be disadvantaged. At the individual level we predicted (3) that repeatability of ICESV would be low if females adjusted this in relation to the environment in different years; and (4) that within-individual variation in ICESV should correlate with the number, and timing, of loss of chicks. Mean ICESV averaged −1.9% (measured as last egg mass relative to mean egg mass), typical of other hole-nesting passerines. However, our results did not support any of our hypotheses: (1) although there was annual variation in ICESV, in all years egg mass decreased through the laying sequence; (2) annual variation in ICESV was not consistent with variation in phenology or breeding productivity among years; (3) marked individual variation in ICESV (last laid egg varied from −20 to +10% relative to mean egg mass within clutches) was repeatable, i.e. individual females did not adjust ICESV in subsequent breeding attempts; and (4) variation in the magnitude of ICESV, alone and combined with hatching asynchrony, did not predict early brood loss. Our study suggests that ICESV is a relatively inflexible component of an individual female’s phenotype that might contribute to variation in individual quality.
Zusammenfassung
Die Variation der Eigröße innerhalb des Geleges ist bei Staren Sturnus vulgaris im Jahresvergleich unabhängig vom ökologischen Kontext
Vogelweibchen stehen potenziell verschiedene Mechanismen zur Verfügung, mit denen sie die Qualität des Nachwuchses anpassen können, so zum Beispiel die Variation der Eigröße innerhalb des Geleges (=intraclutch egg size variation; ICESV). Falls diese eine “adaptive” Strategie darstellt, welche es den Weibchen erlaubt, den Reproduktionsaufwand nach Ablage des ersten Eies nachzuregulieren, wenn im späteren Legeverlauf Informationen über Ressourcen berechenbarer werden, dann würden wir vorhersagen, dass sich die ICESV mit dem ökologischen Kontext ändert, beispielsweise in “guten” gegenüber “schlechten” Jahren. Hier überprüften wir an Staren Sturnus vulgaris bestimmte Vorhersagen, nämlich dass (a) die Weibchen in “schlechten” Jahren (mit späten Legedaten und unterdurchschnittlicher Brutproduktivität) relativ kleine letzte Eier legen sowie dass (b) die ICESV in “guten” Jahren (mit frühen Legedaten und überdurchschnittlicher Brutproduktivität) niedriger ist, so dass spät geschlüpfte Küken keinen Nachteil haben. Auf Individuenebene sagten wir voraus, dass (c) die Reproduzierbarkeit der ICESV niedrig sein müsse, falls die Weibchen diese in Bezug auf die Umwelt in verschiedenen Jahren anpassen und dass (d) die individuenbezogene Variation der ICESV mit Anzahl und Zeitpunkt von Kükenverlusten korrelieren sollte. Im Durchschnitt betrug die ICESV -1,9% (gemessen als Masse des letzten Eies im Vergleich zum Mittelwert), was typisch für höhlenbrütende Singvögel ist. Unsere Ergebnisse bestätigten dagegen keine unserer Hypothesen: (a) obwohl die ICESV von Jahr zu Jahr variierte, nahm in allen Jahren die Eimasse im Legeverlauf ab; (b) die Unterschiede in der ICESV von Jahr zu Jahr deckten sich nicht mit der jahresbezogenen Variation in der Phänologie oder der Brutproduktivität; (c) die deutliche individuelle Variation der ICESV (das zuletzt gelegte Ei wich zwischen −20 und +10% von der mittleren Eimasse der Gelege ab) war reproduzierbar, das heißt, die einzelnen Weibchen passten die ICESV bei folgenden Brutversuchen nicht an; und (d) die Variation in der Ausprägung der ICESV, sowohl einzeln als auch in Kombination mit asynchronem Schlüpfen, lieferte keine Vorhersagen über frühe Brutverluste. Unsere Studie lässt darauf schließen, dass die ICESV eine relativ unflexible Komponente des Phänotyps des jeweiligen Weibchens darstellt, der zur Variation der individuellen Qualität beitragen könnte.
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Acknowledgements
We would like to thank the many post-doctoral students, graduate students and undergraduate students who have contributed to data collection in this project since 2002, especially Oliver Love, Sophie Bourgeon, Raime Fronstin, Allison Cornell, and Melinda Fowler. This work was funded by Natural Sciences and Engineering Research Council of Canada Discovery and Accelerator Grants to T. D. W. and a Simon Fraser University Undergraduate Student Research Award to K. F. G. All the work was conducted under a Simon Fraser University Animal Care permit (no. 1018B-96) following guidelines of the Canadian Council on Animal Care.
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This study was funded by Natural Sciences and Engineering Research Council of Canada Discovery (no. 155395-2012) and Accelerator (no. 429387-2012) Grants to T. D. W. and a Simon Fraser University Undergraduate Student Research Award to K. F. G.
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This article does not contain any studies with human participants performed by any of the authors. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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Gibson, K.F., Williams, T.D. Intraclutch egg size variation is independent of ecological context among years in the European Starling Sturnus vulgaris . J Ornithol 158, 1099–1110 (2017). https://doi.org/10.1007/s10336-017-1473-4
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DOI: https://doi.org/10.1007/s10336-017-1473-4