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Journal of Ornithology

, Volume 159, Issue 2, pp 545–554 | Cite as

Patterns of variation in eggshell colour of Mountain Bluebirds (Sialia currucoides) provide mixed support for the sexually selected eggshell colour hypothesis

  • Jeannine A. Randall
  • Russell D. Dawson
Original Article

Abstract

Determining the functional significance of ornamental traits and whether they are shaped by sexual selection requires understanding how they vary among individuals, and their relationship with other measures of quality. Conspicuous eggshell coloration is a trait that may have evolved through sexual selection as a signal of female quality. This hypothesis is based on the possibility that the pigment used to colour blue-green eggs is limited and potentially costly to allocate to eggshells. We investigated the signalling potential of blue-green eggshell colour by quantifying patterns of within- and among-clutch colour variation in Mountain Bluebirds (Sialia currucoides) to determine if pigment was limited and if egg colour traits are intrinsic to individual females. We also explored relationships over two breeding seasons between eggshell colour and other female traits and investment in eggs. Eggshell colour was highly repeatable within clutches for most measures, and relatively consistent between breeding events in different years, indicating that eggshell colour is an intrinsic trait. Eggs in second clutches within a season had less saturated blue-green colour than in first clutches, and within clutches, later-laid eggs were brighter but generally less blue-green in colour than earlier laid eggs, providing evidence of pigment limitation. However, associations between eggshell colour and other female traits and investment in eggs were less clear, and inconsistent among study years. Our findings suggest that there is potential for blue-green egg colour to function as a signal of female quality, but further study is needed to determine if blue-green eggshell colour is a truly informative signal.

Keywords

Eggshell colour Female traits Quality signalling Blue-green 

Zusammenfassung

Unterschiedliche Farbmuster der Eier des Berghüttensängers ( Sialia currucoides ) bieten eine unklare Unterstützung der auf geschlechtlicher Selektion beruhenden “Eischalenfarben-Hypothese”

Um die funktionelle Bedeutung schmückender, dekorativer Merkmale zu ermitteln und auch, ob sie ein Ergebnis sexueller Selektion sind, muss man verstehen, wie sie zwischen den einzelnen Individuen variieren und mit anderen qualitativen Merkmalen zusammenhängen. Eine auffällige Eierfärbung ist ein Merkmal, das sich durch sexuelle Selektion als ein Signal für die Qualität des Weibchens herausgebildet haben könnte. Diese Hypothese basiert auf der Annahme, dass das Pigment für blau-grüne Schalenfärbung möglicherweise eine begrenzte Ressource ist und es deshalb aufwendig wäre, diese für die Färbung von Eischalen zu verbrauchen. Für den Berghüttensänger (Sialia currucoides) untersuchten wir die mögliche Signalwirkung blau-grüner Schalenfärbungen durch die Quantifizierung der Farbmustervariationen innerhalb eines Geleges einerseits und andererseits zwischen einzelnen Gelegen, um herauszufinden, ob das Pigment begrenzt ist und ob bestimmte Farbmuster für individuelle Weibchen spezifisch sind. Über zwei Brutperioden hinweg untersuchten wir außerdem, ob es Zusammenhänge zwischen den Eierfärbungen und anderen Merkmalen der Weibchen und ihren “Investments” in die Eier gab. Innerhalb eines Geleges wiederholte sich das gleiche Farbmuster immer wieder und war ziemlich konsistent für Bruten in unterschiedlichen Jahren, was darauf hinweist, dass die Farbmuster der Eischalen ein intrinsisches Merkmal sind. Eier einer zweiten Brut in der gleichen Brutsaison zeigten eine weniger satte blau-grüne Farbgebung als in den ersten Bruten, und innerhalb eines Geleges waren die später gelegten Eier generell heller und blasser blau-grün als die vorher gelegten Eier, was auf einen begrenzten Pigment-Vorrat hindeutet. Aber mögliche Zusammenhänge zwischen der Schalenfärbung einerseits und anderen Merkmalen der Weibchen oder ihrer „Eier-Investments“waren weniger klar und über die Untersuchungsjahre hinweg widersprüchlich. Unsere Ergebnisse legen nahe, dass es ein Potential für blau-grüne Eifärbung als Signal für eine hohe Güte der Weibchen gibt; aber nur weitere Untersuchungen können letztlich zeigen, ob die blau-grüne Eifärbung wirklich ein Signal mit Informationswert ist.

Notes

Acknowledgements

We thank S. Proulx for allowing us access to his nest box trail, and L. James (James Cattle Company) and B. Twan (Alkali Lake Ranch) for allowing access to their properties. A. Lacika, S. Sparks, E. O’Brien, and E. Hancock provided assistance in the field, and the Hancock family provided key logistical support. We appreciate the constructive comments of D. Hanley and an anonymous reviewer on a previous version of the manuscript. Funding was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and the University of Northern British Columbia. Our research protocols were approved by the University of Northern British Columbia Animal Care and Use Committee on behalf of the Canadian Council on Animal Care.

Compliance with ethical standards

Conflict of interest

The authors declare that there are no conflicts of interest.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2017

Authors and Affiliations

  1. 1.Ecosystem Science and Management ProgramUniversity of Northern British ColumbiaPrince GeorgeCanada

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