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Sexual dimorphism and offspring growth: smaller female Blue Tit nestlings develop relatively larger gapes

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Sexual size dimorphism results in asymmetric sibling competition, and nestlings of the smaller sex are expected to prioritise the development of those morphological characters that maximise effective sibling competition. In this study, we test the prediction that female Blue Tit Cyanistes caeruleus nestlings, which are smaller than males, preferentially develop relative gape area in a trade-off against growth of mass and head-bill length. We found that whilst male nestlings were heavier, female and male nestlings had similar head-bill lengths, but females had relatively larger gape areas. Therefore, female nestlings were investing relatively more resources in gape area than males because of their overall smaller body size, presumably because gapes are an integral part of the process used by nestlings to solicit food from their parents.


Geschlechtsdimorphismus und Wachstum der Jungen: kleinere weibliche Blaumeisen-Nestlinge entwickeln relativ größere Schnabelöffnungen beim Sperren.

Geschlechtsbedingte Unterschiede in der Körpergröße führen bei Nestlingen zu asymmetrischer Geschwister-Konkurrenz, wobei man von den Nestlingen des kleineren Geschlechts erwarten würde, dass sie diejenigen morphologischen Eigenheiten besonders entwickeln, die ihnen in der Konkurrenz mit den Geschwistern speziell helfen. In unserer Untersuchung testeten wir die Voraussage, dass weibliche Blaumeisen-Nestlinge (Cyanistes caeruleus), die kleiner als ihre männlichen Geschwister sind, die Größe ihrer Schnabelöffnungen bevorzugt entwickeln, im Kompromiss mit dem entsprechenden Wachstum von Schnabellänge und -masse. Wir fanden, dass die weiblichen und männlichen Nestlinge ähnlich große Schnabellängen hatten, obwohl die männlichen Nestlinge schwerer waren, dass aber die weiblichen relativ größere Schlundöffnungen zeigten. Also investieren weibliche Nestlinge bei generell geringerer Körpergröße relativ mehr Ressourcen als die männlichen in die Größe der Schnabelöffnungen. Der Grund hierfür liegt vermutlich darin, dass die Schnabelöffnungen als Sperren ein integraler Bestandteil des Prozesses ist, mit dem die Jungen Futter von ihren Eltern erbetteln.

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We thank Terry Burke, Andy Krupa, Gavin Horsburgh, Clemens Küpper, Maria Elena Mannarelli and Deborah Dawson at the NERC Biomolecular Analysis Facility at Sheffield for helping M.C.M. to determine the sex of the nestlings; Ian Owens, Jan-Åke Nilsson, Tore Slagsvold, Karen Wiebe and Ken Wilson for useful comments; and the Natural Environment Research Council for funding via studentships to M.C.M. (NER/S/A.2003/11263) and to M.D. (NER/S/A/2002/10396).

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Correspondence to Mark C. Mainwaring.

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Communicated by T. Friedl.

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Mainwaring, M.C., Dickens, M. & Hartley, I.R. Sexual dimorphism and offspring growth: smaller female Blue Tit nestlings develop relatively larger gapes. J Ornithol 153, 1011–1016 (2012).

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