Bird nests function to protect parents, eggs or offspring against fluctuations in the environment. In Blue Tits (Cyanistes caeruleus) producing first or replacement clutches, we examined relationships between nest height (a measure of nest size) and several parameters of breeding performance that are often quantified in long-term avian field studies. We artificially increased the cost of reproduction in randomly selected females by removing the whole first clutch and nest material after about 5 days of incubation. This experiment resulted in replacement nests. Experimentally increasing the range of breeding conditions across females, we expected to have more power to detect a positive correlation between nest height and breeding success. However, we found that nest height was not significantly related to any of the breeding traits investigated (clutch size, number of hatchlings, number of fledglings, hatching success, fledging success, chick mass), after controlling for factors frequently investigated in long-term monitoring programs and known to influence breeding performance (female age, year, clutch type as a proxy of egg-laying date). We conclude that the identification of the exact underlying mechanisms causing relationships between nest characteristics and breeding performance will require more experimental study and the exploitation of larger datasets, both within and across avian model systems.Zusammenfassung Vogelnester schützen Elternvögel, Eier oder Jungvögel vor Schwankungen in der Umwelt. Wir haben bei Blaumeisen (Cyanistes caeruleus), die Erst-oder Ersatzgelege produzierten, die Beziehungen zwischen Nesthöhe (einem Maß für Nestgröße) und mehreren Bruterfolgsparametern, die oftmals in Langzeitfeldstudien an Vögeln quantifiziert werden, untersucht. Wir haben für zufällig ausgewählte Weibchen die Kosten der Fortpflanzung experimentell erhöht, indem wir das gesamte Erstgelege sowie Nistmaterial nach etwa fünf Tagen Bebrütung entfernt haben. Dieses Experiment hatte Ersatznester zur Folge. Unsere Erwartung war, dass wir eine positive Korrelation zwischen Nesthöhe und Bruterfolg eher entdecken können, wenn wir die Brutbedingungen für Weibchen experimentell variabler machen. Wir fanden jedoch, dass die Nestgröße nicht signifikant mit irgendeinem der untersuchten Brutparameter (Gelegegröße, Anzahl geschlüpfter und ausgeflogener Jungvögel, Schlupf-und Ausfliegeerfolg, Körpermasse der Küken) zusammenhing, nachdem wir Faktoren, die in Langzeitstudien häufig untersucht werden und bekanntermaßen die Fortpflanzungleistung beeinflussen (Weibchenalter, Jahr, Gelegetyp als Maß für Legedatum), berücksichtigt hatten. Wir schlussfolgern, dass für die Ermittlung der genauen Mechanismen, die Zusammenhänge zwischen Nestmerkmalen und Fortpflanzungleistung verursachen, mehr experimentelle Untersuchungen und grö-ßere Datensätze benötigt werden, sowohl innerhalb als auch zwischen verschiedenen Vogel-Modellsystemen.

Summary 1.A growing number of studies suggest that female ornaments are linked to maternal quality and influence male mate choice. These findings challenge the traditional male-biased view of sexual selection and the hypothesis that female ornaments are the outcome of a genetic correlation with male ornaments. To further test the hypothesis that female traits have a function, it is now essential to investigate their honesty and to determine how signalling and reproduction interact in females. If female traits are honest indicators of quality, then they are likely to have a specific signalling function. 2. We investigated whether carry-over effects of reproduction might ensure the honesty of plumage colour signalling of a bird species with conspicuous UV-blue and yellow coloration, the blue tit Cyanistes caeruleus. Reproductive effort was manipulated by removing clutches, thereby forcing both sexes to reproduce twice and to raise chicks later in the breeding season when food is less abundant. In the year following this manipulation, we investigated the change in plumage in experimental and control males and females. The change was measured in the two putative feather ornaments, the UV-blue cap and the yellow breast, and another feather trait probably less likely to be sexually selected: the wing length. We also tested whether higher-quality females had their coloration less affected by the experiment. 3. We found that control but not manipulated males and females increased their signal towards UV. In addition, in the manipulated group, females that were able to lay more eggs had their UVblue coloration less affected by the treatment. For yellow coloration, we found that manipulated yearlings but not manipulated adults decreased their yellow chroma in comparison with control. Lastly, our results show that the condition of the manipulated females tended to be positively correlated with yellow chroma. 4. These results show that the trade-offs between reproduction and signalling can ensure the honesty of conspicuous plumage traits in female and male blue tits. In addition, they suggest that female traits have the potential to evolve under sexual selection in this and other bird species.

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