Nest decorations: an ‘extended’ female badge of status? Vicente García-Navas a, b , Francisco Valera a , Matteo Griggio c, * a Department of Functional and Evolutionary Ecology, Estaci on Experimental de Zonas  Aridas EEZA-CSIC, Almería, Spain b Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland c Department of Biology, University of Padova, Padova, Italy article info Article history: Received 7 July 2014 Initial acceptance 13 August 2014 Final acceptance 20 October 2014 Published online MS. number: 14-00553R Keywords: feather carrying female competition nest ornamentation Petronia petronia sexual selection signalling Extended phenotypes as signals are widely distributed among animal taxa. For example, many bird species build eye-catching nests or structures, which can potentially mirror the quality or ability of the builder. Rock sparrow, Petronia petronia, nests are usually overly decorated with feathers belonging to different species. Feather carrying in this and other species seems to play a role beyond their supposed thermoregulatory function, that is, to provide insulation to eggs and developing chicks. In this study, we documented for the first time this intriguing pattern of behaviour in the rock sparrow and experi- mentally tested its potential role as a sexually selected or status signal by means of a feather supple- mentation experiment carried out in two distinct populations from Italy and Spain. We found that females were responsible for feather carrying, laid larger clutches and provisioned their young at a lower rate in those nests with experimentally added feathers. Decorated nests sustained fewer intrusions by floater individuals and were defended with greater intensity by both parents than control nests, which supports the role of nest ornamentation as a status signal to conspecifics. Presence of experimental feathers did not significantly increase the frequency with which males provisioned their young but males tended to desert their brood less often and spent more time guarding the brood in experimental nests, indicating that feather presence may also play a role in an intersexual context. Overall, our results allow us to exclude the thermoregulation hypothesis as a likely explanation for the occurrence of these dec- orations and provide partial evidence for the idea that feather carrying conveys information to the partner and potential competitors. Our study thus supports the notion that nonbodily traits serving a direct (naturally selected) function can also evolve a signalling component. © 2014 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Many animals transfer information to conspecifics through morphological or behavioural traits such as gaudy plumages, extravagant body ornaments (antlers, horns) or courtship displays (Maynard Smith & Harper, 2003). From these conspicuous signals, receivers can assess the quality of the bearer (e.g. the fighting ability of a rival or the quality of a potential mate). Some species go further, collecting different materials to build complex structures or decorate their nests, which are used as an extension of their phenotype (‘extended phenotype’ sensu Dawkins, 1982). The extended phenotype concept refers to the potential effects of genes on the environment beyond the individual's body (reviewed in Schaedelin & Taborsky, 2009). For example, certain orb web spiders add extra silk structures to their capture webs whose function may be to deter predators or provide a warning signal for organisms that might destroy the web (Herbestein, Craig, Coddington, & Elgar, 2000; Th ery & Casas, 2009; Walter & Elgar, 2012). Thus, the contemporary signal effect of such decorations is different from their original function (i.e. entangle unsuspecting prey). In some cases, a novel trait can acquire a signalling effect and may explain, for example, the occurrence of odd items such as flowers, stones, snake skins, scats or human-derived material (foil, plastics) in the breeding structures of many species (e.g. € Ostlund-Nilsson & Holmlund, 2003; Schuetz, 2005; Trnka & Prokop, 2011). The pres- ence of this unusual nest-building material may indicate the builder's vigour, technical or harvesting ability and capacity to deter rivals (Mainwaring et al., 2014; Moreno, 2012). Thus, nests not only provide a protective environment for developing eggs and offspring but may also constitute an important signalling device to reveal information for members of the opposite sex (Barber, Nairn, & Huntingford, 2001; Brouwer & Komdeur, 2004; Hoi, Schleicher, &Valera, 1996; Schaedelin & Taborsky, 2006) or potential compet- itors (Penteriani & Delgado, 2008). In this sense, on the basis of the existing literature, we can discern two main functions of nest (or similar structures) decorations as visual cues: (1) to act as a pre- or postmating sexual signal (intersexual context) and (2) to advertise social status to potential intruders (intrasexual context). * Correspondence: M. Griggio, Department of Biology, University of Padova, U. Bassi, 35100 Padova, Italy. E-mail address: matteo.griggio@unipd.it (M. Griggio). Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav http://dx.doi.org/10.1016/j.anbehav.2014.10.024 0003-3472/© 2014 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Animal Behaviour 99 (2015) 95e107