Brighter Great Tit parents build bigger nests

Nest size is commonly believed to be naturally selected,and to covary with requirements of developing eggs andnestlings (Collias & Collias 1984). In many altricial birdspecies, it is an important component of parental effort,as parents devote an important amount of energy andtime to nest building, but by doing so they expose them-selves and their offspring to a substantial predation risk(Martin et al. 2000). In this paper, we studied nest-sizevariation in a Great Tit Parus major population in rela-tion to breeding factors and parental carotenoid-basedplumage characteristics. The Great Tit is a secondary-cavity nesting species and females are solely responsiblefor building the nest (Gosler 1993). Sexual signalling viacarotenoid-based plumage coloration is known to occurin this species (Hill & McGraw 2006). We found thatplumage chroma of female and especially male GreatTits was positively related to nest size, suggesting greaterfemale investment in nest building when mated to malesin good condition.Nest-building behaviour and ultimately the nest itselfare often associated with courtship and pair formation(Collias & Collias 1984, Soler et al. 1998a). Further,nest-building behaviour may be involved in post-matingsexual communication between partners (Soler et al.1998a). In this way, individuals may increase their nest-building effort to signal their quality, or willingness toinvest in the current reproductive event (e.g. Soler et al.2001). Likewise, they may respond to their partner’scharacteristics in a similar way, to promote their invest-ment. However, knowledge of the use of the nest as apost-mating sexual display is limited, and most empiricalevidence comes from species where males are responsi-ble for nest building (Moreno et al. 1994, Hoi et al.1996, Soler et al. 1998b, 2001).Plumage coloration is known to be a sexually selectedcharacter signalling individual quality, and is ofteninvolved in pair formation and courtship (Hill &McGraw 2006). In particular, carotenoid-based plumagecoloration has been the focus of numerous recent stud-ies, and as carotenoids can only be obtained through thediet, they can be reliable indicators of the bearer’s feed-ing efficiency (Senar & Escobar 2002, Senar et al. 2008),body condition (Saks et al. 2003a, 2003b, Ferns & Hins-ley 2008) and survival (Horak et al. 2001). Thus, carot-enoid-based plumage coloration can reflect individualparenting ability (Senar et al. 2002, Doutrelant et al.2008), and consequently could be expected to affect sev-eral components of parental investment and resourceallocation (Clutton-Brock & Godfray 1991). In this way,nest-size variation may be related to parental quality,which in turn may be reflected by the parents’ plumagecoloration.METHODSThe study was performed in a forest area (c.4km