Life-history theory proposes that costs must be associated with reproduction. Many direct costs are incurred during breeding. There is also evidence for indirect costs, incurred after breeding, which decrease survival and future reproductive success. One possible indirect cost identi¢ed in birds is that breeding activity in some way compromises plumage quality in the subsequent moult. Here we propose a mechanism by which this could occur. Breeding activity delays the start of moult. Birds that start to moult later also moult more rapidlyöan e¡ect of decreasing daylength. Could this result in poorer quality plumage? We kept two groups of male European starlings, Sturnus vulgaris, one on constant long days and the other on decreasing daylengths from the start of moult. Decreasing daylengths reduced the duration of moult from 103 § 4 days to 73 § 3 days (p 5 0.0001). Newly grown primary feathers of birds that moulted fast were slightly shorter, weighed less (p 5 0.05) and were more asymmetrical. They had a thinner rachis (p 5 0.005), were less hard (p 5 0.01) and less rigid (p 5 0.05). They were also less resistant to wear so that di¡erences in mass and asymmetry increased with time. There was no di¡erence in Young's modulus. Poorer quality plumage will lead to decreased survival due to decreased £ight performance and increased thermoregulatory costs. Thus, reproduction incurs costs through a mechanism that operates after the end of breeding.

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Flight necessitates that the feather rachis is extremely tough and light. Yet, the crucial filamentous hierarchy of the rachis is unknown-study hindered by the tight chemical bonding between the filaments and matrix. We used novel microbial biodegradation to delineate the fibres of the rachidial cortex in situ. It revealed the thickest keratin filaments known to date (factor .10), approximately 6 mm thick, extending predominantly axially but with a small outer circumferential component. Near-periodic thickened nodes of the fibres are staggered with those in adjacent fibres in two-and three-dimensional planes, creating a fibre-matrix texture with high attributes for crack stopping and resistance to transverse cutting. Close association of the fibre layer with the underlying 'spongy' medulloid pith indicates the potential for higher buckling loads and greater elastic recoil. Strikingly, the fibres are similar in dimensions and form to the free filaments of the feather vane and plumulaceous and embryonic down, the syncitial barbules, but, identified for the first time in 140þ years of study in a new location-as a major structural component of the rachis. Early in feather evolution, syncitial barbules were consolidated in a robust central rachis, definitively characterizing the avian lineage of keratin.

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