Hatching Order and Seasonal Timing of Development Predict Bill Morphology of Nestling and Adult Lincoln’s Sparrows

. Traits under strong directional or stabilizing selection can nonetheless vary considerably if the selective forces impinging on them vary in strength or direction. Although such variation is commonly manifested between unrelated individuals, some of the largest differences occur between siblings because of differences in their developmental experience. This is exemplified by the broods of altricial birds, in which asynchronous hatching produces a competitive hierarchy that influences an individual's developmental rate and fitness. Using Lincoln's Sparrow (Melospiza lincolnii), I examined whether the order of hatching or the seasonal timing of development (another factor that influences offspring fitness and estimated as the date of initiation of the clutch from which an individual hatched) predicted variation in bill shape, a trait often under strong natural and sexual selection. I measured individuals as nestlings and as adults for a metric of bill shape (height/width) that is associated with an adult male Lincoln's Sparrow's song performance. I found that an individual's bill shape during the nestling stage positively correlated with its bill shape during the adult stage. Nestling bill shape declined (became wider and flatter in cross-section) with hatching order, and the bill shape of both nestlings and adults declined with the individual's seasonal timing of development. These findings raise the possibility of a trade-off between life stages in the optimization of songbird bill morphology and may explain the persistence of suboptimal bill shapes among adult songbirds in the presence of strong stabilizing or directional selection.