Nestling Growth Strategies of Two Sympatric Rosefinch Species in a Tibetan Alpine Habitat

How ecological factors, particularly food availability and breeding season length, shape growth strategies in ways that can override allometric constraints is a key question in life-history evolution. Here we address this question by comparing body-mass growth of two Carpodacus species of distinct body size, the streaked rosefinch C. rubicilloides (~40 g) and the pink-rumped rosefinch C. eos (~20 g), which commonly nest in the alpine zones of south Tibet. Although nestlings of both species fledged at the same age and at a similar body size relative to the adults, the larger species grew significantly faster than the smaller one, despite an allometric scaling constraint that predicts the opposite. This counterintuitive pattern may be explained by interspecific differences in (i) nestling food availability and (ii) time constraints on growth. Nestlings of C. rubicilloides fed exclusively on the seeds of a highly productive leguminous plant, whereas those of C. eos consumed a diverse array of small seeds that were expected to require greater searching effort, potentially limiting energy intake. Concurrently, C. rubicilloides experienced a shorter breeding window (~30 days) compared to its congener (~50 days), a difference likely linked to species-specific availability of food resources. These observations suggest that, for closely related species coexisting in harsh alpine environments, the effect of ecological factors on growth can override that of allometric constraints in shaping growth trajectories.