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

## TL;DR

This study compares how two similar but differently sized birds in the Tibetan alpine region grow, finding that ecological factors can override body size constraints.

## Contribution

The study reveals that ecological niche segregation and seasonal constraints can override allometric growth predictions in sympatric bird species.

## Key findings

- The larger streaked rosefinch grows faster than the smaller pink-rumped rosefinch despite allometric expectations.
- Diet and seasonal time constraints influence growth rates differently between the two species.
- Ecological factors can override body size constraints in shaping growth trajectories in alpine birds.

## Abstract

The present research compared the growth trajectories of two sympatric, size-distinct Carpodacus species, namely the larger streaked rosefinch (C. rubicilloides) and the smaller pink-rumped rosefinch (C. eos), aiming to test the prediction that ecological factors can help organisms to overcome the allometric constraints of body size on growth rate. Logistic growth models showed that, although nestlings of the two species fledged at a similar age, the larger species grew significantly faster than the smaller species, a pattern opposite to what would be predicted from body size. This pattern likely stems from a segregation in nutritional niche, whereby the larger species relied mainly on energy-rich legume seeds, whereas the smaller species consumed smaller, more diverse grass seeds that require greater searching effort. In addition, a stronger seasonal time constraint experienced by the larger species may further accentuate this growth divergence. The result supports the initial prediction and contributes to our understanding of avian life-history divergence in alpine environments.

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.