Altitudinal morphometric variation in Rufous-collared Sparrow Zonotrichia capensis (Aves: Passerellidae) in the southwestern Peruvian Andes

Abstract

Ecogeographic rules such as Bergmann's and Allen's provide a framework to evaluate how morphological traits vary across environmental gradients. We tested these rules in the Rufous-collared Sparrow (Zonotrichia capensis), a broadly distributed Neotropical passerine, along an altitudinal gradient in southwestern Peru. We conducted morphometric analysis on 192 individuals from 16 localities distributed across two river basins in the Arequipa region, covering elevations from sea level to 3800 m. Seven morphological traits were measured, including body mass (to test Bergmann's rule) and appendages such as bill length, width, and height, wing length, tarsus length, and tail length (to assess Allen's rule). Generalized linear mixed models were used to evaluate the effects of sex, elevation, and river basin. Our results support Bergmann's rule: body mass increased significantly with altitude. However, we found only partial support for Allen's rule—bill length decreased with elevation, while other appendages showed no clear pattern. Additionally, males exhibited larger body size and longer bills than females. These findings suggest that Z. capensis populations exhibit altitude-associated morphological variation consistent with thermoregulatory expectations, although non-thermal ecological and biological factors may also shape phenotypic diversity across elevations.