# Geography, Environmental Conditions and Life History Shape Patterns of Within‐Population Phenotypic Variation in North American Birds

## TL;DR

This study shows that geography, environment, and life history traits influence how much variation exists within bird populations across North America.

## Contribution

The study provides empirical evidence linking geographic, environmental, and life history factors to within-population phenotypic variation in birds.

## Key findings

- Phenotypic variation in birds is influenced by geographic and environmental factors.
- Life history traits also modulate the magnitude of within-population variation.
- Results support long-standing hypotheses about drivers of intraspecific biodiversity.

## Abstract

Intraspecific variation is a fundamental component of biodiversity, shaping species interactions and coexistence dynamics. While numerous mechanisms have been proposed to shape the degree of phenotypic variation within species, many remain largely untested or poorly explored at broad spatial and taxonomic scales. Using data from nearly 200,000 bird captures from 99 species across North America, we investigated hypothesized drivers of within‐population phenotypic variation, using body mass and wing length as traits of interest. The magnitude of observed phenotypic variation was modulated by a combination of geographic, environmental, and life history factors. This was true whether considering differences in within‐population phenotypic variation within or among species. The impact of these non‐mutually exclusive mechanisms has resulted in substantial variation in the observed magnitude of within‐population phenotypic variation. These results provide empirical evidence for a set of long‐standing hypotheses regarding the processes that regulate observed patterns of this understudied, but important, component of biodiversity.

Intraspecific variation is a fundamental component of biodiversity, shaping species interactions and coexistence dynamics. While numerous mechanisms have been proposed to shape the degree of phenotypic variation within species, many remain largely untested or poorly explored at broad spatial and taxonomic scales. Using data from nearly 200,000 bird captures from 99 species across North America, we show that phenotypic variation within populations was modulated by a combination of geographic, environmental, and life history factors, with support for a set of long‐standing hypotheses.