# Birds That Don't Exist: Niche Pre‐Emption as a Constraint on Morphological Evolution in the Passeroidea

## TL;DR

This study explores why certain bird body types never evolved in a group of songbirds, suggesting competition from other species may have blocked their development.

## Contribution

The study introduces a framework using topological data analysis to detect and explain unoccupied morphological gaps in evolutionary history.

## Key findings

- A morphological gap in Passeroidea songbirds is unoccupied despite being surrounded by similar forms.
- The gap shows no evidence of past occupation or intrinsic constraints, pointing to competitive exclusion.
- Early-colonizing species outside Passeroidea may have preempted key habitats, limiting evolutionary opportunities.

## Abstract

Understanding why some viable body forms never evolve can reveal how ecological and evolutionary forces shape biodiversity. We investigate this question in the Passeroidea, a large group of songbirds, by analysing their morphological trait space using topological data analysis and ancestral state reconstruction. We identify a persistent morphological gap densely surrounded by extant species but unoccupied throughout passeroid diversification. The gap patterns deviate from stochastic expectations and show no evidence of past occupation, rendering undirected trait evolution and extinction unlikely. Similar morphologies exist in other bird lineages, ruling out intrinsic constraints or niche absence. Geographic distributions and traits of passeroids versus non‐passeroid gap occupants point to competitive exclusion as the plausible explanation: early‐colonising territorial specialists outside the Passeroidea may have preemptively occupied key habitats, limiting evolutionary opportunities for later‐arriving lineages. We demonstrate how historical contingency can shape macroevolutionary outcomes and introduce a generalizable framework for investigating structural gaps in trait evolution.

We use topological data analysis to reveal a persistent morphological gap in a major group of songbirds (superfamily Passeroidea). The gap remained unoccupied for millions of years, even though nearby morphologies are common and the same body form exists in other groups of birds. Our results suggest that competitive exclusion has constrained the evolution of this missing form.