# Vocal Complexity Constrains the Dear Enemy Effect: A Comparative Study of Coal Tits and Green‐Backed Tits

## TL;DR

This study shows that birds with more complex songs, like coal tits, can better recognize neighbors and strangers, while those with simpler songs, like green-backed tits, cannot.

## Contribution

The study reveals that vocal complexity influences the ability to exhibit the dear enemy effect in birds.

## Key findings

- Coal tits showed a strong dear enemy effect, responding more aggressively to strangers.
- Green-backed tits did not differentiate between neighbors and strangers in their responses.
- Vocal repertoire size and population-level diversity correlate with the ability to discriminate neighbors from strangers.

## Abstract

The “dear enemy effect,” wherein territorial animals exhibit reduced aggression toward familiar neighbors compared to strangers, is a widespread strategy to minimize energy expenditure on territory defense. However, whether and how this behavioral capacity varies across with differing vocal complexity remains poorly unclear. We investigated neighbor–stranger discrimination (NSD) in two sympatric tit species that exhibit a stark contrast in song repertoire complexity: coal tits (
Periparus ater
) and green‐backed tits (
Parus monticolus
). Acoustic analysis revealed that coal tits possessed a large population‐level song‐type diversity (19 distinct song types) and, crucially, a significantly larger individual syllable repertoire size compared to green‐backed tits (5 song types). Playback experiments showed that coal tits exhibited a robust “dear enemy” effect, responding to strangers with significantly closer approach distance and higher flight frequencies near the nest. In contrast, green‐backed tits showed uniformly low and undifferentiated responses toward both playbacks of familiar neighbors and strangers, indicating a lack of discrimination. This interspecific divergence was underpinned differences in individual repertoire size and population‐level acoustic diversity, with green‐backed tits exhibiting higher vocal similarity among individuals. These results demonstrate that the capacity for fine‐scale NSD is not universal and suggest that constrained vocal systems—characterized by minimal individual repertoires and high acoustic similarity among individuals—may limit the potential for vocal individual recognition, thereby favoring alternative territorial strategies.

This study investigated neighbor–stranger discrimination in two sympatric tit species exhibiting contrasts in song repertoire complexity: coal tits (
Periparus ater
) and green‐backed tits (
Parus monticolus
). Coal tits exhibited robust discrimination, responding with significantly greater aggression to strangers. In contrast, green‐backed tits showed a lack of discrimination. The dear enemy effect is not universal and that vocal repertoire complexity plays a critical role in shaping individual recognition abilities.