Morphological and ecological causes of interspecific aggression between golden-winged and chestnut-sided warblers.

Interspecific aggression is widespread throughout the animal kingdom, yet research that documents the evolutionary and ecological consequences remains limited and unclear. Aggressive behaviors are often indicative of an ecological niche overlap between morphologically and ecologically similar species, which can cause interference competition between animals. Here, I focus on determining the stimulus for aggression between golden-winged (Vermivora chrysoptera) and chestnut-sided warblers (Setophaga pensylvanica) in the southern Appalachian Mountains by exploring two potential explanations: mistaken identity and interspecific competition. I used museum and field data and used both ecological modeling and experimental field methods to investigate the cause of interspecific aggression. First, using museum specimens, I found that the crown plumages of both warblers are two distinct shades of yellow that both warbler species should theoretically be able to distinguish. Next, I studied warblers for two field seasons to document (1) potential fitness consequences and (2) if mistaken identity were to occur using model birds. I found that competition is unlikely to occur between warblers and interspecific aggression is likely a function of mistaken identity. Without net gains from behaving as such, these interspecific aggression between warblers may drive crown morphology to become more distinctive to reduce species recognition errors.