Imperfectly redundant signals: Color, song, and mate choice in chestnut-sided warblers (Dendroica pensylvanica)

Why do so many organisms invest in multiple, multi-modal, sexually selected traits? Two hypotheses explain the evolution of multiple sexually selected traits: (1) the Redundant Signals hypothesis, which states that multiple traits broadcast overlapping information, either to amplify the signal, or to compensate for unreliable signals, and (2) the Multiple Messages hypothesis, which states that each trait broadcasts different information. I tested the redundant signals and multiple messages hypotheses by assessing multiple color and song traits in a colorful migratory songbird, the chestnut-sided warbler (Dendroica pensylvanica). For each warbler, I measured the area and spectral qualities of three plumage patches: the chestnut stripe, black facial mask, and yellow cap. I also measured several singing performance parameters for each of the two categories of songs sung by males of this species. In addition, I explored how color and song traits inform social and genetic mate choice and how female color relates to female defensive behavior. My results indicate that, in general, males with larger and brighter color patches sang their songs more consistently. However, color traits appear to be related to social mate quality, while song traits appear to be related to genetic mate choice. Males and females paired assortatively by color patch area, and colorful males provisioned nestlings at higher rates, while colorful females produced heavier young. Colorful females also exhibited quieter, more cryptic behavior during threats to their nests, while drab females used chipping displays against potential threats. Male warblers with better singing performance experienced more cuckholdry in their social nests, but also sired more extra-pair reproductive young. I interpret these results as evidence for a partial separation in signal content between the two types of sexual signals used by chestnut-sided warblers, with color indicating social mate quality, and song indicating genetic mate quality. And so, color and song traits in chestnut-sided warblers seem to function as imperfectly redundant signals that may also contain multiple messages