Size, Ornamentation, and Flight Feather Morphology Promote Within-pair Paternity in a Sexually Dimorphic Passerine

Morphology in sexually dimorphic species is related to increased opportunity for sexual selection when traits reflect individual quality. In socially monogamous species, it may function to increase variance in reproductive success if exaggerated traits are related to the opportunity to engage in extra-pair paternity (EPP). Nonetheless, it is poorly understood if ornamental versus functional traits are differentially related to the distribution of paternity across individuals. We examined EPP in the Scissor-tailed Flycatcher (Tyrannus forficatus), a sexually dimorphic suboscine passerine, to determine how flight feather morphology (both ornamental and related to flight performance), ornamental coloration, and flight performance were related to paternity for males and females. We assessed paternity at 140 nests across 7 years and found that 73% of nests contained extra-pair young with 59% of nestlings resulted from extra-pair fertilizations. Males that secured paternity with their social mate had larger size (wing chord and tail length), better body condition, and more exaggerated ornamental traits (color, primary notch, and streamer length) than cuckolded males. When compared directly to the social males they cuckolded, however, cuckolders had shorter primary notches and had duller coloration. Elongated flight feather features were associated with greater maneuverability during flight, and longer primary notches and tail streamers were associated with maintenance of within-pair paternity, possibly via flight displays. Females that cuckolded their mates had less exaggerated flank coloration but better body condition than those that did not. We posit that in this system, there is a lack of strong selection on extra-pair mate morphology; rather, high-quality males maintained WPP while females in good condition were more likely to cuckold social males of poorer quality, signaling a role of reciprocal selection in extra-pair mate choice. Determinants of the frequency and extent of extra-pair paternity are central to our conception of mating systems, population genetics, and sexual selection. Morphological determinants can influence faithfulness to a mate and are a key aspect of locomotion, limiting or enhancing an individual’s ability to interact with others. We examined how ornamental (coloration and tail streamers) and functional traits involved in flight (wing and tail structure) correlated with cuckoldry. Male condition, size, and ornamental traits were negatively associated with cuckoldry; yet, social males were cuckolded by males of poorer quality. Flight feather characteristics were positively associated both with flight performance and maintenance of paternity, suggesting a role of flight in cuckoldry. Female body condition was related to cuckoldry and perhaps indicates reciprocal choice by males for healthy extra-pair mates. We posit a lack of strong selection on extra-pair mate morphology in this system; rather, high-quality males maintained WPP while females were more likely to cuckold social males of poorer quality