Seasonal vs opportunistic breeders, a seasonal morphological and endocrine comparative study of the gonadal cycle in birds.

Avian reproduction is usually seasonal and strongly influenced by environmental factors, primarily photoperiod, which is mainly perceived by deep brain photoreceptors in the hypothalamic region and regulates the activation of the hypothalamic-pituitary-gonadal axis. However, certain species have developed adaptable reproductive strategies that differ from the classical seasonal model. In this study, we analyzed seasonal variations and gonadal asymmetry in three dove species with different reproductive strategies: The White-tipped Dove (Leptotila verreauxi) and the Eared Dove (Zenaida auriculata), both native to South America, and the Rock Dove (Columba livia), an introduced species from the Northern Hemisphere. Our results indicate that the White-tipped Dove, a seasonal breeder, follows a photoperiod-dependent reproductive pattern, showing seasonal variations in gonadal size and reproductive activity, as well as testosterone plasma levels, with no evidence of gonadal asymmetry. In the Eared Dove, an opportunistic breeder, the gonadal response is less pronounced, with testicular activity present even during months with short photoperiod, no association with testosterone plasma levels, and no gonadal asymmetry. Finally, the Rock Dove, another opportunistic breeder, exhibits the most variable gonadal response, with no association between photoperiod, gonadal morphology, and testosterone plasma levels. These findings suggest that the White-tipped Dove follows a classical photoperiod-regulated seasonal reproductive cycle, whereas the Eared Dove and the Rock Dove display greater reproductive plasticity, likely influenced by other factors, such as anthropogenic food availability, rather than photoperiod.