Comparison of the Reproductive Biology of Two Neotropical Wrens in an Unpredictable Environment in Northeastern Colombia

Abstract

Abstract Buff-breasted (Thryothorus leucotis) and Rufous-and-white (T. rufalbus) wrens living in a dry forest in northeast Colombia (Parque Nacional Natural Tayrona) are faced with a large year-to-year uncertainty in the arrival time of the rainy season, as well as the amount of rain falling in the first six months of the year. Those factors are thought to be important cues used by those species in their reproductive decisions. In this study, I gathered data on several reproductive parameters (clutch size, nesting success, timing of breeding, renesting attempts) for both species during two years of contrasting rainfall patterns. I collected information on the foraging behavior of both species to identify their main food and to study how rainfall affects the dynamics of those resources. Buff-breasted Wrens fed mostly in the understory, gleaning arthropods from upper and lower leaf surfaces, dry branches, and aerial litter. Numbers of arthropods in those microhabitats depend strongly on the amount of rainfall; understory arthropod levels are low during the dry season and increase with the arrival of the rains. Buff-breasted Wrens timed their reproduction with the arrival of the rains in both years, delaying the onset of breeding significantly and continuing to breed during the dry year (1994). Rufous-and-white Wrens spent a large proportion of their time feeding on arthropods in the leaf litter. Number of arthropods in the litter varied little between dry and wet periods. Therefore, Rufous-and-white Wrens had a more constant food environment despite large differences in rainfall within and between the years of the study. That species started breeding earlier in the dry season and extended its breeding longer than Buff-breasted Wrens. My observations suggest that the evolution of the reproductive strategies in those species was mostly through the change of behavioral parameters rather than physiological reproductive parameters such as changes in clutch size, egg size, or number of broods.