Golondrinas de las Americas: pace of life and life history evolution in Tachycineta swallows

Variation in life histories within and across species is one of the enduring research questions in evolutionary biology. One promising conceptual framework for examining interspecific variation is ‘pace of life’, the hypothesis that closely-related species in different environmental conditions will differ consistently in physiological and behavioral traits linked to their life histories in a way predicted by selection for the maximization of life-time reproductive output. Here we present an overview of our preliminary findings from a large comparative study of Tachycineta swallows across the Western Hemisphere in the Golondrinas de las Americas project. Overall, life histories differ consistently between tropical and temperate species, with tropical species showing smaller clutch sizes, lower parental care, slower growth rates and thermoregulatory development, and lower incubation investment. These patterns are consistent with a slower pace of life in species found closer to the Equator. However, we also find results that confound a simple pace of life explanation. In particular, while time spent incubating is lower in the tropics, tropical species show higher incubation temperatures and greater thermal sensitivity of embryos. We also find higher relative investment in yolk in tropical species. Overall, our results suggest a role of both ecological conditions and evolutionary history in explaining the diversification of life histories in Tachycineta swallows.