Dhondt A.A. (2001) Trade-offs between reproduction and survival in tits. ARDEA 89 (1): 155-166

The existence of trade-offs between reproduction and survival in birds has been documented by comparing variation in these traits across species and by using brood-size manipulation experiments to show experimentally the existence of a cost of reproduction. How natural selection has resulted in variation in adaptive strategies between species, however, is not well studied, and few empirical studies attempt to determine if changes in survival rate drive changes in reproduction, or if the causality is the other way around. I propose that a profitable approach to resolve this problem would be to compare life-history traits in different geographic populations of the same species. Variation in fecundity and adult survival rates across fourteen populations of Blue Tits Parus caeruleus are as large as those among eleven tits Parus spp., so that comparisons among these populations can help identify selective factors that influence reproduction and survival. In a set of populations of Great Tit Parus major and Blue Tit there exists a significant inverse relation between productivity (number of young fledged per female per season) and adult annual survival rates, but not between fecundity and survival rates. I discuss experiments that help to address the question of cause and effect, and conclude that variations in productivity cause variations in survival rates. I finally present different scenarios that propose evolutionary pathways illustrating how populations could evolve from one set of life-history traits to another