ANCIENT ECOLOGICAL DIVERSIFICATION EXPLAINS LIFE-HISTORY VARIATION AMONG LIVING BIRDS

Recent work suggests that life-history strategies lie along a precise line of equilibrium between mortality, fecundity and growth. However, it has proved remarkably difficult to find a convincing ecological exp lanation for why different organisms are at different positions along this line. This is surprising because life-history traits are generall y considered to be closely connected to fitness. Here, we test four ca ndidate variables which represent competing ecological explanations fo r the origin of avian life-history diversity. food type; foraging rang e; developmental mode; and nesting habit. First, we find that over 90% of the variation in key life-history traits occurs among lineages cor responding to the phylogenetic level of between families or above. Thi s suggests that variation among living species is almost entirely due to events which occurred in the ancient evolutionary history of birds. Hence we use minor-axis regression models to quantify the direction a nd magnitude of ancient changes in 'reproductive effort'. We then test for ecological correlates of these ancient changes. Of the four ecolo gical variables tested, only changes in nesting habit are significantl y correlated with ancient changes in 'reproductive effort'. The adopti on of safe nest sites among archaic birds is consistently associated w ith reduced 'reproductive effort'. We suggest that life-history variat ion among living birds is largely due to diversification in nesting ha bit which occurred over 40 Ma sp, between the Cretaceous and Eocene.