AbstractThe shape and intensity of natural selection can vary between years, potentially resulting in a chronic reduction of fitness as individuals need to track a continually changing optimum of fitness (i.e., a “lag load”). In endangered species, often characterized by small population size, the lack of genetic diversity is expected to limit the response to this constant need to adjust to fluctuating selection, increasing the fitness burden and thus the risk of extinction. Here, we use long‐term monitoring data to assess whether the type of selection for a key fitness trait (i.e., lay date) differs between two reintroduced populations of a threatened passerine bird, the hihi (Notiomystis cincta). We apply recent statistical developments to test for the presence or absence of fluctuation in selection in both the Tiritiri Mātangi Island and the Kārori sanctuary populations. Our results support the presence of stabilizing selection in Tiritiri Mātangi with a potential moving optimum for lay date. In Kārori our results favour a regime of directional selection. Although the shape of selection may differ, for both populations an earlier lay date generally increases fitness in both environments. Further, the moving optimum models of lay date on Tiritiri Mātangi, suggesting that selection varies between years, imply a substantial lag load in addition to the fitness burden caused by the population laying too late. Our results highlight the importance of characterizing the form and temporal variation of selection for each population to predict the effects of environmental change and to inform management.