Sequential Partial Migration Across Monarch Generations in Michigan

Running title: Monarch alternative migration: We collected 434 adult monarchs and surveyed milkweeds for immature monarchs in southwest Michigan, USA in order to test the hypothesis that monarchs are temporally variable, sequential partial migrants rather than partial migrants that may be spatially separated. Adult size, wing wear, female egg counts, fat content and sequestered chemical defenses were measured in monarchs across an entire season from spring migrant arrival, through breeding, until autumn migrant departure. We predicted that a population characterized by starting from all migrants and no residents, through breeding residents, to all migrants and no residents should show life history measures consistent with changes in these proportions. Results show that female monarch spring migrants arrive with chorionated eggs and high wing loads in both intact and fat-extracted adults. Wing loads of both males and females decrease during the summer and increase again immediately before autumn departure, when the fat content of all adults increases markedly. The high fat content of spring arrivals is also characteristic of migrants. Cardenolide content of adults showed a similar pattern of high content in spring arrivals, a decrease in the summer and then an accumulation of cardenolide defenses in adults in late summer just before migratory departure. We conclude that these results are consistent with temporally variable, sequential partial migration in a short-lived insect that contrasts with spatially variable partial migration in longer-lived vertebrates.