Data from: Migratory monarchs that encounter resident monarchs show life-history differences and higher rates of parasite infection

Environmental change induces some wildlife populations to shift from
migratory to resident behaviours. Newly formed resident populations could
influence the health and behaviour of remaining migrants. We investigated
migrant-resident interactions among monarch butterflies and consequences
for life history and parasitism. Eastern North American monarchs migrate
annually to Mexico, but some now breed year-round on exotic milkweed in
the southern U.S. and experience high infection prevalence of protozoan
parasites. Using stable isotopes (2H, 13C) and cardenolide profiles to
estimate natal origins, we show that migrant and resident monarchs overlap
during fall and spring migration. Migrants at sites with residents were 13
times more likely to have infections and three times more likely to be
reproductive (outside normal breeding season) compared to other migrants.
Exotic milkweed might either induce these states or attract migrants that
are already infected or reproductive. Increased migrant-resident
interactions could affect monarch parasitism, migratory success, and
long-term conservation.