Background/Question/Methods Knowing the distribution of migratory species at different stages of their life cycle is necessary for their effective conservation. For the Monarch butterfly (Danaus plexippus), although its overwintering distribution is well-known, the available information on pre-migration distribution is limited to the studies estimating the natal origins of overwintering Monarchs in Mexico (i.e. post-migration data). However, the pre-migration distribution and the natal origins of overwintering Monarchs can be equivalent only if we assume that migrating Monarchs have the same mortality rate irrespective of their origins.
Results/Conclusions To estimate Monarchs’ pre-migration distribution, we used data reported by community scientists before Monarchs start their fall migration, i.e. before migration mortality, and controlled for sampling bias. We compared the resulting distribution to distributions estimated using post-migration data (i.e., isotopic-based natal origin assignments) to determine whether migration mortality varies between butterflies originating from different regions. This comparison suggests that Monarchs starting their migration from the Northwest and Northeast breeding region may have a lower mortality rate than those coming from other regions. In contrast, those that originated from the Southwest breeding region appear to have a higher mortality rate than other Monarchs. Our pre-migration distribution map will be useful in future studies estimating the rates, distribution, and causes of mortality in migrating Monarchs.