Characteristics of monarch butterflies (Danaus plexippus) that stopover at a site in coastal South Carolina during fall migration

While the annual fall migration of eastern North American monarch butterflies (Danaus plexippus) to wintering sites in central Mexico is a well-known and frequently-studied phenomenon, one aspect of this behavior that remains poorly understood is the nature of their migratory stopovers. Like migrating birds, monarchs must stop frequently during their journey to rest and refuel (i.e. obtain food), and why they choose to stop and for how long are important pieces of information, yet these have rarely been examined for monarchs. In this study we utilized data from a long-term monarch migration tagging operation in South Carolina to address certain aspects of this knowledge gap. Monarchs are tagged at this site each fall and recaptured individuals are also noted. Here we compared the characteristics of these recaptured individuals (n=407 over 13 years) to those that were never recaptured (n=12,989), focusing specifically on their wing size and wing condition, which was scored on a 1-5 scale. We also looked for evidence that stopover lengths are influenced by size or condition. The overall recapture rate at this site was 3.1%, although there was a small degree of annual variation in this rate (ranging from 1.3 5.6%). Males were recaptured twice as often as females. Recaptured monarchs did not differ from non-recaptured monarchs in wing size, but did have greater wing damage and wear than non-recaptured individuals. The recapture rate was the highest (8.5%) for monarchs with the most worn and damaged wings, while the rate was the lowest (2.9%) for monarchs with the freshest wings with no damage. Furthermore, monarchs with highly damaged and worn wings tended to remain longer at the stopover site than those with no damage or wear. Taken together, these results indicate that wing condition influences whether or not monarchs remain at a stopover site and for how long. In addition, they suggest that monarchs with poor wing condition may have a slower pace of migration owing to their more frequent and longer stopovers.