Butterfly migration along the Georgia coast and recommendations for optimal surveying

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Krishna Sharma, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, Joseph E. Colbert, Odum School of Ecology, University of Georgia, Christa F. Hayes, Hayes Environmental Concepts, GA and Elizabeth King, Center for Integrative Conservation Research, University of Georgia, Athens, GA

Background/Question/Methods
Insect migrations involve the massive transport of energy and nutrients across entire landscapes, which can have cascading ecological effects. The southeastern Atlantic coast is a migration route that hosts an overlooked diversity of migratory invertebrate species. A novel coalition of site managers, the Butterflies of the Atlantic Flyway Alliance (BAFA), is currently surveying migratory butterflies to document this understudied phenomenon. From 2018 through 2020, BAFA conducted surveys at 52 sites along the Georgia coast. Our first research objective was to characterize Georgia coastal migration in terms of butterfly diversity, plant resource use, habitat preference, and environmental conditions. Our second research objective was to assess how time of day and survey duration affect sample precision. For eight days we conducted seven-hour surveys of gulf fritillaries (Agraulis vanillae). We used a temporal window analysis, extracting survey samples ranging from ten minutes to two hours long and comparing these to total average daily rates with R2 as a metric of survey sample precision.
Results/Conclusions
Over eight days, we counted 96,000 gulf fritillaries. We also noted the migration of cloudless sulphurs (Pheobis sennae), monarchs (Danaus plexippus), common buckeyes (Junonia coenia), zebra longwings (Heliconius charithonia), and long-tailed skippers (Urbanus proteus). Several butterfly species were seen co-feeding on Baccharis halimifolia and other coastal plants. When the sky was overcast, gulf fritillaries would immediately halt migration. Around mid-afternoon, gulf fritillaries shifted from being predominantly in beach habitat to predominantly in dune habitat, implying their use of beach as dispersal habitat and dune as feeding and roosting habitat. Northerly and easterly winds correlated with higher migration rates.
Butterfly migration rate had a roughly parabolic curve throughout the day, peaking around early afternoon. Surveys that began at 11:00 or 14:00 (EST) most accurately represented average daily migration rate. Survey duration did not have a consistently increasing effect on sample accuracy. Our results indicate that future research would greatly benefit from surveying at consistent times of day, and that short survey intervals can still offer accurate measures of butterfly migration rate. This study will inform future migratory butterfly surveying protocol. This study also conveys the ecological importance of the southeastern Atlantic coast for insect migration.