Original Research ARTICLE
Is the timing, pace and success of the monarch migration associated with sun angle?
- 1Department of Ecology and Evolutionary Biology, University of Kansas, United States
- 2Monarch Watch, Kansas Biological Survey, University of Kansas, United States
- 3Department of Biology, University of Toronto Mississauga, Canada
- 4Upper Midwest Environmental Sciences Center, United States Geological Survey (USGS), United States
- 5Geosciences and Environmental Change Science Center, United States Geological Survey (USGS), United States
- 6Department of Ecology, Evolution and Organismal Biology, Iowa State University, United States
- 7Great Lakes Science Center, United States Geological Survey (USGS), United States
A basic question concerning the monarch butterflies’ fall migration is which monarchs succeed in reaching overwintering sites in Mexico, which fail—and why. We document the timing and pace of the fall migration, ask whether the sun’s position in the sky is associated with the pace of the migration, and whether timing affects success in completing the migration. Using data from the Monarch Watch tagging program, we explore whether the fall monarch migration is associated with the daily maximum vertical angle of the sun above the horizon (Sun Angle at Solar Noon, SASN) or whether other processes are more likely to explain the pace of the migration. From 1998 to 2015, more than 1.38 million monarchs were tagged and 13,824 (1%) were recovered in Mexico. The pace of migration was relatively slow early in the migration but increased in late September and declined again later in October as the migrating monarchs approached lower latitudes. This slow-fast-slow pacing in the fall migration is consistent with monarchs reaching latitudes with the same SASN, day after day, as they move south to their overwintering sites. The observed pacing pattern and overall movement rates are also consistent with monarchs migrating at a pace determined by interactions among SASN, temperature, and daylength. The results suggest monarchs successfully reaching the Monarch Butterfly Biosphere Reserve (MBBR) migrate within a “migration window” with an SASN of about 57° at the leading edge of the migration and 46° at the trailing edge. Ninety percent of the tags recovered in Mexico were from monarchs tagged within this window. Migrants reaching locations along the migration route with SASN outside this migration window may be considered early or late migrants. We noted several years with low overwintering abundance of monarchs, 2004 and 2011–2014, with high percentages of late migrants. This observation suggests a possible effect of migration timing on population size. The migration window defined by SASN can serve as a framework against which to establish the influence of environmental factors on the size, geographic distribution, and timing of past and future fall migrations.
Keywords: Danaus plexippus, Migration, Monarch butterfly, Monarch Watch, Phenology, Sun angle
Received: 11 Apr 2019;
Accepted: 30 Oct 2019.
Copyright: © 2019 Taylor, Lovett, Gibo, Weiser, Thogmartin, Semmens, Diffendorfer, Pleasants, Pecoraro and Grundel. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Dr. Orley R. Taylor, University of Kansas, Department of Ecology and Evolutionary Biology, Lawrence, 66045, Kansas, United States, email@example.com