Spring recolonization rate of monarch butterflies in eastern North America: New estimates from citizen-science data

Monarch butterflies in eastern North America return each year from their overwintering site in central Mexico to recolonize their range in the eastern US and Canada. Current knowledge of the spring recolonization rate (i.e the rate of advancement) comes from observations of eggs and larvae along a latitudinal gradient. Journey North is a website-based program whereby citizen-scientists report their first sightings of adult monarch butterflies each spring. We used 7 years of this sighting data combined with GIS technology to derive new estimates of the rate of monarch recolonization based on the cumulative area occupied and the rate of movement of the advancing wave front. We further used these data to examine differences between and within years (we divided each season into 12 time intervals) in the recolonization rate. Across all years, the average rate of recolonization was 71.6 km/d. We found no statistical difference between the annual (season average) estimates, which ranged from 66.6 to 78.1 km/d. Within seasons, we observed 4 distinct phases of the spring migration, consistent with previous work, and there was significant variation within years (i.e. between time intervals) in the rate of recolonization, with the migration wave front progressing the fastest (over 100 km/d) from 20 March to 9 April, and from 19 May to 29 May. Based on these new estimates, we conclude that the spring migration of monarch butterflies in eastern North America progresses faster than previously estimated, and the temporal patterns observed each year are remarkably constant. Volume 59 2005 Number 1 Each spring, monarch butterflies (Danaus plexippus) in eastern North America return from their overwintering site in central Mexico to repopulate their breeding range, ultimately reaching most US states and Canadian provinces east of the Rocky Mountains. This fascinating journey is made even more complex by the fact that recolonization is completed by two successive generations of monarchs (Cockrell et al. 1993, Malcolm et al. 1993, Brower 1995). Many monarchs at the overwintering colonies mate before migrating north in the spring (Van Hook 1993, Oberhauser and Frey 1997). As they migrate northward, the mated females then lay eggs on the newly emerging milkweed plants. Although the exact distance traveled by these first-generation females is difficult to ascertain, evidence suggests that this recolonization process is accomplished in two or three phases. Adults returning from Mexico travel as far north as 35° latitude, or the southern portion of the United States (Cockrell et al. 1993) before they die. The offspring and grand-offspring derived from these eggs then complete the second and third phase of the journey northward to the northernmost areas of the monarchs' breeding range in the northern US and southern Canada. This recolonization process has been referred to as the 'successive brood' migration strategy (Malcolm et al. 1993). Much of what is known about the timing of monarch recolonization is the result of a comprehensive study published by Barbara Cockrell, Stephen Malcolm and Lincoln Brower (Cockrell et al. 1993). In this study, detailed observations of spring oviposition timing were recorded over three successive years at many locations throughout the monarchs' eastern range. The authors provided evidence for the successive brood migration strategy, and demonstrated that the rate of recolonization northward does not proceed at a constant speed, but varies due to the lag of larval development of the second brood midway through the migration. A decade later, our knowledge of the monarchs' spring migration has improved, largely due to the internet-based, citizen-science program called Journey North (http://www.learner.org/jnorth/). Established in 1994, Journey North is a web-based citizen science program where school children, educational groups and other volunteers throughout North America report the first sighting they make of an adult monarch butterfly each spring. The date and location of the sighting are recorded by the participants online and these data are archived by Journey North staff. At regular intervals during the spring, the sightings are plotted on a map of North America by Journey North staff, allowing the online participants to view their sighting along with other sightings from around North America. When all observations are combined and plotted by Journey North staff, viewers can see the northward progression of the monarchs' spring migration via the timing of participant sightings. Although the Journey North program is used primarily as an educational tool for teachers, the data generated by the program have demonstrated their scientific value in increasing our knowledge of this critical phase of the monarch life cycle. We used the first 6 years of the Journey North data in a previous paper to document the state-by-state pattern of spring migration in the eastern North American population (Howard and Davis 2004). By using the timings and locations of monarch 'first sightings' reported by Journey North participants throughout the spring migration range we showed that there was an annual and nontrivial eastward movement of monarchs from southern Texas along the southeastern states before the migration wave front proceeds northward. In the present paper, we used 7 years of Journey North data combined with GIS technology to derive updated estimates of the rate of spring recolonization based on both northward and eastward movement from southern Texas (which we used as the 'starting point' for the migration). We also used these revised rate data to determine if the rate of recolonization varies between and within years.