3 years ago



North takes place with

North takes place with two to three more generations being produced there before migration south begins in August (Cockrell et al. 1993, Howard and Davis 2004, Flockhart et al. 2013). The small number of monarchs that migrate to Cuba and the Caribbean apparently do not return to North America (Dockx 2002, Dockx 2007, 2012, Knight and Brower 2009) perhaps because they do not experience the suite of environmental conditions required to trigger migration (Guerra and Reppert 2013). The fall migratory route of eastern monarchs has been studied since the 1930s (Urquhart and Urquhart 1978) and monitoring continues through the present via several citizen science projects (Howard and Davis 2008 and references therein). Monarchs east of the Rockies follow one main “central” flyway from southern Ontario and Midwest states south-southwest through the states of Kansas, Missouri, Oklahoma and Arkansas to Texas and Northern Mexico (Howard and Davis 2008). There is also a second flyway along the easternmost states and coastal areas. A large gap without monarch roost sightings exists between the central and eastern/coastal flyway (Howard and Davis 2008, see Figure 5, below). During spring migration, monarchs do not congregate in roosts and monarch occurrence is largely coincident with breeding habitat and the seasonal development of milkweed (Solensky 2004a). Figure 5. Central and eastern northward migratory flyways of monarchs east of the Rockies. Dots represent observations of roost sightings from Journey North data. The dashed line represents an apparent gap in monarch flyways. The star represents the overwintering sites in inland Mexico. Figure 2 from Howard and Davis 2008, original caption omitted. Monarch ESA Petition 24

Monarch butterflies in western North America migrate to overwintering sites in coastal California and coastal Mexico (Figures 6, 7, 8). Monarchs have historically aggregated in the fall and winter at more than 450 wooded sites scattered along 620 miles of the California coast from northern Mendocino County to as far south as Baja California, Mexico (Lane 1993, Leong et al. 2004, Jepsen and Black in press), although in the past ten years, only 72 of these sites have hosted more than 1,000 butterflies (Figure 7). In the fall of 2013, only 22 sites hosted more than 1,000 butterflies. Smaller aggregations of monarchs consisting of tens to hundreds of butterflies have been reported from Arizona and southeastern California (Monroe et al. 2013, California Natural Diversity Database 2012, Xerces Society 2013). Figure 6. Winter and potential breeding range of western monarchs. Dots represent western monarch overwintering sites. Shaded areas represent the most likely locations of breeding grounds for migratory monarchs based on late-summer milkweed occurrence and thermal conditions. Lines within state boundaries represent climatic regions. Figure 1 from Stevens and Frey 2010, original caption omitted. Monarch ESA Petition 25

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