Mortality of migrating monarch butterflies from a wind storm on the shore of Lake Michigan, USA

When monarch butterflies (Danaus plexippus) in eastern North America undertake their annual fall migration to wintering sites in central Mexico, they face numerous obstacles, and many do not survive the journey. Large water crossings in particular have long been known to be a source of mortality during migration; before the winter destination of eastern monarchs was known, Beal (1946) reported that he often found monarchs and other insects ‘cast up’ on the shore of Lake Erie. On one afternoon in September 1943, he collected 57 monarchs ‘ just above the water line’ over 1.5 miles (2.41km) of beach (Beall, 1946). Other evidence that water crossings are risky comes from the monarch’s reluctance to cross water during unfavorable winds (Schmidt-Koenig, 1985) and the fact that monarchs tagged along the Atlantic coastline have an extremely low recovery rate at the Mexican overwintering site (Garland & Davis, 2002; Brindza et al., 2008; McCord & Davis, 2010). Large water bodies therefore appear to lead to substantial mortality of migrating monarchs. What is missing, however, from the collective evidence for the effect of water barriers, is first-hand accounts of mortality at such barriers. In this report, we summarize a series of observations submitted to the citizen-science program, Journey North (http://www.learner.org/ jnorth/), regarding a mass mortality of migrating monarch butterflies at a location on the shore of Lake Michigan (Fig. 1) following an intense wind storm. The storm in question was actually three backto-back low-pressure systems that swept through the Midwest region of the US beginning on October 14, 2011. The national weather service for the Grand Rapids (MI) area described the systems as follows: The first low produced rain and cloud cover resulting in temperatures around normal from the 14th through the 17th. The rain was light with multi-day totals mostly under an inch. Strong winds of 25 to 35mph (blowing westerly) were also felt across the area with winds gusting between 40 and 50 mph from late on the 14th into the 15th. The second low pressure system, which was more intense than the first, came a day later. Temperatures fell to below normal through the 22nd as heavy rain and strong (westerly) winds again lashed the area. Rainfall totals for the storm, mostly on the 19th and 20th, ranged from over an inch to nearly 3 inches across southwest lower Michigan. Winds gusted to between 40 and 50 mph with isolated sites experiencing gusts to near 60 mph. A third system came a day later; thunder and hail were reported at times from the 23rd through the 29th. This was accompanied by light to moderate rainfall. Temperatures were around or below normal through the end of October 2011 (National Climatic Data Center, 2012). The magnitude and duration of these storms can also be seen in a chart of the daily average and maximum wind speeds from the