Red Admirals continue to become more abundant in the upper Midwest and southern Ontario, despite several intervening interludes of cold weather. They migrated in on days when warmer, moister air was flowing up from the south. The first of two migration waves occurred on April 15-16, resulting in an abrupt increase in both butterflies and observers noticing them for the first time this year. Locally, I observed Red Admirals flying consistently northward on the 16th, as well as a threefold increase of Red Admirals in territories late that afternoon, as compared with a previous survey a week earlier. After a cooler period, a second and perhaps larger wave arrived in various parts of the Midwest from April 21 -24, again resulting in a further increase in the local population. Observers in Minneapolis, southern Wisconsin, southern Michigan, and southern Ontario reported large increases in Red Admirals, and some substantial northward migrations occurred in southern Ontario, raising the question of whether Red Admirals were actually crossing Lakes Erie and Ontario.
A subsequent period of cooler weather has quieted their activity recently, with far fewer seen during a local survey on April 29th than during a previous one on April 23. Whether this is due to the butterflies sheltering from the cool weather or to an actual decrease in numbers remains to be seen.
-- Royce Bitzer, April 30, 2019