Although Red Admirals were first seen migrating southward in central Iowa in early September, their migration remained sporadic and sparse here until the end of the month. This year, unlike during 2017, most of the migrating Vanessa have been staying to the north, in Minnesota, Wisconsin, and Michigan, for much of September. That is, until the weather patterns changed during the past week....
Larger numbers of Red Admirals first appeared in Ames on September 30, when an unseasonably warm and humid air mass over the Midwest led to the development of gusty south winds that kept the butterflies, along with some roosting Monarchs, from proceeding further southward. Some of these butterflies, seen feeding on fallen apples in an orchard, were an intermixture of fresh and partly-worn individuals, perhaps from two different generations.
A cold front moving eastward into this warm, humid air mass spawned strong to severe thunderstorms during the afternoon and evening of October 1, which was followed by cool overcast for the next two days. October 4 then dawned mostly sunny, a cool, crisp day with light to moderate winds alternating between northeasterly and southeasterly. And that's when the migrants appeared again - a nearly equal mixture of Red Admirals and Painted Ladies, along with three Monarchs, flying generally south-southwest from 1 to 3 p.m. through cool air that never got warmer than 60 F.
October 5 was another thundery day of heavy rain, followed by a cool sunny day on the 6th, when the Vanessa butterflies were once again out and about and showing signs of moving. Then the migration really picked up on the 7th. As the air warmed above 60 F after 10 a.m., the butterflies started moving, and by 12 p.m., the southward migration was definitely underway. Being mostly Red Admirals this time, the migrants arrived on average once every four minutes across a 100-foot east-west line, with sometimes two or three coming in a minute around the 1 p.m. peak. The migration then gradually decreased and ended for the day shortly after 3 p.m. At that point, I went back to the orchard that I had visited on September 30 and found about 30 Red Admirals there along with a few Polygonia comma, all feeding on fallen apples (pictures at https://www.inaturalist.org/observations/34054397). They remained active there until after 4:45 p.m., when the area was becoming shaded and the butterflies started departing for the evening.
Although migrating Vanessa butterflies are not as obvious as Monarchs, when they are as abundant as they have been here recently, they are readily spotted flying generally southward. Go out to an open area on clear, crisp days with nearly-calm to moderate northerly or westerly winds and watch for dark or orange butterflies smaller than Monarchs flying from near the ground to eye level to 30 feet/10 meters overhead. Even if you do not see any flying, look at the fallen fruit beneath fruit trees for Red Admirals and other butterflies taking advantage of the feast.
-- Royce Bitzer, October 8, 2019