Date: 09/08/2019

Number: 15000

I recorded 15,000 butterflies per hour flying at various altitudes above the bluffs on Lake Erie's north shore. They are all flying in the same direction - south west towards Pt Pellee (the southernmost geographic point in Canada). This butterfly swarm continued for about three hours in the morning and tapered off to 350 per hour in the afternoon. Weather conditions ideal - sunny, 65- 70 F with little to no wind.

Editor's note: contacted observer for more details on this sighting

On Sunday Sept 8, for a 2 hour period from 10:00 AM until noon hour a river of orange snaked along and high above the bluffs overlooking Lake Erie at Clearville Park. I have a trailer at that location with a roof top viewing deck. All butterflies move in southwest direction and parallel to the lake shore. The bluffs are 80 feet above the water surface. The butterflies were in the air column above the bluffs with many butterflies so high they where just specks. I confirmed they were monarchs through binoculars. This is caused by light winds from the west that blow the butterflies up against the lake. The butterflies have no need to go over the water here because their destination is Pt Pelee 60 miles to the south-west which offers the shortest distance over the lake to Ohio. Their direction of travel exactly parallels the lake which also just happens to be the direction they need to go to get to Mexico. The lake shore acts like a funnel. I noticed that there were also high numbers observed along the east shore of Lake Michigan and also on Lake Erie further to the north-east from my location. When weather conditions are right I have overserved this funnelling of butterflies along the lake in previous years migrations.

These conditions didn’t last long because once the on-shore breeze starts up the butterflies disperse over a wider area so the numbers observed drop. This same phenomenon occurs for migrating green-darner dragonflies at Clearville Park. They all of a sudden appear by the 10s of thousands. Its quite a site. Green-darners migrate to the gulf coast of the US and Mexico.

The 15000 is my best hourly estimate. For two 10 minute periods I count as many as I can see and if I see a huge clump of them I estimate how many are in the clump. I compare the totals for each period and continue observing throughout the hour to see if the rate changes. I then used this information to estimate hourly totals. When the on-shore breezes picked up the numbers of butterflies dropped to the lower values and this continued for several hours in the afternoon. The following two days the wind conditions changed again and butterflies observed drop to 25 per hour or less.

Clearville Park, ON

Latitude: 42.5 Longitude: -81.7

Observed by: Steve
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