Clouds of Yellow Butterflies

I first noticed the increase in numbers last week.  I spotted this puddle of yellow butterflies in my neighbor's driveway.

Yesterday I saw them nectaring on clover in my backyard.

Today, they were everywhere - by the thousands - in alfalfa fields and prairies along the roads and highways of west-central Wisconsin.

They made the Red Admiral emergence in early July seem insignificant.

Their bodies littered the roadsides.

I expected to see lots of birds feasting on this bounty of butterflies - but spotted only a few juvenile starlings dodging traffic to get a taste.  I was surprised to see the birds grab the butterflies, shake them, then drop them and fly away.

While I've never noticed Clouded Sulphurs before, they're among the most common butterflies in North America - from Alaska to Guatemala.  Their caterpillars feed on legumes - around here, that's clover and alfalfa.  The adults feed on flower nectar. 

From above, the males have a solid black border on their forewings.  The black border on the female forewings are dotted with yellow or white spots.  The underwings of both male and female have a double white dot and a row of black dots along the margin.

Why so many this year?  Scientists attribute it to the weather: warm spring and adequate moisture.