Harvesting insects, or excrement from insects like aphids might be a likely cause.

Maybe they are cleaning up for you.

M. D. Vaden of Oregon

Have a close look at your Willow, the new growth is probably covered with aphids, these are probably what is turning the leaves brown. The ants are milking the aphids (their own personal dairy herd) get rid of the aphids and the ants will disappear.
A......

Aphid farming is often what ants are doing.

An effective way of controlling aphids is to spray the tree with a fairly strong stream of water. It will knock many of the aphids off, breaking their mouth parts in the process, and causing them to starve. You will need to do this consistently at 3-5 day intervals to prevent the next generation of aphids from getting established. Usually a two weeks of diligence will reduce the numbers to the extend that ladybugs and other beneficials can get to work and take over.

I noticed about a week ago with the ongoing rains that my willow looked split in the bark. Upon viewing more closely I found the bark to be split and pulled away from the trunk and red ants taking refuge under the torn bark. I at first started thinking about all the high winds and thought it may have twisted the tree to make this happen. That didn't explain the ants though. the tree is healthy as far as I can see