What Attracts Monarchs to a Roost?

People often observe monarchs roosting on the same trees — and even the same branches — from one night to the next. Here is how one woman described what she saw:

"Of the hundreds of trees in our yard, the monarchs consistently manage to find the very same tree--AND the same branch--on which the monarchs roosted on nights before. At first I thought it was just chance, but as the incoming butterflies select the same places night after night, I really wonder if the others leave a trail of some sort."

What attracts monarchs to a roost? How could you find out?

Does a scent — or something else monarchs can sense — attract monarchs to the same branch each night?

(Science standards >>)