Like the baker, that tries to sell his bread saying that all of it is good, we can tell you that butterflies are always good but we would lie. Suddenly there are days at the endings of March or begginings of April, when they´re all gone. And it goes from one day to the other. It is possible that we go on the first Monday of April and we find them, and then the next day ...nothing on Tuesday.
We have to repeat to you that Nature doesn´t have a contract with us. Butterflies don´t have an specific date to iniciate or finish departure. When they leave they don´t do it all at once, they take up to one month of continues departure. And it´s the same when they arrive: they take more than one month to do it. In the season Nov 2006- Apr 2007 there were yet many by April 18 th!. But there has been seasons when by March 30th there´s not one single butterfly there.
You have an English saying: "You can´t see the forest for the trees" Even if you come to see just some hundreds of thousands, think of what it means: "some hundreds of thousands" of butterflies around you. Even if there´s only one million of them, your eyes can´t see them all at once. The phenomenum is so big that goes beyond our visual posibilites.
What we mean is that this goes beyond human Logics.