Here's a dirty thought ...

The Science Daily reports that Getting Dirty May Lift Your Mood because in a study at the University of Bristol:

Treatment of mice with a ‘friendly’ bacteria, normally found in the soil, altered their behavior in a way similar to that produced by antidepressant drugs, reports research published in the latest issue Neuroscience.

Is this the reason some of the birds in my yard 'roll' in the sandy soil?