Crow Jokers?

What motivates crows?  When crows, who otherwise avoid the property, take a deep interest in it one day, should one be nervous?  Are they omens of a bad day?  I harbored these thoughts as a pair of loud American Crows dropped by, stared at a corner of the yard, and in turns dropped down to the grass.  I never was able to ascertain what was going on.  But I thought, just perhaps, the crows were playing a joke on me.  They had that amused, cocky look about them, and didn't fly off when I "investigated"... and they are very clever, very intelligent birds.  Makes me wonder...