This spreading behavior is called Anting.  It often occurs over ant nests,
although some of the times when I have walked over to the spot after the
jay left, I could see no sign of an ant nest or special concentration of
ants.  The majority of times I have seen this behavior it has been with
jays of one species or another, but I have seen several unrelated species
anting.

Good Observation and

Good Birding,

Rick Hollis
3524 Cumberland Ridge Road NE
North Liberty, IA 52317-9328

319-665-3141