I also photographed a Least Flycatcher at Paynes Prairie Preserve in Gainesville. Empidonax are notoriously tough identifications without a call or song. Luckily, this bird was giving the "pit" call typical of Least Flycatcher, and the accompanying field marks add up. Any other empid would be very rare here.
I hope this works, as I plan to use this technique to nail down other tricky field identifications such as Fish Crow and other empidonax flycatchers, among others.