Chinquapin
The tune is Chinquapin. This jam happened at the Clifftop festival 2013. You mostly see and hear Rhys Jones fiddle + Mark Olitsky banjo. The other fiddler is Steve Selin from the Evil City. The video starts and ends in the middle of the second part, key of D. The semi-fretless banjo is tuned open A. Also David Jones, guitar; Jason Zorn, mandolin; Mike Levy, Bass. They also played Shady Grove. Rhys Jones played this on his Mississippi Square Dance CD [clip at amazon]. Steve Selin played it with Evil City String Band [at the band's site, click Sounds, then scroll to this song].
Notes
Key of A. The two parts are similar and opposite. The first part is mostly an A chord but the second part is mostly a D chord. Fiddles play drone strings often. Fiddles can cross-tune (AEAE) though standard (GDAE) works too. Part 1 feels monotonous till the flourish at the end. Part 2 is just as monotonous and, while it has no flourish at the end, it does resolve on the A chord. This is a good tune for banjo noodling.
Chinquapin?
In north and central USA, the word refers to an acorn from the Chinkapin Oak [wikipedia] and is unusual for being edible and sweet. In south eastern USA, the word refers to a nut from the Allegheny Chinquapin [wikipedia], or dwarf chestnut tree. In north western USA, the word refers to a nut from the Giant Chinquapin [OSU] evergreen tree. In southern USA, the word refers to a fish also known as the red-ear sunfish [wikipedia]. In old time music, the word refers to this tune.
Not to be confused with...
Chinquapin Hunting. Different tune. [OTF]
Chinquapin Pie. Different tune. [Amazon]
Mayfield a.k.a. Chinquapin. Different tune, same name. Here is a beautiful rendition on solo banjo [utoob]. Here is an outdoor jam session [utoob]. Here is an indoor jam [utoob].
Chinky Pin. Different tune, similar name. Solo mandolin [utoob]. According to Phillips Collection, the Chinky Pin tune also goes by several other names.