Fwd: Swallows in Riverton

[Utah Birds]

Karen Robbins sent in a special sight report for some swallows that are nesting in the sandy cliffs on the west bank along the Jordan River in Riverton.  They have yellow beasts ranging from pale to "lemon yellow" and are shaped like a Northern Rough-winged Swallow.  (She thinks they might be exhibiting some kind of mutation).

Here's her report:

http://utahbirds.org/RecCom/OutOFRange/O_2014_01.htm

If there's anyone who is familiar with these swallows who could shed some light on what kind of birds these are, it would be much appreciated.