Usually nest building begins immediately after males arrive. The slow start may be due to the weather. Temperatures have barely made it past 60 this week and morning temperatures have still been in the 30s and 40s. The next couple days are supposed to be much warmer, so I expect things to take off quickly.
Nest started at A6 by the first male bk/wh, gn/al who moved across the field. This nest is now 3 inches tall.