Prairie Arrival Order Does not Match Nesting Dates

Photo above shows earliest nesting on the left and latest to start nesting on the right.

Timeline of dabbler arrival:

Mallard/Pintail
Shoveler
Wigeon/Gadwall
Blue-wing

Timeline of nesting dates:

Pintail/Mallard
Shoveler
Blue-wing
Wigeon
Gadwall

It is not evident why Gadwall and Wigeon wait so long to start nesting.

-Frank