From Birding North Dakota, by Dan Svingen and Ron Martin.
When to Visit (pg 7):
The best birding for most species in North Dakota is late May thru early July.
If you wish to witness the wonders of migration, visit North Dakota in April and October, as it is then the big flocks of waterfowl are present.
Spring migration for songbirds peaks in mid to late May.
Fall songbird migration generally peaks in September.
Birding in November and December is the best chance of finding rare waterfowl, raptor, and gull species.
(Page 16 mentions peak songbird fall migration is 1st and 2nd week of September.)
Specialty Species (pg 10):
These are species that are difficult to find elsewhere. (Has locations, which I have not listed as it was quite lengthy.)
Greater Sage Grouse
Nelson's Sharp-Tailed Sparrow
NOTE: I'm not sure if this publication is still available or where it can be found.
I think it was developed/funded by Game and Fish North Dakota (gfnd). Publish date is before 2004.
gfnd has a list of publications available.
A birding guide is available by mail, but it doesn't give details.