Links below are taken from the IOS (IL Ornithological Society) website

Illinois Rare Bird Documentation questions and form:
Going to this link will explain some of the questions and not needed to be 
repeated in the body of this post.

Checklist of Birds of IL and their status
(Far too long to repeat in the body of this post).

I see they do not have a difference in status of occurrence of specific
birds one season vs. another, so do not know what to tell you about the
status of spring Hudsonians vs. fall birds in IL.
Maybe one of the IORC members (IL Ornith Record Committee) from S. IL
can enlighten us on this subject.  ;-)

The IORC prefers documentation done ASAP of the initial sighting of the bird 
and a photo is VERY important.
The more evidence the better (!!).

They do not have a SPARKS or CACHE system but believe their Seasonal Reports 
are more detailed.

And YES, they do want birders to document rarities seen on the IL side of 
RMBS, Chain of Rocks, Horseshoe Lake, Carlyle Lake, Two Rivers NWR, etc, 
etc, etc, etc......any and all deemed to be on the "IL side".
See the IORC's Review List of Rare Birds needing documentation (much like MO 
Bird Record Committee's list):

Birds out of season (like the Hudsonians) I do not know anything about and 
someone else will have to advise.

Think most of this is correct despite the late hour but if not, I am sure 
someone will make the proper corrections.
(spelling and grammar does NOT count)

Hope this helps.

Nitie-nite kidz,
Charlene Malone
St. Louis co.

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