My name is Tracy and I'm posting on behalf of my dad (he's not very computer savy) who loves to spend time in the backyard planting stuff.
I did some rehab and release on a squirrel that found it's way to our home last year and he and all his squirrel friends are now visitors in the backyard. As a result we found that planting sunflowers was a fun thing for them. We had some success with our fist couple of plants (the bugs scarred me a bit but I got over it quick enough). We did notice a problem, and it has continued even though we switched to black oil sunflowers (he happened to have that variety)
Here are some pictures that better describe the issue.
I told him that I think is related to overwatering, but he swears he isn't overwatering.