Hi Heidi,

" Is this the work of birds? "

That is probably what is happening -- a bird lands on the bloom and is simply too heavy for the stem to hold up. As many times as it has been happening on your sunflowers, I am a little surprised you haven't seen that happen. It has happened on my zinnias, but rarely. That is because I use reflective pinwheels to "scare" the birds away from my zinnias. I don't want seed-eating birds like finches to eat my zinnia seeds. If you want to keep birds off of your sunflowers, you might want to try the reflective pinwheels, or some other "bird scarer".

ZM

Heidi - could be some kind of borer, too. Dissect one to see if you see a worm in the stem near the break.

Alex

could also be vermin of an appropriate size... squirrel ... chipmonks .. mice ... rats .. yuck .. lol ..

and dont forget those climbing rabbits ..

ken

They definitely creep me out, I know.

Squirrels turned out to be the culprits in my garden this year (Lemon Queen sunflower).

good God.. finally ... a group with a sense of humor ... lol ... ken