sunflowers - when are they ready?

[spish]

this year i grew lots and lots of sunflowers, and now they are starting to droop their heads and die off.

when is the right time to chop off the heads and let the seeds 'dry'? i dont want to cut them off too early if the seeds arent ready...how do i know when its time?

 

[woodmort]

Never quite sure, usually I get to them about a day after the birds have picked them clean.

 

[kla37]

When the petals start to turn brown. I put big paper bags over the tops to keep the birds off of them and cut the heads off when they are pretty dry.