Analysing the hats in portraits and from extant examples in museums provides a lot of insight into how the bergere was constructed and worn, according to period, the hair styles, the trims, the size of the brim, and the height of the crown.
Even watching movies provides some insight so long as you don't assume they are correct. Some movies do quite well in their research and some are just big hodgepodges of whatever costuming items they could find.
Museum websites are rewarding particularly in terms of the structural makeup of the hat.
The shallow crown varies in height from almost non-existent to more substantial and perhaps this was in response to the changing hairstyles throughout the century.
Popular throughout the 18th century the bergere's chief features were that it could be worn and trimmed in a multitude of ways.
Hairstyles of the period ranged from quite modest and close to the head becoming bigger and more impressive as the century wore on.
Compare the relative simplicity of the bergere styling in the movie Dangerous Liaisons with the later modes from the movie The Duchess where they are worn atop huge wigs.
A classic bergere worn by Glenn Close in Dangerous Liaisons.
And then there is this hat. My favourite.
Then of course there is Miss Knightly in The Duchess. More attitude and more altitude on bigger hair! These bergeres have a higher crown though it is still not high by modern standards and the trims have become assymetrical rather than the puffed ribbon all around and feature ostrich plumes.
Other museum examples include bergeres covered completely with colourful dyed guinea fowl feathers (V&A), intricate woven straw in geometric patterns (Met), and applied raffia embroidery (V&A) just to name a few.
So while the basic straw bergere was very popular, women of the 18th century liked variety just as much as their modern sisters.
Milliners complied and I suspect produced far greater variety than we often assume.
Women themselves used their imaginations as they always have to embellish hats to their own tastes.
The few surviving samples merely give us a tiny window on how varied those tastes could be.