Just as if a royal eagle, chained to a perch, should be forced to entreat a sparrow, to become its purveyor.'
'Royal eagle' - The bird imagery represents Rochester and how he used to be a dominant, strong and independent, and now he is reduced to a mere sparrow.
'Chained to a perch' - Though he was in a good position before (in society), Jane knows that he was restricted and unhappy then too by the shackles of his marriage even then.
'Should be forced' - This may be showing the punishment that he is facing for deception and atempting bigamy. It also shows that he can't be a controlling figure anymore, since this time, he is the one who is being forced.
'To entreat' - This decreases his status and shows how he went down from his original height and position at the top of the food chain to a meagre position near the bottom.
'Sparrow' - A sparrow is a fairly normal bird, and this shows the decrease of his power and strength. Jane compares him from a great and mighty eagle and how now he is an insignificant sparrow. This also shows his dependency and his vulnerability.
Bronte's message - Bronte allows Jane to use the bird motif whereas so far in the novel, it has always been Rochester who has condescended to use this for Jane. This shows that linguistic reversal of roles, and the shift of power dynamics. The difference over here though, is that Jane is not using this to her advantage, or patronising Rochester.