This video clip of Mimulus aurantiacus shows the closure of the two, flattened stigma lobes as a response to mechanical stimulus. This feature has been termed movement herkogamy, and functions to separate male (anthers) from female (stigma) parts of the same flowers.
Rapid closure of the stigma lobes occurs upon contact by a hummingbird's bill as it probes the flower and may function to prevent pollen from being deposited onto the stigma of the same flower (intrafloral selfing). However, stigma closure may also function to move the stigma lobes out of the way, such that the bird may more effectively pick up pollen and transfer it to other flowers.
(Thanks to Josh Kohn for editing this blurb.)