Thanks for sharing your personal experience with hummingbird numbers!
Until humans can design and improve upon something like this
for hummingbirds or other new data is discovered it remains a mystery.
Perhaps habitat changes are happening to the land around you. New construction? New cell phone towers? New pesticides? Natural land destruction? Wide usage of red dye in hummingbird food? Introduced species? Or just a natural ebb and flow which our human perspective cannot understand yet? So much remains a mystery to us unfortunately! There's a funny scene from the movie Idiocracy which laments how the greatest minds and resources were focused on conquering hair loss and prolonging erections instead of understanding our natural world.
There's a whole thread about this decline question currently:
In any case, that red mimosa sounds lovely and I sure wish I could see a video or a photo of it.