Ouzel's Bob and Dip

Why does the dipper bob and dip?
Nobody knows
A trait of aquatic birds
Over whose feet water flows?
This dipper parent duo
Breeding residents of Silver Tip
Hid
In amongst the nests of cliff swallows
Built of the mud of stream-banks and bison wallows
Not too high, nor low

No predator can breach
This south-facing wall
Its pool below
Invites
We float downstream, feet first
Grazing tops of rocks
Montana drowned
In Cambrian times
And left corals
Jack Horner excites
Over its trilobites,
older than dinosaurs
Secrets of the
Sandstone
Quartz
Lava
Birds chose andesite ledges
Approached in formation flight
Solving the same problem
Darwin would say
Like dipper and cutthroat trout.
But trout don't bob and dip.