From a distance, the dense flocks looked like dark ribbons:
With a magnified view, it was possible to see many birds sitting on the water, as well as large numbers in flight.
Occasionally, the birds sitting on the water would take off all at once:
Although the sitting flocks stood out, scanning other areas of the ocean revealed impressive, although sparser, concentrations of birds flying south:
The birds in these photographs are Sooty Shearwaters (Puffinus griseus). It was very difficult to count the number of shearwaters visible off Bodega Head this afternoon.
I tried to make a quick estimate of the number of birds in the second photo of this post. Here it is again (below) if you'd like to try counting yourself. [Click on the picture of a slightly larger version.]
My first pass through the flock in the picture above resulted in a count of ~550-600 shearwaters. This was a very small portion of the total number of birds visible across the entire seascape. I'm going to throw out a very rough estimate of at least 10,000 Sooty Shearwaters in view from Bodega Head at ~3:30 p.m. today.
If you're near the coast, it's worth watching for this spectacle of shearwaters.
P.S. I first wrote about Sooty Shearwaters and their amazing migrations on 19 August 2012. You can review that post here.
ADDENDUM (24 August 2015): And here's another view of an incredibly dense shearwater flock (all of those little black specks). Can you tell where this picture was taken?
View from Dillon Beach, looking towards Tomales Point (northern tip of Point Reyes). Taken on 23 August 2015 (Thanks, Stephanie!). [Clues for the location: extensive dune grass in the foreground, looking straight across to a steep rocky promontory, and a navigational buoy just off the tip.]