I am not sure as to the gender of these Mergansers. I suspect they are mostly juvies with a mix of males and females. However, they could be mostly males as 1st year males are similar to adult females except for having a white dash above their gape. The male gradually acquires male plumage by the summer, after the 1st winter. In other shots of this same group, I can see the white dash on many of them.