The title is probably an accurate assessment of the relationship between these birds, but coexistence is an association a bit looser than friendship, which is the challenge topic.
On a technical note: This seems a bit overexposed. The detail on the white duck is washed out, and the the shadows below the other duck's head blur the boundary between the bird and the water. You might be able to bring some of the detail back by adjusting the curves or reducing the contrast.
I think I would have to put this one down to unfortunate lighting as the duck at the rear is over exposed and the one at the front is suffering a little under exposure as he has a dark head unable to see the strong greens he has. still the composition is great.
Woudl have been a fantastic shot but (and i'm sure you'll hear this more than once) the white duck seems to be overexposed. All detail seems to be lost.