Thank you Rob for the detailed answer. I think that the first two links are of the same observation, and indeed the second one was observed late afternoon at 18:20 (just less than an hour before sunset at that date).
Here are the photos of the holotype of A. c. monicae
held in the Paris museum. You are right, the bird (breeding adult) is not very white but light grey. The second and third photos show also the black on the head.