Do herring gull(Larus argentatus) and lesser blackheaded gull (Larus fuscus) show seasonal isolation?

I was reading a book, LIFE by Sanjib Chattopadhyay and there this fact was presented but as the book contains flaws I'm doubtfut of the information.

the link below certrianly makes that statment but note the refrenecs provided and the dates. I would expect that recent solid scientific evidence for this is scant at best. … mp;f=false