The Weekly Photo Challenge is Prolific.
For several years in the 1990s I was part of a small group surveying and recording an inland Gull roost on a reservoir in Cheshire, England. Numbers peaked in the winter although there was birds there every month.
The majority of the birds were Black-headed Gulls. Typical numbers were between 13,000 and 22,000 Black-headed Gulls in the winter.
The photo shows part of the roost in the process of moving to a different section of the reservoir.