These two shots were taken in early April and early June, respectively. They are to the best of my understanding first year Herring Gulls. My suspicions are that the first bird is a local bird while the second is a migrant. Most of the first year Herring Gulls I saw in April were very similar to the first bird. Now, the second bird typifies the first year Herring Gulls, I have seen. I cannot resolve how any transition would account for the difference in appearances. The only explanation I can come up with is the difference exists because they are from different populations. The second bird seems much darker than the first and it is not the darkest in the flock.
Herring_Gull_04_2613m by
Grandpa577, on Flickr
Herring_Gull_06_0847m by
Grandpa577, on Flickr