I've noticed that some birders are surprised by the difference in size of gulls of the same species. There can be a massive size difference between the sexes. Some female Herring Gulls are only slightly larger than Common Gulls and males can be almost as big as a Great Black-backed Gull. The Glaucous Gull I saw yesterday was probably a female, because although it was bigger than the other gulls it lacked the brutish appearance of a large male. The photo below demonstrates the difference between the sexes very well.
|Male and Female Glaucous Gull, Spitsbergen|
The variation between the sexes can also make interpreting field guide information difficult. Particularly if the features illustrated are of a male rather than a female. I have found myself confused by the appearance of a Caspian or Yellow-legged Gull only to realise later on that the females are slightly different structurally. Of course this comes about when you micro inspect the rarer individuals rather than just going through the common ones looking for something different.