Birds, like people, frequently wear different clothing in different seasons. Below are three Heermann's Gulls in their adult breeding plumage. The photo was taken July 3, 2009. Note the white heads.
I've spent much of the day trying to distinguish between different types of gulls (Shush! We all have our hobbies). CA Gulls have black feet, Western Gulls have pink feet, and there is a world of difference in the shading of the primary feathers for these gulls and others (Thayer's, Herring, Glaucous-Winged).
In comparison, Heermann's gulls are so easy to identify. They have black feet, white heads, reddish beaks, and below you can make out the red tint of their eyes. Note that the beak barely has some black shading at the tip.
Their winter clothing includes a grey head. These pictures were all taken yesterday. Here you can see that the molt is halfway complete. The bill also has a darker tip during the non-breeding season.
The molt has progressed further in this guy.
According to my field guide, Heermann's Gulls frequently steal food from other birds. The bird below had other Heermann's Gulls trying to steal his food!
Sadly, all the photos of the chase were blurry. Rest assured, dear readers, he was able to keep the food.