There are two statements that will immediately and significantly damage a birder’s credibility: “I’m sure of the ID, because the bird looked exactly like the picture in my field guide.” and “It couldn’t be that species because it doesn’t look like the bird in my field guide.”
The fact is, no bird looks exactly like the picture in the field guide. Field guide illustrations are either an artist’s interpretation or a photo of a particular individual at one moment in time. Every bird is slightly different from every other bird. Rather than looking for birds that are an exact match to a picture, our goal in field identification is to combine elements of size, shape, color, pattern, sound, and behavior into a recognizable species.
Here we have a “textbook” Thayer’s Gull in winter plumage. The head is round, giving the bird a petite or gentle expression. The eye is dark, the underside of the primaries show a lot of white, and the bill is that characteristic greenish-gray color with a bright yellow tip.
This Thayer’s Gull is not quite as round-headed as the previous bird, and the the bill is more yellow, but everything else seems OK. Head shape will vary with the bird’s position, and males tend to be more “robust” than females. So we have a little variation on this bird, no need to panic.
The forehead on this bird is really flat, like that of a Herring Gull, and the bill is noticeably longer. The white underside of the outer primaries is still good for Thayer’s. If you zoom in, you can see the pink orbital ring, also good for Thayer’s. While some Herring Gulls show dark flecks on the iris, their eyes never appear this dark. It appears that we have a very butch Thayer’s Gull.
And now we know why some birders avoid gulls. The pale eye suggests Herring Gull, but up to 20 percent of Thayer’s Gulls can have a pale iris. The head is pretty round, suggesting Thayer’s. The bill is neither too big or too small, and is very yellow. No orbital ring is visible. What we can see of the underside of the primaries is white, but we can’t see it all. So do we have a robust Thayer’s Gull with a light iris, or a very demure Herring Gull?
Here is a similar bird, but the bill is a little smaller and you can see a pink orbital ring, making this a good pale-eyed Thayer’s Gull.
All this variation cannot be covered in a standard field guide. For complicated groups like gulls, more detailed identification guides are very useful. Then you can say, “The bird looked similar to the one in the gull guide.”