Ultimately, many of our identifications are reinforced by comparisons - comparisons among species in the field. These comparisons solidify for us the differences between Species A, Species B and so on and so forth.
Consider these two below:
The bird on the left is a fairly typical 2nd cycle Herring Gull. To the experienced birder that has looked through enough Herrings, the bird on the right is different. It has softer brown primaries with faint pale edges. The underside to the left wing is noticeably pale. The wing coverts and tertials have an overall icy-marbling appearance. The head is rounded, and the bill - which has retained smudges of black along the cutting edge - is thinner towards its base.
The bird on the right is a fairly typical 2nd cycle Thayer's Gull. Here's this beauty in flight:
|Thayer's Gull (2nd Cycle). Will County, Illinois. 15 February 2016.|