Sometimes your field guide can mislead you. Case in point: A young couple was asking about a small bird they had seen in their yard. After careful observation they described it as a uniform dark gray all over. They even went one step further, scoured their field guide, and narrowed the identification down to one of two species. Either a Starling or a Catbird. (A more experience birder would have quickly ID’ed it based on the basic difference in overall shape.)
They mentioned that their Sibley field guide shows a large rusty patch under the rump of a catbird. Their bird did not have a rusty patch, so they concluded it must be a Starling. What the Sibley field guide doesn’t say is that you usually don’t see this rusty patch on a catbird. It is there, but doesn’t always show. In fact I can only remember seeing the rusty patch one or twice in the last five years.
So, the presence of a field mark can help identify a species. But the fact that you don’t see a particular field mark doesn’t exclude a bird from being that species.
Too bad they didn’t hear it call … then they’d know the difference for sure!