birdsoup

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.