Black-capped and Carolina Chickadees are another case where too similar species are not necessarily closest relatives (see the Handbook of the birds of the World for more information).  The chickadee and tit group contains a number of very similar species.  Mountain Chickadee for example is generally considered closer to Black-capped than Carolina is.  My impression is that the dividing line in the Saint Louis area has been moving north for some time.  Black-capped Chickadee used to be common at the feeders at Busch Wildlife and it is now rare there in my experience.  I am not sure if the change is due just to a northward shift or is also related to the changes in habitat in the surrounding areas over the years.
David Becher
Saint Louis
> Date: Fri, 21 Nov 2008 07:08:00 -0600
> From: [log in to unmask]
> Subject: Re: chickadees in MO
> To: [log in to unmask]
> In St. Louis area, I believe the line is essentially the Missouri
> River. The contact zone veers southwest in the western part of the
> state.
> Sometimes there isn't much difference between hearsay and heresy.
> The old definition of a species heavily relied on infertile young as
> a basis for determining separate species. That criterion, when
> applied to Baltimore and Bullock's Orioles in their contact zone in
> the upper midwest (a zone created by the planting of trees in the
> former barrier of the plains) gave us the short-lived species,
> "Northern Oriole." Subsequent DNA work showed that they are not even
> the closest of relatives.
> The definition of species is changing. News creeping into the
> general birding population is that recent DNA work will change our
> perception of species greatly. Look-alikes are often not as closely
> related as they seem. Behavior is often learned (including songs and
> feeding techniques).
> As for chickadees, it will be interesting as this DNA work gets a
> full explanation.
> Edge Wade
> Columbia, MO
> [log in to unmask]
> member, ASM
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