Barriers to dispersal

[NJPharris@aol.com]

In a message dated 12/19/01 4:05:14 AM Eastern Standard Time, 
tommy.tyrberg@aerotechtelub.se writes:

> Things aren't quite that simple. Even rather narrow water gaps are
>  actually rather strong barriers to bird dispersal, especially for
>  landbirds.

Yeah.  Some species are particularly weird that way.  The Wrentit (_Chamaea 
fasciata_), for instance, is the only North American babbler (timaliid), and 
its ancestors must therefore have come over the Bering Land Bridge at some 
point.  However, they are not now found anywhere north of Oregon, since they 
apparently will not cross the Columbia River!

--Nick P.