Which major geographical area is excluded from the Common Yellowthroat's vast breeding range across the US?

Answer

The most arid western regions

The breeding distribution of the Common Yellowthroat is extensive, covering nearly all of North America from southern Canada south into Mexico and Central America. Within the United States, this range is remarkably broad, encompassing nearly every state. However, the primary exception noted is the exclusion of the most arid sections found in the western part of the country. Birds inhabiting the northern limits of this wide range undertake long migrations southward for the winter, often reaching Central America or even the northern limits of South America.

Which major geographical area is excluded from the Common Yellowthroat's vast breeding range across the US?
speciesbirdanimalevolutionYellowthroat