Which environment in Texas acts as an eastern barrier preventing further nesting expansion for *T. verticalis*?

The Pineywoods region

Despite the Western Kingbird's noted behavioral and habitat plasticity—allowing it to use man-made structures and thrive in grasslands and scrub—its eastward nesting expansion in Texas is curtailed by a specific geographical feature known as the Pineywoods region. This area appears to function as a natural eastern boundary for successful breeding. The halt suggests that although the birds can adapt to using artificial perches, the fundamental ecological requirements related to the broader habitat structure—perhaps humidity levels, high forest density, or competitive interactions with Eastern Kingbirds in those wetter environments—impose an evolutionarily prohibitive constraint on successful nesting beyond that point. This illustrates the balance between contemporary behavioral flexibility and deeper ecological limitations.