Do you know any bird species that have used the Verde River Valley as a tunnel to extend its range from the low desert into Cottonwood? I do.
The Verde River, as it runs past Dead Horse SP and Tavasci Marsh, drops approximately 1300’ over a 65 mile run until it empties into Horseshoe reservoir just North of Phoenix. At times the river runs through a valley and at times it runs through a canyon and all the while it is approximately 2000’ to 3000’ lower than its walls. From the standpoint of physical geography, the Verde has made itself a very long tunnel with an open sky roof through relatively high country.
To a student of biogeography, this is like opening up a long gateway for species range expansion. This student will look for indicator species that will characterize the phenomenon. In our case here in the Verde he needs to look no further than the Cactus Wren, State Bird of Arizona. The Cactus Wren is a Lower Sonoran species that likes to nest in cavities in Saguaro Cactus. Lower Sonoran Desert is where you find large cactus, Palo Verde Trees and Phoenix. It is hot. However, on the north side of the East/west mountains in Arizona you find the Upper Sonoran zone characterized by Pinon and Juniper trees. It is somewhat cooler. It is not typical habitat for Cactus Wrens. Still, if you look carefully enough, you will find a few of these low desert birds here in the Verde Valley. They are not here in significant numbers, but they are present and, in fact, we have a nesting pair in our neighborhood here in Clarkdale.
The spread of Cactus Wrens into the Verde Valley is a long story. I mean it took many generations for the species to extend its range up to here. It is likely that, when young Wrens were fledged in the low desert, a few moved a very short distance upstream to find new territories. If you give them hundreds of generations there is enough time for the species to move to the very limits of their ecological tolerances. And here they are. However, it would be extremely unlikely that this species movement would have occurred over the mountains and high country. It is one thing to select for very slight adaptations to slightly cooler, less cactus habitats and quite another for the species to adapt to cold, tree covered areas. Therefore, the Cactus Wren has extended its range through the Verde tunnel. This is by no means the same a migration route nor is it a journey undertaken by a given individual. It is a true range extension. I could not even guess when this occurred. I do not know if it commenced in recorded history.
However, there is one caveat to the story. There have been increasing reports of birds and insects extending their ranges northward in the present time. That means these are documented extensions. Can you guess the reason? If you said “global warming” you would be correct. With that one thought in mind, you are free to speculate about other low desert species extending their ranges northward through the Verde tunnel.
I need to add a disclaimer here. This is a concept that I think is very reasonable but I have not seen actual science to back it up. It appeals to my Biology education and experience.