Both species are expanding their range and hybridizing in the periphery.
In March 2018 I received a message from Alexander Worm. He had just completed his Master thesis and informed me that “Scissor-tailed Flycatcher (Tyrannus forficatus) and the Western Kingbird (T. verticalis) are able to hybridize and produce viable offspring.” Recently, the genetic analyses behind this statement were published in the journal Ibis. Let’s have a look at the results!
A Scissor-tailed Flycatcher © Gary Kramer | Wikimedia Commons
Sampling
The Scissor-tailed Flycatcher and the Western Kingbird are widely distributed across North America and their ranges overlap in several regions. Some putative hybrids have been reported in Texas, Oklahoma, Colorado and California. To see if hybrids actually occur and if they are fertile, Alexander Worm and his colleagues collected samples from a contact zone combined with several museum specimens, culminating in a final data set of 84 individuals.
The distribution of Western Kingbird (dark blue) and Scissor-tailed Flycatcher (light blue) in North America. Highlighted counties reflect sampling locations. From: Worm et al. (2019) Ibis
Mosaic Hybrid Zone
Based on eight microsatellites and one mitochondrial marker, the researchers found convincing evidence for gene flow between these species. The analyses did not uncover first generation hybrids, but there were several backcrosses (in both directions).
The system can be regarded as a mosaic hybrid zone where two sympatric species interbreed in a patchy distribution. Moreover, both species are expanding their range (mostly it is only one species expanding into the range of another) and hybridizing at the periphery. This unusual situation offers many opportunities for further research.
A Western Kingbird © Becky Matsubara | Flickr
References
Worm, A. J., Roeder, D. V., Husak, M. S., Fluker, B. L., & Boves, T. J. (2019). Characterizing patterns of introgressive hybridization between two species of Tyrannus following concurrent range expansion. Ibis, 161(4), 770-780.
This paper has been added to the Tyrannidae page.
Avian Hybrids 
My wife and I have both witnessed a number of Kingbirds here in Central Texas. I don’t know if they are hybrids, but they all seem to lack the white tail feathers of a Western Kingbird and appear to be Tropical Kingbirds advancing from Mexico as many other birds have done such as White Wing Doves. Check out the Topical Kingbirds.