Jeff,
I have to agree with tanks on this one. TX Indigos are still pretty common in parts of South Texas, but the problem is that most of the good habitat is either being destroyed or is inaccessible to the public.
I worked on some land SE of Alice as a summer job doing brush surveys (it sucked!). However, the one good part of the job was the number of indigos I saw. They were the most conspicuous snakes over much of the ranch and I rarely went more than a day or two without seeing one (or more). And I wasn't looking for them. If I really needed to see one, there were several whose favorite basking spots I knew and I could go to those areas in the morning and find one basking 5 days out of 7.
If you hike around cattle tanks in the morning in the brush country, or hike along the Rio Grande banks in the morning, you will undoubtedly find an indigo. For such a large snake, they are surprisingly inconspicuous among the dead black mesquite limbs. When I was trying to find my first, a friend told me to go to this ranch and walk along the river and watch for a mesquite limb to move. It worked.
If you just look by driving roads, you wouldn't expect to see very many. I don't think they wander onto well traveled roads that often.
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Chris Harrison
San Antonio, Texas