Patricia, you will notice that Company "G" was originally the fourth company of the Eighth Texas Infantry "Battalion" and known as Company "D", added to Hobby's original three companies on June 20, 1862 in Refugio County. The "Regiment" was organized shortly after by consolidating Hobby's Battalion with Shea's Artillery Battalion from Calhoun County (Indianola). At that time Company "D" became Company "G" and was headed by Captain Edwin E. Hobby, brother to Colonel Alfred M. Hobby.

A roster that I have, and something that is not unusual, has a number of names spelled similar to your Lockwood Birdsell.
Birdsell, L.
Birdull, L.
Burdell, L.
Burtchell, Lockren

Your ancestor likely saw action at the Battle of Corpus Christi and subsequent capture of the Federal gunboat under J. W. Kittridge in the summer/fall of 1862. Also at the Battle of Saint Joseph's Island May 1863.

Hobby's troops were forced out of Fort Esperanza in late November 1863 and ordered to Galveston in December 1863 where they spent the remainder of the war in coastal defense.

My GG-Grandfather was a member of Artillery Company "B".

Hope some of this gets you a starting point for further research.