There are good reasons why very large flying creatures are rare, and they are the obvious ones, weight/strength ratios, lift, air density, etc. Really large and heavy birds are associated with mountainous terrain, thermal updrafts, steady tradewinds, cliffs overlooking the oceean, etc, that make those designs feasible. We know really big birds and flying reptiles existed from their fossils, but I understand many of the pterosaurs were marine, and the big flying birds were all related to condors.

Big birds of this type are not impossible, but their habitats would be limited to only a few suitable areas, and they would not be very maneuverable or nimble near the ground in areas that were predominantly flat. Also they would be very hard to hide. My guess is this is just another Texas tall tale.