Was in south Texas last month:
-Saw two Whooping Cranes from the first overlook on to the mudflats when coming from the visitor centre (
https://www.google.com/maps/@28.3006919,-96.8079656,1341m/data=!3m1!1e3). Also another three from the big observation deck at the end of the road - that viewing deck is a marvel of over engineering. Also note that the visitor centre is not open every day, but the reserve is.
-Sunset Lake just west of Corpus Christi had a very nice range of waders including LB Curlew, Piping Plover, Semip Plover and Snowy Plover.
https://www.google.com/maps/@27.8640473,-97.3368832,17z
-There is a long-staying Painted Redstart (not Bunting!) at Falfurrias Rest Stop (
https://www.google.com/maps/@27.0942446,-98.1459377,679m/data=!3m1!1e3). Even if the bird is gone it is well worth stopping here - it is more a nature reserve then car park! Also saw Green Jay, Yellow-throated Warbler, Eastern Bluebird, Coopers Hawk, Inca Dove, Black-crested Titmouse & Fox Squirrel. The north end has a nice pool where birds come to drink and there is a trail through woodland at the south end. Saw quite a few White-tailed Hawks between here and Edinburg.
-Only had three days in the Lower Rio Grande and visited Estero Llano (Least Grebe, Pauraque, Sora Rail, Eastern Screech Owl in nestbox by path and Green Kingfisher), Santa Ana (Grey Hawk) and Sabal Palms (RT Becard). All three would warrant a full day. From what I heard from local birders, Tropical Kingbird had a good breeding season last year and seems quite widespread. Saw/heard one definite at Mission Nature Park (
https://www.google.com/maps/@26.1926436,-98.3354703,698m/data=!3m1!1e3) where Hook-billed Kite have been regularly seen this winter.
-From McAllen it is a long drive to Big Bend! My route was Fort Davis - Del Rio (4-5 hours) Del Rio - Laredo (another 4 to 5 hours) Laredo - McAllen (about 3 hours). From Fort Davis it is another 100 miles south to Big Bend. Note there is very little civilisation between Del Rio and Alpine along Route 90! Also all shops/restaurants were closed by 8pm in Fort Davis (seems to be a common time for places to close in small towns).
-Between Laredo and Del Rio, saw 116 Mountain Plovers here:
https://www.google.com/maps/@28.9705157,-99.2496486,667m/data=!3m1!1e3. Could be gone by mid-April though (?). View from Route 57 only.
-In Laredo, Lake Casa Blanca Park is well worth visiting, as is North Central Park for Seedeaters and if the Slaty-backed Gull is still hanging around.
-Spent a day around Davis Mountains SP, but didn't see or hear any Montezuma Quails unfortunately. Did have great views of Javelina at the water tank by the feeders.
Hope this helps!