Oaxaca Slopes

Oaxaca is an exceptionally diverse state in Mexico, stretching from the Pacific Ocean to the edge of the coastal plain that borders the Gulf of Mexico. It has rugged, primarily mountainous terrain, in the center of which sits the high Oaxaca Valley. We had birded this valley in 2021, and one week ago finished a 10-day tour with Field Guides, Inc. that sampled other key regions of the state.

The tour first visited the Pacific Slope, where many West Mexican endemics reach the southeastern extent of their range. The birds encountered varied significantly with elevation; our efforts were concentrated around 8800 feet, where White-throated Jay and Red-headed Tanager were highlights; 7900 feet, where the elusive Aztec Thrush was an excellent find; 4900 feet where we saw Gray-headed Woodpecker, a flicker that was one of my favorite birds of the tour; 3500 feet, where we first encountered the outrageous White-throated Magpie-Jay as well as Happy Wren and Cinnamon Hummingbird; and 900 feet, where Fan-tailed Warbler and Yellow-winged Cacique were seen very well.

On the immediate Pacific Coast, we explored Huatulco National Park, which had excellent birding. Highlights of the day included Red-breasted Chat, Lesser Ground-Cuckoo, Colima Pygmy-Owl, and Citreoline Trogon.  Unfortunately, Eileen had a bad fall, bashing both forearms into a rocky road surface. She scraped her forehead, palms, and knees, and damaged her glasses. Her arms were so painful that leader Dan Lane kindly arranged for the in-house doctor to visit. He and his team took x-rays and administered intravenous painkillers. Though the x-rays from their portable device showed no evident fractures, more extensive testing once we got home revealed that Eileen had broken the radius bones in both arms at their narrowest point, the radial neck. Eighteen days later, as I write this, she is on Meloxicam and Acetaminophen, but is still in considerable pain, and has restricted use of her arms. Nonetheless, that night, Eileen carried on and was rewarded with beautifully seeing and hearing Pacific Screech-Owl, her 5000th bird!

We also spent half a day on a small boat, looking for oceanic species. The continental shelf is very close to shore here, and the cold upwelling can be good for birds. Sadly, water temperatures were high and we had only a few pelagic birds, such Black Storm-Petrel. However, we were entertained by hundreds of Pantropical Spotted Dolphins; saw numerous Olive Ridley’s Sea Turtles, including a mating pair; and watched in awe as Munk’s Devil Rays (Mobula munkiana), with three-foot wingspans, repeatedly leapt 10 feet or more out of the water!

The Isthmus of Tehuantepec is the narrowest part of Mexico, where the Pacific Ocean is just 125 miles south of the Gulf of Mexico. There is a significant break in the mountain ranges here, and winds blow through this gap, from the Gulf towards the Pacific Ocean, much of the time. Quite a number of species reach the northern or southern edges of their range in this region. On the Pacific side of the isthmus, we birded at salt pans featuring Collared Plover and Reddish Egret, and in scrub yielding Cinnamon-tailed Sparrow and the exquisite Rose-bellied Bunting.

A full day’s drive took us to the Atlantic Slope, where the town of Tuxtepec served as our base of operations for four nights. We generally saw fewer life birds on this half of the trip, partly because of the heavy overlap of the avifauna with Belize, which we visited a month earlier. It is only about 500 miles away and lies on the same coastal plain.

We spent one morning in a karst formation, searching for Sumichrast’s Wren, a very range-restricted endemic that forages among limestone outcrops. We heard the bird very well and glimpsed it several times as it scurried, mouse-like, between hiding places. An afternoon at a marsh over the border in the state of Veracruz turned up Veracruz Wren at the extreme southern edge of its very limited range; Aplomado Falcon; and the ochraceous fuertesi subspecies of Orchard Oriole.

The second day of birding was in foothills, with a fabulous look at the scarce Black-and-White Hawk-Eagle, an unexpected Woodhouse’s Jay, and Yellow-bellied Tyrannulet, a lifer that was a real treat – this cute bird is usually found in the canopy of tall trees, where it is hard to see well, but our bird was at eye level in plain view. Rainy weather curtailed higher-elevation birding in the afternoon. We got somewhat higher on the third day but cold rain and fog again interfered; Gray-collared Becard and Scaled Pigeon were consolation prizes.

Thankfully, on the last day, we had better weather and were finally able to explore the interesting cloud forest habitats, which receive their moisture, much of it as fog, from the Gulf. We greatly enjoyed listening to the haunting songs of Slate-colored Solitaire, which are reminiscent of our Wood Thrush in North America, but are longer and more complex. Other new birds included Unicolored Jay and the bizarre Pheasant Cuckoo.

As soon as we passed over the crest of the mountains (around 9600 feet elevation), the habitat abruptly became Madrean in character, dominated by pine and oak, and strongly resembling that of the Chiricahua and Huachuca Mountains in Southeast Arizona. This transition is due to nearly all of the moisture of the humid Gulf air having been wrung out as the air rose along the Atlantic Slope, a classic rain shadow effect.

In total, we saw well 333 species of birds, an impressive total for such a short trip. Of these, 36 were lifers for Eileen and 31 for me. We detected just 7 mammal species: Pantropical Spotted Dolphin, Common Bottlenose Dolphin, White-nosed Coati, Red-bellied Squirrel, Greater Sac-winged Bat, Parnell’s Mustached Bat, and Mexican Free-tailed Bat. The latter three were identified by ultrasonic recordings of echolocation calls. We also had a nice selection of plants and scattered herps and insects, though the constant procession of birds, including many wintering North American songbirds, left us scant time to photograph other groups. With the major exception of Eileen’s serious fall, this was a very enjoyable trip and an exceptionally interesting one from a biogeographical perspective.