Mexico - thorn forest, dry forest

Many people think of the tropics as always hot and wet; however, there are many areas of the tropics that show strong seasonal variability between dry and wet seasons. This variability is caused by monsoons or other climactic conditions. Plants in such variable climates have adaptations to cope with the annual cycle of change, e.g. many lose their leaves in the dry season. Thorns are another brilliant adaptation that not only reduce herbivory but also deflect sunlight, keeping the plant cool. You’ll find some large thorns on the trees and shrubs of Mexico’s dry thorn forests and some large areas where thorn forests dominate the landscape, especially south of the true deserts around 20°N - 30°N. Most of the world’s deserts in fact occur 20° – 30° from the equator due the air currents that cause warm tropical air to descend. The exceptions like the Great Basin in the U.S.A and the Gobi in Asia are the product of large rain shadows, i.e. the Sierra Nevada and the Himalayas suck out the moisture coming from the coast leaving the leeside dry. Anyway, i digress… the thorn forest, like other tropical dry forests are influenced by the dry air around them and the long dry season and can seem brown and harsh… until you look a little closer! Tropical dry forests and a very threatened ecosystem throughout the world, because the sparse vegetation in the dry season leaves them vulnerable to clearing and other forms of human incursion… but at least the thorn forests have a little self-defence (yes, they hurt!)

With a tiny global range confined to Western Mexico’s thorn forests and surrounding arid landscape, the Balsas Screech-Owl is the perfect place to start this entry. This is a large screech-owl and their bouncing ball song sounds A LOT like the closely-related Western Screech-Owl. Nonetheless, to me though they look quite different to a Western Screech-Owl, being larger in size with a long tail and gorgeous brown eyes. This was one of only a few owl species of owl calling regularly in September (we heard 16 in 3 hours!!). This may have had to due with the heat but I suspect it's more likely an artifact of tropical breeding phenology.


Since we started with an owl why not run with that theme. This next character, the Mottled Owl, is found in a variety of forest types from northern Mexico all the way down to northern Argentina from the lowlands up as high as 2500m asl. Their large repertoire of explosive hoots are commonly heard, even around human habitation, making then arguably one of the most characteristic Neotropical owls. This photo was taken in dry forest – can you see the acacia leaves? Mottled owls are among the smallest owls in the genus Strix (round-headed woodland owls), little more than half the size of a Barred Owl for example.


Sticking with the owl theme, here is a lamentably poor shot of a Ridgway’s Pygmy-Owl in thorn forest (a taxon i believe should be split from the Ferruginous Pygmy-Owl – see my “Colombia – brown and rufous” post from January to compare)


The similar Colima Pygmy-Owl also occurs in thorn forest, although the bird in this photo is a little higher up in semi-deciduous forest. You’d be hard pressed to identify the 6 species of Pygmy-Owl found in Mexico unless you listened to their call first. Fortunately they call a lot and many of them are diurnal or crepuscular, so you can find them (the ones that are nocturnal like Central American Pygmy-Owl are super tough to see). At 14cm, the Colima Pygmy-Owl is very small (Ridway’s averages about 17.5cm but is quite variable), has a longer tail than Ridgway’s (which you can just see here) and has a spotted not streaked crown, which you can’t see so well here because the bird’s head is angled up while calling (note the throat feathers puffed out).

Shifting gears, here are two super characteristic birds of the dry forests of western Mexico – the two magpie jays: the main photo is of Black-throated Magpie-Jays and the inserted bird on the bottom left is their sister species, the White-throated Magpie-Jay, for comparison. Famous for the LONG tails and crests, these jays are truly a rhapsody in blue and this photo does them no justice! The Black-throated Magpie-Jay is endemic to northwestern Mexico and graces the front cover of the Howell and Webb field guide. The White-throated Magpie-Jay has a more southerly distribution along the Pacific slope. Mostly allopatric, they do have a narrow range of overlap in Jalisco and Colima and they occasionally hybridise.

This photo is of a Lesser Roadrunner, the smaller, more southerly sister species of the Greater Roadrunner (the only two species in the genus Geococcyx). If you are familiar with the Greater Roadrunner, you notice the Lesser Roadrunner’s buffier underparts and shorter, thinner bill. The Lesser Roadrunner is also smaller but otherwise they are very similar. The Lesser Roadrunner is a Mexican near-endemic with a mostly west coast distribution, except for a disjunct population in the Yucatan peninsula. In this photo you can see the zygodactylous toes (two toes point backward) found in groups like cuckoos, woodpeckers, and owls (though in owls the fourth toe is reversible rather than being fixed in the backward position like the others).

The dry open forests are home to many woodpeckers. This composite shows two closely related species of Melanerpes woodpeckers, both of which are endemic to western Mexico. On the left, two male Golden-cheeked Woodpeckers interacting and on the right a male Grey-breasted Woodpecker perched on a cactus. Both of these species occur in dry forest types but the Grey-breasted has a more interior and very restricted distribution. You’ll notice the difference in crown coloration and facial markings.

The Lineated Woodpecker is not a bird I was expecting to find in thorn forest so I was surprised to see this female on a snag at the thorn forest ecotone. In addition to dry forests, this species is found in a wide variety of forest types including mangroves. This species is a congenitor of the larger Pileated Woodpecker.

Moving back to the passerines, here’s the beautiful Cassin’s Kingbird, sitting in the thorn forest canopy. This is arguably one of the most distinct of the yellow-bellied type Tyrannus kingbirds.

The beautiful Golden Vireo is a west Mexican endemic found mostly in drier forest types although here seen in semi-humid forest.

The Rufous-backed Thrush, a.k.a Rufous-backed Robin, is another west Mexican endemic although they do occasionally stray northward across the Mexican border into the ABA area. They are very common and use a variety of forest types including thorn forest. Like the American Robin, they are attracted to fruiting bushes and trees as shown here.


The Rusty Sparrow is one example of many sparrow species that utilize the dry forests and scrub


There are also Passerina buntings in this habitat type. The remarkably gaudy Painted Bunting on the right is a winter visitor to western Mexico but breeds in northeast Mexico. The Orange-breasted Bunting is, you guessed it, yet another west Mexican endemic. In case you are wondering by now, west Mexico’s dry forest habitats are surrounded by barriers: the continental divide to the east, the true desert to the north and dense rainforest to the south. These barriers are the biographical trigger for such extraordinary endemism, having separated the gene pools of many ancestral species to evolve in isolation on either side of their respective “fences”. This is also why some of the endemics, especially the ones with more specialized habitat requirements, are so range restricted. Of course, some other species have surmounted these barriers and have ranges that extend into northern South America, e.g. some coastal birds and waterbirds find suitable habitat and a ready means of dispersal along the coast.


You can view these and other photos in a larger format at: http://artusophotos.com/