Naturalist Newsletter of December 3, 2007
issued from Yerba Buena Clinic just outside
Pueblo Nuevo Solistahuacan, Chiapas, MÉXICO
about 1740 meters in elevation, ± LAT. 17° 11' 27"N, LONG. -92° 53' 35"W
North American birders often think of nuthatches, titmice, chickadees and Brown Creepers as similarly small, curious, conspicuous birds that often flock together, especially in the winter woods. Here, despite our high-elevation, semideciduous forests sharing so many species with eastern North America's forests, that cheerful bird-assemblage just doesn't exist.
Supposedly North America's Brown Creepers do indeed occupy the Chiapas highlands, but I've not seen them. They occur in upland forests as far south as Nicaragua.
However, nuthatches, titmice and chickadees aren't represented at all in Chiapas. Among the nuthatches both White-breasted and Pygmy Nuthatches follow the cool uplands from the US border deep into south-central Mexico, but they don't make it across the lowland Isthmus of Tehuantepec to here.
Similarly, Mexican and Mountain Chickadees, as well as Bridled, Plain and Black-crested Titmice all extend across the US border into Mexico, but none has made the leap across the lowland Isthmus of Tehuantepec.
I've wondered why this might be so. Maybe it's because when North America's migrants overwinter here they fairly saturate the ecological niche for birds foraging on small, arboreal invertebrates. Remember that North America's winter coincides with our dry season, and the dry season is our least buggy time of year. Maybe, at least during the dry season, there's just not enough bugs to go around for both winter visitors and permanent-resident nuthatches, titmice and chickadees.
Or maybe the Isthmus of Tehuantepec's lowlands have been just too extensive, hot and humid for upland- loving nuthatches, titmice and chickadees to make it across.
Whatever the reason, the lack of those birds in our chilly forests with abundant Sweetgums and Blackgums feels as strange as it does seeing bromeliads and orchids growing on North American tree species here in Mexico's southernmost state.