NO NUTHATCHES, TITMICE OR
CHICKADEES IN CHIAPAS
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. |