Geography Reference
In-Depth Information
l ights favor birds with slender and pointed wings, so the blackcaps heading
to Britain have rounder wings than those traveling farther to Spain. Dietary
dif erences favor distinctively narrower and sharper beaks in northern mi-
grants as well. Blackcaps wintering in the south feed mainly on fruits that re-
quire a wide gape to swallow. In the north, a general diet of bugs, seeds, and
fat, much of it from bird feeders, favors a narrow bill. Northern and southern
blackcaps also dif er somewhat in color; the beaks and feathers of northern
birds are browner than those of southern birds. This may be a result of dif er-
ences in molt pattern or simply may rel ect the random heritage of the few
founders that pioneered the northern route.
The recently and rapidly evolved behavioral and physical dif erences be-
tween northwest-migrating and southwest-migrating blackcaps rel ect distinct
gene pools despite their coincident breeding grounds. Genetically, these two
lineages are more distinct from one another than they are from more geo-
graphically distant blackcaps. Separate arrival on the breeding grounds and
selection against hybrids allow the unique migratory heritage of blackcaps to
persist and enables British subirdia to adorn the northern migrants with
round wings and narrow beaks. Subirdia, it seems, not only is capable of sus-
taining diversity, but also may actually create it.
Northwest-migrating blackcaps have not yet been designated a new spe-
cies. Systematists—those ornithologists who make such decisions—consider
these birds a race or an “ecotype” of
Sylvia atricapilla.
Certainly they are
more. They are ecologically and genetically distinct, pursuing a new evolu-
tionary trajectory that is deeply connected to the actions of city people. Many
other species are doing likewise. Adjustments in migratory behavior and as-
sociated morphology are known in at least i fty species, such as the house
i nches that live on the East Coast of North America and American crows that
inhabit the northern prairies. As the urban tsunami crests, expect many spe-
cies to settle in deeply with humans. The adjustments they make to our way of
life may change course or strengthen, but they are unlikely to recede. Perhaps
