Males of
European H. rustica subspecies swallows have nearly
the palest ventral color and the most exaggerated tail
streamer lengths of all of the barn swallows, while swallows
from the North American populations are substantially
more colorful, with streamers that are among the shortest
of all subspecies (Figures 1 and 2). Intriguingly, populations
from the two Middle Eastern subspecies (savignii
along the Nile and transitiva throughout Israel, Lebanon,
Jordan, and Syria) have combinations of dark plumage
coloration with streamer lengths that are almost the
same as those of the European subspecies. Populations in
northern Asia (H. rustica tyleri ) possess intermediate values
of streamer lengths and feather color relative to their
conspecifics while H. rustica gutturalis, which occurs
throughout much of Asia, has among the least exaggerated
features of all, with the palest ventral color and shortest
tail streamers. Though differentially sexually dimorphic
with respect to both streamer lengths and color, differences
in female morphology are highly concordant with
differences in males throughout the entire range of this
species complex. Ongoing research is focused on determining
the underlying causes of these fascinating phenotypic
differences.