March 10, 2010
Whiskered Screech-Owl showing length of rictal bristles
Whiskered Screech-Owl showing length of rictal bristles
As recently as the early '80s birding literature indicated Whiskered Screech-Owl, along with Arizona's other small, insectivorous owl species, Elf Owl and Flammulated Owl, withdrew southward from its breeding territory in winter.  Now, however, Whiskereds are known to be permanent residents in pine/oak woodlands in all of Arizona's Sky Island mountains, primarily above 5,000 feet, and they are common in that habitat.  How and why can they survive in this environmental niche in winter when cold weather brings a decrease in insect biomass which causes Elfs and Flammies to migrate?

The answer must lie in the fascinating adaptations by which evolution has slotted all species into very specific and exquisitely differentiated environment niches.  Whiskereds (90 grams) are considerably larger than Elfs (40 grams) and Flams (60 grams), but considerably smaller than the Western Screech-Owls (150 grams) with which ornithologists now think they share a common ancestor.  Quite likely Elfs and Flams, because of their smaller body mass, have evolved the instinct to depart for warmer climates when insect populations drop in winter.  Whiskereds, twice and half again as large respectively as Elfs and Flams, are apparently able to survive colder temperatures on less protein (yes, insects are protein), but Whiskereds too are known to migrate altitudinally downslope in their canyons in extreme (for southern Arizona) cold.

Even more fascinating than body weight, however, are two more specific physical features by which Whiskereds are distinguished from both the slightly smaller and slightly larger owls which overlap their geographic range.  Their name "whiskered" derives from measurably, even visually, longer rictal bristles, the stiff, lacy extensions of the facial feathers emanating from near the base of the bill.  It is thought their function is to provide tactile information about the proximity of prey items, perhaps even acting to funnel prey into the mouth.  Longer bristles would be better for a species that is insectivorous like Elfs and Flams but, unlike them, non-migratory, and for a species that takes mostly insects unlike Westerns which take mostly rodents.

This difference from Westerns' prey preference also explains why Whiskereds have smaller feet, shorter claws and toes, and shorter bills than their close relative.  Whiskered's slightly higher, denser woodland habitat is also quite likely the reason for the divergent feather patterns of these two species' breasts and bellies.  Both have light underparts with dark vertical shafts crosshatched by horizontal streaks.  These streaks are thicker and closer together on Whiskereds, giving them a bold, spotted look, lighter and farther apart on Westerns, giving them a muted, less obvious pattern.  Form always follows function.  Bold and spotted would provide more camouflage in denser woods.

The best rule of thumb in considering how closely related avian species diverge is to remember there are really only two kinds of birds--predators and predator food, and small owls are both.  The features which distinguish Whiskered Screech-Owls from Arizona's other three small "eared" owls have certainly made them more efficient predators and less obvious predator prey for Great Horned Owls and accipiters within their relatively small and specific habitat niche.

Avian adaptation is just one more reason why birding is so fun and fascinating.