Wing Loading in 15 Species of North American Owls David H. Johnson 1 Abstract.—Infor mation on wing morphology is important in under - standing the mechanics and ener getics of flight and in aspects r e- lated to reversed sexual size dimorphism in owls. I summarized wing span, wing area, wing loading, root box, and aspect ratio calculations from the available literatur e and from 113 owls examined in this study. Wing loading estimates for 15 species ranged fr om 0.211 to 0.545 g/cm 2. Measur ements were available for both male and female owls in 12 species; males of all species had a lower wing loading. In five species with sufficient sample sizes, males had significantly lower wing loading (18 percent on average) than females of the same spe- cies. Root box ar ea (the area between the wings) averaged 15.4 percent of the combined wing and r oot box ar eas. Aspect ratio, the ratio of the wing span to mean wing width, ranged fr om 4.84 to 8.90. Infor mation is pr esented for the following species: Bar n (Tyto alba), Short-ear ed (Asio flammeus), Long-ear ed (A. otus), Gr eat Horned (Bubo virginianus), Barred (Strix varia), Gr eat Gray (S. nebulosa), Norther n Spotted ( S. occidentalis caurina), Snowy (Nyctea scandiaca), Eastern Scr eech- ( Otus asio), Western Scr eech- ( O. kennicottii), Flammulated (O. flammeolus), Norther n Pygmy- (Glaucidium gnoma), Norther n Saw-whet (Aegolius acadicus), Bor eal (A. funereus), and Burr owing (Speotyto cunicularia) Owls. The type of habitat a flying or gliding animal narrow, slim, unslotted high-speed wings, chooses to live in, as well as its way of exploit- designed for fast flight in open habitats. ing the habitat, ar e closely related to its body Shearwaters, albatrosses, and other seabir ds size, wing form, flight style, flight speed, and have long, narr ow, flat, high-aspect-ratio wings, flight ener getics. Natural selection is likely to designed for long-distance gliding. Last, act towards a wing structur e that minimizes storks, eagles, and vultur es have high-lift or the power requir ed to fly at the speed and style slotted soaring wings, which in lar ge birds optimal for the animal, and is assumed to produces a very efficient soaring wing result in some near -optimal combination of (Feduccia 1996). Most owls have r elatively these variables. The optimal flight speed varies large, rounded, and slotted wings. Savile with the flight goal and the type and abun- (1957) characterized the Easter n Scr eech-owl dance of food. T o understand how flying (Otus asio) as having a slotted high-lift wing. animals work, their physiology, morphology, The “slotting” is a r esult of the abrupt narr ow- ecology, and wing function must be known. ing (ter med attenuation or emar gination) in the distal end of up to five or six of the longest Although ther e are many styles of wings, primaries. While this attenuation is limited in ornithologists generally r ecognize four basic some owls, it is quite pr onounced in others (see wing types (Savile 1957). Woodpeckers, galli- Averill 1927). W ithers (1981) suggests that naceous species, and most passerines have wing-tip slots have evolved because of biome- short, br oad elliptical wings, designed for chanical limitations to the bending str ength of maneuvering thr ough dense vegetation. Swifts, large, low-aspect ratio bir d wings that could swallows, falcons, and plovers have long, have detrimental aer odynamic consequences. Wing loading is a metric used in deter mining 1 Washington Department of Fish and W ildlife, the speed, dynamics of lift, and tur ning radius 600 Capitol Way North, Olympia, W A 98501- of birds (also bats and aircraft). It is expr essed 1091. as the relationship between body mass and 553 2nd Owl Symposium total wing area, calculated by dividing the association with other activities during a weight of a bird by the sur face area of both demographic study on this species. Bor eal wings. Wing loading is expr essed by grams per Owls were sexed by their behavior during square centimeter (g/cm 2) (Clark 1971). Owls’ radio-telemetry studies during the nesting wings are broad, with a large area in compari- season (e.g., males delivering pr ey to nest site; son to the weight of the bir d, giving them a low females incubating and br ooding) (G. Hayward, wing-loading relative to other bir ds. pers. comm.). Gr eg Hayward (unpubl. data) submitted wing ar eas, weights, and capture Another expr ession of wing morphology is dates for all of the Bor eal Owls examined in called aspect ratio—the ratio of wing span to this study. Data on a lar ge sample of Barn mean wing width. Thus, long and narr ow Owls (Tyto alba) was drawn from Marti (1990); wings designed for high speed, have a high differences in his methodology should be noted. aspect ratio, while short, br oad wings designed Summary wing loading and aspect ratio calcu- for low speed and maneuverability, have a low lations were determined using a weighted aspect ratio. In general, wing length is some- mean, that is: (mean of males plus mean of what shorter in those bir d species which hunt females)/2. in cover, and longer in those which hunt in open country or ar e highly migratory. Wing span—Wing span is defined as the dis- tance (mm) fr om one wing tip to the other , with The objective of this paper is to summarize the the wings spread horizontally as far out as they relevant literatur e and to pr ovide mor e specific will go (fig. 1). Wing span measur ements were infor mation on wing span, wing loading, r oot taken with the owls placed on their backs atop box, aspect ratio, and male/female compari- a tape measure. sons for the owls of North America. Wing area—Owls were held with their body METHODS AND MA TERIALS facing downward and a single wing spread over a paper on a board or table (see Pennycuick Wing data for eight species was extracted fr om 1989, p. 11). W ing area was measured by the limited literatur e on this topic. Data for tracing ar ound each fully extended and flat- this study was obtained from the following tened wing. Starting wher e the front of the locations: Gr eat Gray Owls (Strix nebulosa) from souther n Manitoba, Bor eal Owls (Aegolius funereus) from Idaho, a Flammulated Owl (Otus flammeolus) from Colorado, and nine species of owls from Or egon. With the exception of eight Great Horned Owls (Bubo virginianus), the owls from Or egon came from the west side of the Cascade Mountains. Except for the Norther n Spotted Owls (Strix occidentalis caurina), Bor eal Owls, and one Barred Owl (S. varia), all owls examined were dead. Between 1988 and 1997, over 250 owl carcasses were examined. The owls were trauma-killed, the vast majority resulting fr om vehicle collisions. Only fr esh specimens in excellent condition wer e used; owls with broken wing bones, tor n skin tissue, a pronounced keel suggesting cause of death by starvation, other damage, or br oken or Figure 1.—Wing span is the wingtip-to-wingtip molted wing feathers were discarded. The sex distance (in mm), with wings spread out to of dead owls was determined through inter nal the sides to their fullest extent. WL = wing sexing (e.g., by looking for testes or ovaries). length, as measured (in mm) from the wing The sex of live Norther n Spotted Owls and the root line to tip of the longest primary. RL = Barred Owl was determined by their vocaliza- root line, a straight line depicting the inter- tions and by examination for the pr esence/ face between the wing and the owl’s body. absence of a brood patch (if during the nesting RB = root box, (in cm2). This (reduced) season). Norther n Spotted Owl wing measur e- tracing is from a male Western Screech-owl ments were acquir ed during 1988-1989 in (Otus kennicottii ). 554 wing meets the body, a line was drawn follow- Wing length—Wing length was determined by ing the outline of the individual feathers. After measuring the perpendicular distance (mm) tracing the wing to wher e it again met the body from the wing r oot line to the tip of the longest (at the inside edge of the secondaries), the bir d primary feather (fig. 1). was lifted off the paper and a wing root line was drawn to the starting point, thus closing Root box—The area (cm 2) between the wings the wing polygon. Both wings fr om each owl (fig. 1). were measured (except in Bor eal Owls, where only one wing was measured). Wing tracings Aspect ratio—The aspect ratio is a simple were digitized into a geographical infor mation measure of the shape of the wing. It is the system (e.g., ArcInfo software) and the total ratio of the wing span to mean wing width. area (cm 2) calculated. Ar eas of paired wings Wing width is the distance fr om the leading were generally within 0-3 per cent of each other; edge to the trailing edge, measur ed along the the larger wing area was doubled and used in direction of flight. The mean wing width was subsequent analysis. T racing wings requir es determined by first summing the ar ea of both two people and some practice; the slight dif fer- wings with the area of the root box.