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Aegolius ridgwayi

Unspotted Saw-whet Owl

  • Order: Strigiformes
  • Family: Strigidae
  • Polytypic 3 Subspecies

Authors: Enríquez, Paula L., M.C. Arizmendi, C. Rodríguez-Flores, and C. Soberanes-González

Identification

Summary

Owls of the genus Aegolius are small owls that lack "ear" tufts, and that have relatively large heads and short tails. Unspotted Saw-whet Owl has a simple plumage pattern; the upperparts and the upper breast are brown, and the lower breast and the belly are ochraceous buff. Additionally, Unspotted Saw-whet Owl has a narrow white border to the brown facial disks, and has prominent white "eyebrows".

Similar Species

Unspotted Saw-whet Owl is unique within its range; no other small owl is unpatterned (that is, totally lacking streaks or barring in the plumage). Unspotted Saw-whet Owl is closely similar, however, to the allopatric Northern Saw-whet Owl (Aegolius acadicus). Adult Northern Saw-whet Owl easily is distinguished by its white underparts, heavily streaked with rufous brown, and by the extensive white spotting on the scapulars and wing coverts. Juvenile Northern Saw-whet Owl has a plumage pattern that is very similar to that of Unspotted Saw-whet Owl, but perhaps is paler above; Unspotted Saw-whet Owl also is completely unspotted above, and the facial disk has a narrow white border (the border of the facial disk is dark in juvenile Northern Saw-whet Owl).

Vocalizations

Owls communicate through vocalizations (calls) that are used to defend territorial areas or attract females. Each owl species has its own repertory. There is little information about the vocalizations of Unspotted Saw-whet Owl. The territorial songs of males are a series from 4 to 10 light notes, higher than the vocalizations of pygmy-owls (Glaucidium spp.) or of similar calls of screech-owls (Megascops ssp.). This song is transcribed as "a whistled hoo hoo hoo… etc.,10/3-5 sec" (Howell and Webb 1995). The female gives a "a high, slightly hissing ssirr" (Howell and Webb 1995). Other vocalizations include "a surprisingly loud shriek and a shrill short chipper" (Howell and Webb 1995).

The song of a species of tree frog, Anotheca, is described as "amazingly owl-like", and can be confused with the song of Unspotted Saw-whet Owl (Stiles and Skutch 1989).

Additional recordings of vocalizations of Unspotted Saw-whet Owl can be heard at Macaulay Library and at xeno-canto.

Nonvocal Sounds

None reported.

Detailed Description (appearance)

The following description is based on Ridgway (1914) and on König and Weick (2008):

Adult: Upperparts generally sepia brown. The tail is darker brown. Remiges grayer brown, narrowly edged with paler brown; inner webs of innermost secondaries have irregular spots of dull whitish near the margins. Facial disk brown, with narrow white border. Lores, chin, and "eyebrows" whitish. Throat and upper breast sepia brown. Lower breast and belly cinnamon buff. 

Juvenile: Similar to adult, but plumage generally softer and more downy. There sometimes is faint pale streaking on the breast.

Bare Parts

Iris: yellow, honey yellow

Bill: gray, dark horn

Tarsi and toes: flesh colored

Bare parts color data from Howell and Webb (1995), König and Weick (2008).

Measurements

Total length: 18 cm (Stiles and Skutch 1989), 18-20 cm (König and Weick 2008), 20.5-21.5 cm (Howell and Webb 1995)

Linear measurements:

wing length: 133-146 mm (n = ?; sexes combined?; König and Weick 2008)

wing length: male, 138.0 mm, 138.8 mm (n = 2; Wetmore 1968)

                      female, 152 mm (n = 1; Wetmore 1968)

tail length: ca 64 mm (n = ?; sexes combined?; König and Weick 2008)

tail length: male, 66.1 mm, 67.7 mm (n = 2; Wetmore 1968)

                       female, 70.5 mm (n = 1; Wetmore 1968)

culnen length (from cere): male, 11.8 mm, 12.3 mm (n = 2; Wetmore 1968)

                     female, 13.8 mm (n = 1; Wetmore 1968)

tarsus length: male, 25.9 mm, 26.5 mm (n = 2; Wetmore 1968)

                    female, 29.8 mm (n = 1; Wetmore 1968)

Mass: ca 80 g (Stiles and Skutch 1989, König and Weick 2008); 89-92 g (P.L. Enríquez, personal observations)

Molts

Molt peak has been recorded during the rainy season, with molt throughout the body (July, n = 2;P.L. Enríquez, personal observations). Fat also was evident during July (P.L. Enríquez, personal observations).

Geographic Variation

Three subspecies of Unspotted Saw-whet Owl have been described, although König and Weick (2008) note that "the lack of sufficient material for comparative studies makes subspecific splitting very difficult, as nothing is known about individual variability"; consequently, they regard the species as monotypic.

The three described subspecies are:

tacanensis Moore 1947; type locality Volcán Tacaná, Chiapas, Mexico, 3000 m.

Highlands of Chiapas, Mexico. Said to differ from rostratus by "having (A) a more pointed wing; (B) a smaller bill and shorter culmen; (C) very short buffy streaks on forehead, instead of none in rostratus, or long white ones in [Aegolius] acadicus" (Moore 1947: 142). Described on the basis of a single specimen.

rostratus (Griscom 1930); type locality Sacapulas, Rio Negro Valley, central Guatemala

Occurs in Guatemala and El Salvador. Similar to nominate ridgwayi, but differs by having a larger and heavier bill, darker brown suborbital region, and white (less buffy) forehead. Described on the basis of a single specimen. Marshall (1943) suggested that the reported larger size of the bill in rostratus is an artifact resulting from an error of comparing two different measurements of bill size, with sexual dimorphism in bill size a potential further compounding factor.

ridgwayi (Alfaro 1905); type locality Cerro de la Canelaria, near Escasú, Costa Rica.

Occurs in Costa Rica and Panama.

Systematics

Aegolius ridgwayi presumably is closey related to Aegolius acadicus (Northern Saw-whet Owl), as these two species have complementary geographic distributions, and all plumages of ridgwayi are similar to that of juvenile acadicus; some authors have suggested that the two might be conspecific (e.g., Peters 1940, Briggs 1954). 

Briggs (1954) described a specimen, said to have been collected in Oaxaca, Mexico, that resembles both juvenile of acadicus and all plumages of ridgwayi, but "in wing and tail markings is exactly intermediate". Binford (1989: 279) questioned the reported locality for this specimen, and further suggested that its characters may prove to be "within the individual variation of Oaxaca acadicus or [of] some population of ridgwayi occurring east of the Isthmus [of Tehuantepec]". König and Weick (2008: 446) regard the type (and only known specimen of?) tacanensis, from Chiapas, to be a hybrid between acadicus and ridgwayi, and  further raise the possibility that rostratus also represents a hybrid (König and Weick 2008: 447).

Recommended Citation

Enríquez, Paula L., M.C. Arizmendi, C. Rodríguez-Flores, and C. Soberanes-González. 2012. Unspotted Saw-whet Owl (Aegolius ridgwayi), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=215576