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Scientific name: Aegolius ridgwayi

Name in English: Unspotted Saw-whet Owl

Common names in Spanish: Lechucita Parda, Lechucita de Alfaro, Mochuelo Moreno

Global distribution: from Mexico southern mountains to western Panama

Distribution in Costa Rica: highlands of Talamanca Mountain Range and Turrialba and Irazú Volcanoes  (from 2.500 m.s.n.m.)






The species was described in 1905 on the Washington Biological Society academic journal under the name of Cryptoglaux ridgwayi. This genus, which means "little hidden owl," was given to species that are now within the Aegolius genus.

First collection

The USWO was first collected in 1903 at Cerro La Candelaria - near Escazú - by Costa Rican naturalist Anastasio Alfaro, who was founder and director of the National Museum of Costa Rica (1887)for many years.

Alfaro named the species ridgwayi in honor to Robert Ridgway, an American ornithologist and secretary of the Smithsonian Institute who made great contributions to the natural history of Costa Rica.


Photo of the specimen collected by Alfaro property of  the Smithsonian Museum of Natural History collection.

Table 1
Records of the subspecies of Aegolius ridgwayi made for the first time in chronological order


Height: 18 cm

Wing:  140 mm,  151 mm

Wingspan: ♀ 514 mm

Weight: ♂ 85 g, ♀ 115-126 g


Since this species has never been studied before we did not know what size bands should be used, but with the measurements we have made we were able to order the correct size bands for this species.

The bands have an email adress (uswocr@gmail.com) and a numeral series. The series for the males is M001-M100 and for the females H001-H100.



Head and upper region uniformly brown

Nape golden-brown

Facial disc border, supercilium and chin between white and buff

uswo ilustr2.png

External primaries and alula with white  leading edge

Chest between cinnamon and brown

Buff belly

Iris between yellow and greenish yellow

Cere and bill mostly blackish with pale sides

Pinkish-yellow toes

Talons mostly black but some are pale pink



Age is determined by studying the molt cycle of the individual (the relative age of its flight feathers) and one of the techniques we use is to determine this is by looking at the feathers under ultraviolet light


New feathers have porphyrines which reflect UV light and makes them glow pink.  In older feathers the porphyrines have degraded due to exposition to sunlight and when UV light is cast upon them they look dark and opaque.  Immature individuals are the only ones going to have one generation of feathers and they will all appear an even pink under UV light.  Adults can have up to three generations of flight feathers.




Their diet is composed mainly of small rodents, shrews and large insects such as beatles and moths.  We have observed them catching rodents both on the ground and on tree trunks, while the insects were captured in mid-air and in all cases they used their talons. 


They tend to hunt from low perches, between 1-3 meters above the ground, from where they observe and listen carefully for their prey.

Part of its diet has been discovered by the finding of pellets. Pellets are a mass of undigestible food parts that birds of prey regurgitate such as bones, hair and insect exoskeletons.



Pellet dissection | Many skeleton parts of small rodents and beetle exoskeletons were found.


The most commonly heard and recorded vocalizations are those of the males since in the majority of cases they occur in response to playback and it is generally the male who responds more aggressively in territorial defense.  However, we have recorded another nine different vocalizations from both sexes, noting the respective behavior involved with each type of call.

USWO maleSong
00:00 / 00:11
USWO Canto Macho.png

Male spectrogram

USWO femaleCall
00:00 / 00:06
USWO Canto Hembra.png

Female spectrogram

USWO maleAggressive call
00:00 / 00:13


All known nests except one have been in natural cavities such as knot holes or broken trunks and branches that have rotted out.  One nest in Guatemala was reported in an old artificial nesting box.  Nest height is anywhere between two and 15 meters above the ground.  The female lays two white, quite round eggs and she incubates them for 28 – 30 days.  During this period the male brings food to the female.  The time from hatching to fledging is still unknown.  As is the case with many Neotropical birds, nest predation seems to be high.


Nest: natural cavities- broken trunks, knot holes

Clutch size: 2 eggs

Eggs: white, oblong

Incubation period: 28-30 days

Parental care: female lays eggs, male carries food

Nesting success: none of the monitored nests have been successful due to predation

We use RECONYX camera traps for monitoring the nests, this way we interfere as little as possible on their behavior.

The cameras' batteries and cards are changed when the owls are not present.


The home range (the entire area a species uses) on average is 55 hectares, but this varies depending on gender, age and time of year.  The habitat used by this owl can vary depending on its age with younger individuals using disturbed areas such as agricultural land and deforested areas, whereas adults will keep within natural habitats and pristine areas.  The greater part of the population of the subspecies A. r. ridgwayi appears to be in oak forests above 2.800m, elfin forests and paramo.




The IUCN lists this species to be of Least Concern but our data shows a strong preference from this species to use paramo and elfin forests which are two habitats at high risk due to rapid climate change.


Ernesto M. Carman is the forerunner of the systematic study of Unspotted Saw-whet Owl (Aegolius ridgwayi) in Costa Rica, starting the continuous search for the species since 2010. Subsequently, other members joined and a project dedicated to the study and conservation of the USWO was formally formed in 2014. 







  • Radiotelemetry

  • Nest monitoring

  • New nestbox design


Natural history behavior, diet and reproduction.

Bioacoustics: recording and analyzing its vocalizations.

Ecology: Home range and distribution.


Observation and photography of the USWO for people interested.  These tours are led to different sites so as to reduce the impact on individuals and giving participants a fair chance of observing the species.  During breeding season visits are restricted and no playback is allowed. 


Involving the general public, authorities and landowners to gain their interest.


Public presentations, participation in ornithological meetings, publishing of results and audiovisual material on social media.


The main objective of our project is to carry out research on the species' natural history and ecology to understand its needs and be capable of proposing conservation guidelines.  The USWO is an "umbrella species"; conservation actions to preserve this owl will benefit many other species, both endemic and migratory, that live in the highlands of Central America

Woud you like to see this owl?



© Photos & videos by Ernesto Carman, Pablo Siles, Diego Quesada and Marco Molina.