| Summary: | I conducted the first systematic study ever on distribution, abundance, habitat selection, morphology, breeding biology, and diet of the endemic and threatened Bearded Screech-Owl (Megascops barbarus) in the Central Highlands of Chiapas, Mexico from 2002 to 2004. The Bearded Screech-Owl was more likely to be detected in moist forests and the average of number of owls detected per linear trail was 1.65±0.61 owls/Km in nine locations. The owl presence appeared to be associated with both moist oak and pine-oak forests at the regional level and at Huitepec Reserve where there was greatest owl density than reference sites. Within these forest types, canopy cover was the most important measured habitat variable indicating habitat selection of Bearded Screech-Owls. Slope more than 40.3 ± 5.28% combined with N-NW aspect tended to have higher owl presence, probably because steep slopes with northwest aspect are more humid and also have fewer disturbances by human activities. Estimated mean home range of 8 Bearded Screech-Owls was 22.38 ± 4.21 ha. Mean home range marginally varied between different locations, but was not significantly different between males (N=5) and females (N=3). Morphological measurements from 39 individuals indicated that this species showed reversed sexual dimorphism, with females being heavier, having longer tail than males, but smaller tarsus and culmen. Moulting occurred in the rainy season (July to October), with the primary and secondary feathers being moulted simultaneously. Subcutaneous fat was moderate to abundant in the dry season only (December to May). The first nest recorded for this species was found in a natural cavity of an old Quercus laurina tree. Four roosting sites were located, two in Clethra macrophylla and two in Pinus ayacahuite. Feces analysis and remains collected in the nest indicated that the Bearded Screech-Owl feeds mostly invertebrates, including Melonontidae (Coleoptera), and fewer of Orthoptera, Lepidoptera, and Arachnida. Stable isotopes of δ¹³C and δ¹⁵N analyzed from body and tail feathers from 24 individuals showed ranges of 3.74[per thousand] and 3.99[per thousand] respectively, which indicated large variation in the diet among individuals during the period of feather growth. Although there was no significant variation in stable isotopes between sexes, females had wider variation in both isotopes signatures than males. Stable isotopes from feathers did not show spatial variation, but using feathers from museum specimens and my field stuffy there were significant differences (P < 0.01) in δ¹⁵N signature values over time. This study represents a first step towards our understanding of the ecological requirements of the endemic and threatened Bearded Screech-Owl in the Central Highlands of Chiapas. === Land and Food Systems, Faculty of === Graduate
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