To know the taxonomy of prey is important to establish inter and intraspecific interactions, the role of the predator in its environment and understand its ecology and natural history, the diet of a species could be inferred base on bill form or knowledge of congeners preys, nevertheless the diet may vary due to factors of temporality, geography or availability of prey, even in nearby or similar geographical areas could be individual specialization (^{Bolnick et al. 2003}). Despite an increase in publications on owls’ diets in recent years (e.g., ^{Cadena-Ortiz et al. 2018}, ^{Orihuela-Torres et al. 2018}), the knowledge of natural history for Ecuadorian Strigiformes still low (^{Freile et al. 2017}). Only three studies have been done on diet composition of short-eared owls (Asio flammeus) in Ecuador; two from the Galapagos Islands (^{De Groot 1983}, ^{Piedrahita and Wagner 2017}) and the other one from Pichincha province (^{Pozo-Zamora et al. 2017}). Asio flammeus has wide distribution worldwide (^{König and Weick 2008}), in Ecuador occurs along the Andes, principally between 3000 to 4000 masl, in different habitats such as paramo, dry valleys and adjacent to agricultural fields (^{Freile and Restall 2018}).

• ^{Bolnick et al. 2003}
  The ecology of individuals: Incidence and Implications of individual specialization

  The American Naturalist, 2003
  The ecology of individuals: Incidence and Implications of individual specialization
• ^{Cadena-Ortiz et al. 2018}
  Notas sobre la dieta, desarrollo de jóvenes y distribución del Búho Estigio Asio stygius (Strigiformes: Strigidae) en Ecuador

  Revista Ecuatoriana de Ornitología, 2018
  Notas sobre la dieta, desarrollo de jóvenes y distribución del Búho Estigio Asio stygius (Strigiformes: Strigidae) en Ecuador
• ^{Orihuela-Torres et al. 2018}
  Diet of the Spectacled Owl (Pulsatrix perspicillata) in Zapotillo, southwestern Ecuador

  Revista Brasileira de Ornitologia, 2018
  Diet of the Spectacled Owl (Pulsatrix perspicillata) in Zapotillo, southwestern Ecuador
• ^{Freile et al. 2017}
  The Owls of Ecuador

  Neotropical Owls, 2017
  The Owls of Ecuador
• ^{De Groot 1983}
  Origin, status and ecology of the owls in Galapagos

  Ardea, 1983
  Origin, status and ecology of the owls in Galapagos
• ^{Piedrahita and Wagner 2017}
  Food habits of Galapagos Owls: a preliminary study

  110th annual meeting of the German Zoological Society, 2017
  Food habits of Galapagos Owls: a preliminary study
• ^{Pozo-Zamora et al. 2017}
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

  Revista Ecuatoriana de Ornitología, 2017
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador
• ^{König and Weick 2008}
  Owls of the world, 2008
  König, C., F. Weick. 2008. Owls of the world. 2a. ed. Christopher Helm. London, Great Britain.
• ^{Freile and Restall 2018}
  Birds of Ecuador. Helm Field Guides, 2018
  Freile J., R. Restall. 2018. Birds of Ecuador. Helm Field Guides. London, Great Britain.

Here we report the diet of the short-eared owl based on n = 163 pellets collected in January and July 2017, and in July 2018, inside tussock of Poaceae grass (Figure 1A) scattered in a small area ca. 20 m², in La Ovejeria grasslands (00°31’ S, 78°14’ W, 4050 masl), on the Antisana Ecological Reserve border, Napo Province. During collection we saw short-eared owls flying over us. In one occasion we saw up to six owls close by the grass tufts, which could be a breeding site of short-eared owl family groups (^{Olsen et al. 2019}).

• ^{Olsen et al. 2019}
  Short-eared Owl (Asio flammeus)

  Handbook of the birds of the world alive, 2019
  Olsen, P.D., G.M. Kirwan, D.A. Christie, J.S. Marks. 2019. Short-eared Owl (Asio flammeus). In J. Del Hoyo, A. Elliott, J. Sargatal, D.A. Christie, E. De Juana (eds.). Handbook of the birds of the world alive. Lynx Editions. Barcelona, Spain. Available from: Available from: http://www.hbw.com/node/55128 (accessed on 3 January 2019).

Pellets were dried to the environment for c. a week, each one were measured, weighted and manually disintegrated, the bones and arthropod remains were separated, and we determined the minimum number of individuals in the sample by counting the homologous jaw or skull remnants for vertebrates, and elytra, heads and mandibles of arthropods, items found in pellets were identified using literature (^{Hershkovitz 1962}, ^{Voss 2003}, ^{Weksler and Percequillo 2011}, ^{Ruedas et al. 2017}, ^{Brito et al. 2019}) and by direct comparison to specimens deposited at the Instituto Nacional de Biodiversidad (Inabio), the weight of prey species also was taken from Inabio data. For our data and ^{Pozo-Zamora et al. (2017)} data, we estimate niche breadth using Levins’s measure: B = 1 / ∑ p i 2 , where p_i is the proportion of individuals found of prey-item i on the diet, and Levins’s standardized niche breadth: B_est = (B - 1)/(n - 1), where n is the number of prey-items (^{Feinsinger et al. 1981}, ^{Krebs 1999}).

• ^{Hershkovitz 1962}
  Evolution of Neotropical Cricetine rodents (Muridae) with special reference to the Phyllotine group

  Fieldiana Zoology, 1962
  Evolution of Neotropical Cricetine rodents (Muridae) with special reference to the Phyllotine group
• ^{Voss 2003}
  A new species of Thomasomys (Rodentia: Muridae) from eastern Ecuador, with remarks on mammalian diversity and biogeography in the Cordillera Oriental

  American Museum Novitates, 2003
  A new species of Thomasomys (Rodentia: Muridae) from eastern Ecuador, with remarks on mammalian diversity and biogeography in the Cordillera Oriental
• ^{Weksler and Percequillo 2011}
  Key to the genera of the tribe Oryzomyini (Rodentia: Cricetidae: Sigmodontinae)

  Mastozoología Neotropical, 2011
  Key to the genera of the tribe Oryzomyini (Rodentia: Cricetidae: Sigmodontinae)
• ^{Ruedas et al. 2017}
  A prolegomenon to the systematics of South American cottontail rabbits (Mammalia, Lagomorpha, Leporidae: Sylvilagus): Designation of a neotype for S. brasiliensis (Linnaeus, 1758), and restoration of S. andinus (Thomas, 1897) and S. tapetillus (Thomas, 1913), 2017
  A prolegomenon to the systematics of South American cottontail rabbits (Mammalia, Lagomorpha, Leporidae: Sylvilagus): Designation of a neotype for S. brasiliensis (Linnaeus, 1758), and restoration of S. andinus (Thomas, 1897) and S. tapetillus (Thomas, 1913)
• ^{Brito et al. 2019}
  Mamíferos del Ecuador. Versión 2019.0, 2019
  Mamíferos del Ecuador. Versión 2019.0
• ^{Pozo-Zamora et al. (2017)}
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

  Revista Ecuatoriana de Ornitología, 2017
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador
• ^{Feinsinger et al. 1981}
  A simple measure of niche breadth

  Ecology, 1981
  A simple measure of niche breadth
• ^{Krebs 1999}
  Ecological methodology, 1999
  Krebs, C.J. 1999. Ecological methodology. 2a. ed. Addison-Wesley. New York, EUA.

The n = 163 pellets collected were of the following sizes: length (mean = 46.4 mm, range = 24.3 - 79.2 mm), width (mean = 17.7 mm, range = 15.6 - 20.4 mm), dry weight (mean = 3.2 g, range = 1 - 6 g). Measurements were similar to those previously reported for three temperate Andean forests in Ecuador (^{Pozo-Zamora et al. 2017}). The mean number of prey items/pellet was 2.5 ± 2.6 (range 1- 6). In the 163 pellets of short-eared owl we found 242 prey items grouped in only six taxa, five mammals was 92% of the prey and the rest was beetles (Order Coleoptera). The breeds of paramo rabbits (Sylvilagus andinus) was the most important prey (Figure 1C) in terms of biomass and frequency (Table 1). Niche breadth of A. flammeus in Antisana highlands, 4050 masl, with six prey items in 163 pellets, showed low values (B = 2.72; B_est = 0.34) also the values of three temperate Andean forests, x - = 2868 masl, with 22 prey items in 52 pellets (Pozo-Zamora et al. 2017) (B = 6.76; B_est = 0.27).

• ^{Pozo-Zamora et al. 2017}
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

  Revista Ecuatoriana de Ornitología, 2017
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

The short-eared owl in highlands and temperate Andean forests has B_est lower than 0.60, which means a predator specialist (^{Krebs 1999}), with narrow trophic niche (^{Jaksic 1989}). However the absence of items previously reported in the diet of short-eared owl in our data, such as birds (e.g., ^{Diéguez 1996}, ^{Martínez et al. 1998}, ^{Baladrón et al. 2014}, ^{Torres et al. 2014}, ^{Pozo-Zamora et al. 2017}, ^{Piedrahita and Wagner 2017}), suggests further specialization in our locality or individual specialization (^{Bolnick et al. 2003}).

• ^{Krebs 1999}
  Ecological methodology, 1999
  Krebs, C.J. 1999. Ecological methodology. 2a. ed. Addison-Wesley. New York, EUA.
• ^{Jaksic 1989}
  What do carnivorous predators cue in on: size or abundance of mammalian prey? Acrucial test in California, Chile, and Spain

  Revista Chilena de Historia Natural, 1989
  What do carnivorous predators cue in on: size or abundance of mammalian prey? Acrucial test in California, Chile, and Spain
• ^{Diéguez 1996}
  Aves depredadas por Asio flammeus suinda en Saladillo, Provincia de Buenos Aires

  APRONA Boletín Científico Asociación para la Protección de la Naturaleza, 1996
  Aves depredadas por Asio flammeus suinda en Saladillo, Provincia de Buenos Aires
• ^{Martínez et al. 1998}
  Food habits and hunting ranges of Short-eared Owls (Asio flammeus) in agricultural landscapes of southern Chile

  Journal of Raptor Research, 1998
  Food habits and hunting ranges of Short-eared Owls (Asio flammeus) in agricultural landscapes of southern Chile
• ^{Baladrón et al. 2014}
  Presas del nuco (Asio flammeus) en un área de pastizal pampeano de Argentina

  Boletín Chileno de Ornitología, 2014
  Presas del nuco (Asio flammeus) en un área de pastizal pampeano de Argentina
• ^{Torres et al. 2014}
  Presas del Búho Campestre (Asio flammeus) en un Agroecosistema Subtropical de Paraguay

  Nuestras Aves, 2014
  Presas del Búho Campestre (Asio flammeus) en un Agroecosistema Subtropical de Paraguay
• ^{Pozo-Zamora et al. 2017}
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

  Revista Ecuatoriana de Ornitología, 2017
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador
• ^{Piedrahita and Wagner 2017}
  Food habits of Galapagos Owls: a preliminary study

  110th annual meeting of the German Zoological Society, 2017
  Food habits of Galapagos Owls: a preliminary study
• ^{Bolnick et al. 2003}
  The ecology of individuals: Incidence and Implications of individual specialization

  The American Naturalist, 2003
  The ecology of individuals: Incidence and Implications of individual specialization

We found a positive correlation between the mass of the prey and its frequency in the pellets (Pearson test r = 0.77; t = 2.43; df = 4; P = 0.07), which suggests a specialization of the owl for capturing large prey, with which they obtain more biomass with less effort. The two prey of greater biomass (S. andinus and Phyllotis haggardi) also predominate in frequency on the owl diet. Sylvilagus andinus reproduces all year round (^{Vallejo 2017}) and have high density in Ecuadorian highlands (^{García et al. 2016}), being a permanent source of food for raptors. Our biomass dominance of S. andinus was agreed with ^{Pozo-Zamora et al. (2017)}, and it was also the main prey of variable hawk (Geranoaetus polyosoma) in our same study area (^{De Vries et al. 2014}).

• ^{Vallejo 2017}
  Sylvilagus andinus

  Mamíferos del Ecuador. Version 2019.0, 2017
  Vallejo, A.F. 2017. Sylvilagus andinus. En J. Brito, M.A. Camacho, V. Romero, A. Vallejo (eds.). Mamíferos del Ecuador. Version 2019.0. Museo de Zoología, Pontificia Universidad Católica del Ecuador. Disponible en: Disponible en: https://bioweb.bio/faunaweb/mammaliaweb/FichaEspecie/Sylvilagus%20andinus (consultado el 21 enero de 2019).
• ^{García et al. 2016}
  An assessment of the populations of Sylvilagus brasiliensis andinus in Páramos with different vegetation structures in the northeastern Andes of Ecuador

  Neotropical Biodiversity, 2016
  An assessment of the populations of Sylvilagus brasiliensis andinus in Páramos with different vegetation structures in the northeastern Andes of Ecuador
• ^{Pozo-Zamora et al. (2017)}
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador

  Revista Ecuatoriana de Ornitología, 2017
  Primeras observaciones de la dieta del Búho Orejicorto Asio flammeus bogotensis (Strigiformes: Strigidae) en Pichincha, Ecuador
• ^{De Vries et al. 2014}
  Breeding and feeding biology of the Gurney’s Hawk Geranoaetus poecilochrous in the paramo of Antisana, Ecuador

  Revista Ecuatoriana de Medicina y Ciencias Biológicas, 2014
  Breeding and feeding biology of the Gurney’s Hawk Geranoaetus poecilochrous in the paramo of Antisana, Ecuador

Due to the absence of studies of mammalian densities in our study area, we still cannot define whether the short-eared owl’s foraging behavior would be opportunistic (when it ingests the prey in the same relative abundances of its environment) or selective (when it ingests some or all of the prey in different proportions to those present in the hunting area) (^{Jaksic 1989}); but the studies on the short-eared owl diet in South America were agreed with the dominance of mammals as prey (^{Rau et al. 1992}, ^{Martínez et al. 1998}, ^{Cirignoli et al. 2001}, ^{Baladrón et al. 2014}, ^{Torres et al. 2014}). Remains the necessity of long term studies and samples in more locations along the distribution of the owl, to elucidate patterns and have a better characterization on the diet of this owl in Ecuador.

• ^{Jaksic 1989}
  What do carnivorous predators cue in on: size or abundance of mammalian prey? Acrucial test in California, Chile, and Spain

  Revista Chilena de Historia Natural, 1989
  What do carnivorous predators cue in on: size or abundance of mammalian prey? Acrucial test in California, Chile, and Spain
• ^{Rau et al. 1992}
  Food habits of the Short-eared Owl (Asio flammeus) in southern South America

  Journal of Raptor Research, 1992
  Food habits of the Short-eared Owl (Asio flammeus) in southern South America
• ^{Martínez et al. 1998}
  Food habits and hunting ranges of Short-eared Owls (Asio flammeus) in agricultural landscapes of southern Chile

  Journal of Raptor Research, 1998
  Food habits and hunting ranges of Short-eared Owls (Asio flammeus) in agricultural landscapes of southern Chile
• ^{Cirignoli et al. 2001}
  Diet of the Short-eared Owl in northwestern Argentina

  Journal of Raptor Research, 2001
  Diet of the Short-eared Owl in northwestern Argentina
• ^{Baladrón et al. 2014}
  Presas del nuco (Asio flammeus) en un área de pastizal pampeano de Argentina

  Boletín Chileno de Ornitología, 2014
  Presas del nuco (Asio flammeus) en un área de pastizal pampeano de Argentina
• ^{Torres et al. 2014}
  Presas del Búho Campestre (Asio flammeus) en un Agroecosistema Subtropical de Paraguay

  Nuestras Aves, 2014
  Presas del Búho Campestre (Asio flammeus) en un Agroecosistema Subtropical de Paraguay