Cadena-Ortiz, Solórzano, Noboa, and Brito: Diet of the Short-Eared Owl (Asio flammeus) in the Antisana highlands, Ecuador



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).

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).

Figure 1

Habitat in the Antisana highlands, Ecuador 4000 masl (A), Short-Eared Owl (B), paramo rabbits principal prey (C) (photo: JB, July 2018).

1870-7459-huitzil-20-02-e535-gf1.jpg

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/pi2, where pi is the proportion of individuals found of prey-item i on the diet, and Levins’s standardized niche breadth: Best = (B - 1)/(n - 1), where n is the number of prey-items (Feinsinger et al. 1981, Krebs 1999).

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; Best = 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; Best = 0.27).

Table 1

Composition of the diet of Short-Eared Owl (Asio flammeus) based on n = 163 pellets, collected in the Antisana highlands, Ecuador, in January and July 2017, and in July 2018.

Taxa Mass (g) Number of individuals % Biomass %
RODENTIA
Cricetidae
Akodon mollis 15 4 2 60 1
Phyllotis haggardi 20 93 38 1860 19
Microryzomys altissimus 16 12 5 192 2
Thomasomys sp. 12 3 1 36 0
LAGOMORPHA Leporidae
Sylvilagus andinus (breeds) 70 111 46 7770 78
COLEOPTERA 1 19 8 19 0
TOTAL 242 9937

The short-eared owl in highlands and temperate Andean forests has Best 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).

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).

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.

Acknowledgments

We thank Paolo Piedrahita for contents and Julie Watson for English grammar revisions.

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Notes

[1] Sergio Alvarado Orellana

[2] HC and MFS: drafted the manuscript. HC, MFS and JB conducted fieldwork. MN and JB: processed and identified the samples. HC: elaborate the final version of the manuscript. All of the authors reviewed and approved the final manuscript.

[3] Cadena-Ortiz, H., M.F. Solórzano, M. Noboa, J. Brito. 2019. Diet of the Short- Eared Owl (Asio flammeus) in the Antisana highlands, Ecuador. Huitzil 20(2): e-535. DOI: https://doi.org/10.28947/hrmo.2019.20.2.436



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