Factors associated with the detectability of owls in South American temperate forests: Implications for nocturnal raptor monitoring

Abstract

Owls occur at relatively low densities and are cryptic; thus, monitoring programs that estimate variation in detectability will improve inferences about their presence. We investigated temporal and abiotic sources of variation associated with detection probabilities of rufous‐legged owls (Strix rufipes), a threatened forest specialist, and austral pygmy‐owls (Glaucidium nana), a habitat generalist, in temperate forests of southern Chile. We also assessed whether detection of 1 species was related to the detection of the other species. During 2011–2013, we conducted 1,145 broadcast surveys at 101 sampling units established along an elevational gradient located inside and outside protected areas. We used a multi‐season occupancy framework for modeling occupancy (ψ) and detection (p), and ranked models using an information‐theoretic approach. We recorded 292 detections of rufous‐legged owls and 334 detections of austral pygmy‐owls. Occupancy was positively associated with elevation for rufous‐legged owls but constant (i.e., did not vary with covariates) for pygmy‐owls. Detectability for both owls increased with greater moonlight and decreased with environmental noise, and for pygmy‐owls greater wind speed decreased detectability. The probability of detecting pygmy‐owls increased nonlinearly with number of days since the start of surveys and peaked during the latest surveys of the season (23 Jan–7 Feb). Detection of both species was positively correlated with the detection of the other species. We suggest both species should be surveyed simultaneously for a minimum of 3–4 times during a season, survey stations should be located away from noise, and observers should record the moon phase and weather conditions for each survey. © 2014 The Wildlife Society.

Occurrence patterns and niche relationships of sympatric owls in South American temperate forests: A multi-scale approach

“…six new sites in year two). The term ''site'' used here is referred to as a ''sampling unit'' in Ibarra et al (2014).…”

“…We chose this watershed because its rural road and trail system is extensive and accessible, and its landscapes are representative of the Andean portion of Chile's temperate forests (Ibarra et al, 2014 (<4 months) with a mean annual precipitation of 2500 mm, most falling as snow at higher altitudes (>750 m of altitude). Elevation ranges from 200 to above 2800 m, with forests distributed from 200 to 1500 m of altitude.…”

“…We randomly selected 13 of these 19 basins and placed the first site for all basins near the headwater (within 1 km of tree-line). We systematically established the remaining sites every 1.5 km descending the drainages (Ibarra et al, 2014).…”

“…1c) level of peak performance for particular niche resources, than G. nana. To examine owl occurrence patterns and test our predictions, we used occupancy models that account for the likelihood that owls occurred at some sites without detections (i.e., were present but not detected, Ibarra et al, 2014). Our models allowed us to identify key niche resources to which owls are associated, and thus can provide reliable recommendations for owl conservation.…”

Are avian predators effective biological control agents for rodent pest management in agricultural systems?

“…Since rodent species possess a rather complex biology and behaviour (Leirs 2003) and a great deal of avian predators exhibit elusive behaviour and occur in relative low densities (Ibarra et al, 2014), monitoring and obtaining results can be rather difficult.…”