Field evaluation for playback surveys: species-specific detection probabilities and distance estimation errors in a nocturnal bird community

Abstract

Capsule: During playback experiments, the distance from the surveyor to the call influences the chances of detection across nocturnal species in farmland and woodland habitats. Aim: To evaluate how distance affects surveyor detection capability, expressed as the probability of hearing broadcast calls and of estimating their distances correctly, in a nocturnal bird community. Methods: We conducted a playback field experiment in farmland and woodland areas within an agricultural landscape in winter and summer 2020. Recorded Vocalisations of five elusive species (Little Owl Athene noctua, Tawny Owl Strix aluco, Long-eared Owl Asio otus, Common Nightingale Luscinia megarhynchos, Water Rail Rallus acquaticus) were broadcast at various distances to a surveyor, who attempted to detect them and, if successful, to classify them into predefined distance zones. were broadcast to a surveyor who attempted to estimate the distance to each call. Binomial GLMMs were used to estimate detection probability as a function of distance, and the effects of habitat and season on this relationship. Results: The distance of the broadcast call from the surveyor had a significant effect on detection probability in both habitats. In woodland, the probability of hearing calls was significantly higher in winter, while estimating distances correctly was generally higher in summer. An increase in field experience improved our detection capability, mainly in terms of distance estimation, whose errors were mostly overestimations. Long-eared Owl was the only species for which estimates were consistently poor. Conclusions: The probability of hearing calls and distance estimation accuracy varied between species. While a 200 m radius around the surveyor fitted better for the Little and Tawny Owl, this was 100 m for the other species. Most Little Owl and Tawny Owl calls were detected within a 200 m radius of the surveyor, while this was 100 m for the other species. For a multi-species community-level study, playback surveys are thus likely to be most representative of a 200 m radius surrounding the surveyor where the probability of detection is highest, while estimates of distance from the observer are likely to be inaccurate in most cases. Field evaluations such as this should be implemented prior to actual playback surveys.

Tawny Owl Strix aluco Distribution in the Urban Landscape: The Effect of Habitat, Noise and Light Pollution

“…Based on a previous test of the playback methodology for the Tawny Owl and other nocturnal species (Orlando et al 2021), we assumed that the effective detection radius was 200 m, i.e. the distance at which detectability decreases rapidly.…”

“…Furthermore, this radial distance is reliable given that the average size of urban Tawny Owl territories is estimated to be around 20 ha, equivalent to an area with a c. 250 m radius (Galeotti 1994). Therefore, inferences made in an area with a 200 m radius should be quite representative of Tawny Owl territories (Orlando et al 2021). Within the study area, there was a distance of at least 500 m between sample points, enough to avoid potential territory overlap.…”

“…Details on the computation of predictor variables.The steps below were followed to compute the percentage of urban cover within a 200 m radius around each sample point, employing the most recent land-use map of the Piedmont Region available (‘Mappatura del Consumo di suolo in Piemonte 2017’, https://www.webgis.arpa.piemonte.it):– Being a WMTS (Web Map Tile Service), the land-use map was converted in a vectorial format and overlayed on a satellite image showing the study area.– A circular buffer zone with a 200 m radius (i.e. the detectability radius; Orlando et al 2021) was set around all sample points to consider only the land-use within this distance.– Each point (with its buffer) was selected, exported and saved. Then, they were all clipped individually with the vectorial land-use map, so that urban land-use could be calculated in each point separately.– In each point, urban land-use was selected and the area calculated in m 2 using the field calculator in the layer's attributes table.…”

“…-A circular buffer zone with a 200 m radius (i.e. the detectability radius; Orlando et al 2021) was set around all sample points to consider only the land-use within this distance. -Each point (with its buffer) was selected, exported and saved.…”