448 A. T. Marques et al.
For all models, there was no significant autocorrelation in model residuals (Supplementary Fig. 6).
Discussion
Spatio-temporal collision patterns Both bustard species exhibited clustered spatial patterns of collision with power lines. These patterns are probably related to the species’ distribution and abundance across the region, mainly determined by the availability of open habitats in flat areas with lower levels of human disturbance (e.g. Pinto et al., 2005; Silva et al., 2007; Equipa Atlas, 2008; Moreira et al., 2012). The influence of spatial distribution was also evident when comparing collision patterns: al- though there were high risk areas where collisions with power lines were recorded for both species, little bustard fatalities were spread more widely throughout the study area, and there were some localities where only collisions of little bustards were recorded. This could be explained by the larger and more widespread population of little bus- tard (Fig. 1a). Fatalities of both species were recorded inside and outside Special Protected Areas. Temporal patterns in collisions may be explained by dif-
ferences in flocking behaviour, activity and flight patterns. In the case of the little bustard, the peak of mortality dur- ing the post-breeding season is probably related to (1) the migratory movements that some individuals perform from the breeding grounds to areas with higher food resources (Silva et al., 2007; García de la Morena et al., 2015) and the use of stop-over sites in areas with poor habitat conditions during such periods (Alonso et al., 2019), (2) the increased distance travelled daily searching for food (Silva et al., 2014), (3) the higher frequency of flights at heights with an in- creased collision risk (Silva et al., 2014), and (4) the species’ gregarious behaviour during this season. The collision pat- tern of the little bustard also varied across space: there was (1) a high number of collisions outside Special Protected Areas during summer (July–September), when birds leave their breeding grounds (Silva et al., 2007), (2) a peak of mortality during the autumn (October–December) within Special Protected Areas, probably associated with the return to the breeding grounds after the onset of rains (García de la Morena et al., 2015), and (3) a third mortality peak during the beginning of the breeding season inside Special Pro- tected Areas, which coincides with the onset of the mating period (March–May) when males settle in breeding sites and females move between lekking areas. Although the great bustard’s annual distribution in Alen-
tejo is not well known, there is evidence that it is influ- enced by the birds’ behaviour and local movements, as in the little bustard. The majority of fatalities occurred in late summer and autumn (September–November), when a large number of birds leave their breeding grounds in search
of areas with higher food availability and individuals tend to gather in larger flocks (Rocha, 2006). This temporal pat- tern also matches the higher frequency of flights crossing transmission power lines observed by Marques et al. (2007).
Factors influencing collision risk
The proportion of open farmland in the surrounding area was the main factor influencing collision risk in both spe- cies, suggesting that the presence of power lines within bus- tard habitat is themajor collision driver. Collision events are less frequent during migratory journeys, when birds cross other habitat types, probably flying high over power line wires. The threshold for the proportion of open farmland that correlated with an increased collision risk differed be- tween species: little bustard collisions weremore likely when .20% of the surrounding area was open habitat, whereas this threshold was 50% for the great bustard. This may reflect differences in habitat requirements between the species, with the great bustard requiring more open areas (Suárez-Seoane et al., 2002). The variable expressing the proportion of available habitat at a larger landscape scale (5 km buffer) was more important than the dominant habitat in the close vicinity of the power line (1 km buffer), although the latter was also relevant for the little bustard. The configuration of the power line also influenced col-
lision risk. For the little bustard, the large configuration with four levels of wires and a bigger collision risk area (larger distance between top and bottom wires), posed a higher risk than the small and medium configurations, both with just two levels of wires. Also, the comparison between the two horizontal configurations (with similar distance be- tween top and bottom wires) showed that higher lines pose a greater risk to little bustards than lower ones. Although the effect of power line height on collisions is strongly dependent on flight altitude and may be species- specific, higher power lines and the vertical configuration generally pose a higher collision risk, as they are a larger barrier to birds in flight, which tend to gain altitude to fly over the obstacle rather than passing below (Luzenski et al., 2016; Murphy et al., 2016; Bernardino et al., 2018). This behaviour increases the likelihood of collision with the earth wires, which are thinner and less visible than the phase conductors (Bernardino et al., 2018). Although power line configuration has been suggested as a factor determin- ing collision risk in several previous studies, our findings present the first robust evidence of this effect. For the great bustard, the large configuration also ap-
peared to be riskier than the small one, although the differ- ence was not significant. However, the three configurations were not evenly represented in this species’ range within the study area, and our dataset may thus not be adequate to test the effect of different power line configurations on great bustard collisions. Most transmission lines crossing
Oryx, 2021, 55(3), 442–451 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319000292
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