444 A. T. Marques et al.


use is dominated by agricultural areas (c. 40%), mainly permanent crops such as olive groves or vineyards and open areas occupied by non-permanent crops and pastures (c. 47% of the agricultural areas). Forests (c. 20%) aremainly silvo-pastoral sparse woodlands dominated by holm oak Quercus rotundifolia or cork oak Quercus suber (INE, 2011; DGT, 2014). The climate is typically Meso-Mediterranean and Thermo-Mediterranean, characterized by hot, dry and long summers and mild, wet winters (Rivas-Martínex et al., 2002).


Alentejo harbours the entire Portuguese breeding popu-


lation of the great bustard (c. 1,150 birds; Pinto et al., 2005) and 90–95% of the little bustard’s breeding population (c. 8,900 males; Silva et al., 2018). The main breeding areas of both species are inside Special Protected Areas (Fig. 1a,b), but key areas used during the post-breeding sea- son and in winter are not protected (Silva & Pinto, 2006; Silva et al., 2007, 2014). The Special Protected Area status requires the implementation of management actions that include the mitigation of bird collision risk, specifically through the installation of wire-marking devices in new power lines. The total length of the transmission power line network


in the region is 1,239 km (Fig. 1b), primarily 150 and 400 kV power lines. Wires are marked in some sections, particularly inside Special Protected Areas. Wire-marking devices are generally installed on both earth wires, assembled in an al- ternated way, to produce a visual spacing effect, in profile, of 1.5–5 m. The majority of marking devices are large (30 cm diameter) white and red/orange spirals, but smaller (10 cm diameter) grey or white and red/orange spirals and rotating flappers are also used.


Methods


Data compilation We compiled data from the nine studies on bird collisions with transmission power lines conducted in Alentejo during 2003–2015, all promoted by Redes Energéticas Nacionais, the company managing the Portuguese transmission net- work. These studies included a national assessment of bird mortality at transmission power lines, two studies fo- cusing on wire-marking effectiveness, performed by non- governmental organizations and by the public admin- istration, and six monitoring programmes of single power lines carried out within the scope of an environmental impact assessment processes (Supplementary Table 1). Atotal of 280 kmof power lines, c. 23%of the transmission


grid in the region, were systematically surveyed at least once per season during the course of these studies. Sampling effort was uneven across studies (study duration was 12–67 months and intervals between sampling were 15–90 days;


Supplementary Table 1). Collision data also varied in spatial and temporal detail, with the lowest resolution data being the number of carcasses per 2 km power line section. Be- cause of this variation in sampling effort and data reso- lution across studies, we divided the total sampled power line into 144 sections of c. 2 km each (mean 1,946 ± SD 3.19 m; range 755–2,767 m) and used the presence/absence of mortality of each species per section in our analysis (except for the temporal analysis, where we used all available data; see next section). None of the studies reported mal- functioning of marking devices, such as devices fallen off the wire, flapper twist or colour changes.


Seasonal and spatial patterns of collision


We compiled the locations of power line sections with re- ported bustard mortality events for each species and esti- mated seasonal patterns, separately for power line sections inside and outside the Special Protected Areas network. The birds congregate in the protected areas during the breeding season, so we expected collision patterns to vary over time. Only studies with regular sampling at intervals of 15 or 30 days reported bustard fatalities with detailed temporal data, and therefore the seasonal data presented here result from a constant annual survey effort.


Collision risk modelling


To identify the main drivers of bustard collisions with power lines we selected variables representing habitat availability in the area surrounding the power lines, power line config- uration, wire marking and survey effort (Table 1). Great and little bustard are grassland species with strict habitat re- quirements, typically occurring in open and flat or gently undulating landscapes (BirdLife International, 2017). Both are short distance migrants, performing seasonal move- ments of up to 260 km (great bustard)/300 km (little bustard). The majority of birds (65% in the great bustard, 89–96% in the little bustard) move from their breeding grounds to areas with higher food availability during late spring and summer (Rocha, 2006; García de la Morena et al., 2015; Palacín et al., 2017; Alonso et al., 2019). Collisions are more frequent during the post-breeding and wintering seasons (Silva et al., 2014; Palacín et al., 2017), when bustards make most migratory movements. Collision events could thus be associated with the land- cover types crossed during migrations, in addition to the ha- bitats the birds use when not migrating. We extracted land- cover information from the 2007 Carta de Ocupação do Solo (DGT, 2007) and combined different categories into three broad classes, which were probably stable during the study period: open farmland, forest and agroforestry (Table 1).We used the dominant land-cover type (in terms of total area) in


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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