This study represents the first detailed analysis of the recent population decline of the Canarian houbara bustard in Fuerteventura, the island that once represented the main stronghold of this endangered subspecies. Our study is relevant because this decline could be the first step of an irreversible extinction process of houbara bustards in the Canary archipelago. To quantify the decline we compared the numbers of males counted in the island at the beginning and end of the study period (respectively, 1998 and 2021). Counts of displaying males have been used in similar species to estimate population trends because they are highly visible and faithful to their display sites during the mating season, whereas females are much more criptic, mobile and elusive (e.g., Mahood et al. 2020; Bretagnolle et al. 2022). We found that male numbers decreased by a 58% over the last twenty years in Fuerteventura, and that 65% of the display sites identified in 1998 have been abandoned. We also investigated the relative importance of possible causes of population decline by comparing their incidence at all display sites known at the beginning of the study period, and analysing which of these sites disappeared and which remain occupied nowadays. Among all predictor variables analysed, two appeared to be of particular relevance for the observed population reduction, namely NDVI and density of power lines.
As in numerous previous studies (Pettorelli et al. 2006; Wiegand, 2008; Álvarez-Martínez et al. 2015; Weber et al. 2018), habitat quality was estimated using NDVI, an index that represents plant productivity and is a good indicator of food availability in a species that feeds primarily on plants, and whose chicks need sufficient abundance of invertebrates to grow and survive (Collins 1993; Medina 1999; Martín and Lorenzo 2001). NDVI measures the difference in the reflection of near-infrared and visible light, providing an indicator of vegetation vigor and density. This non-invasive and efficient method allows researchers to assess large-scale habitat quality, aiding in understanding animal behavior, population dynamics and the impact of environmental changes on food availability (Zlinszky et al. 2015; Boult et al. 2018). Interestingly, previous studies also show that although male territories generally include enough vegetation where males can feed or hide, the amount of food resources is not greater at display sites than elsewhere in the vicinity (Abril-Colón et al. 2022b; Ucero et al. 2023). Moreover, food abundance may critically affect the breeding success of females, who must raise their chicks in places with sufficient food. Males of this species aggregate on exploded leks (Collins 1984; Hellmich 2003; Hingrat et al. 2004, 2008; Hingrat and Saint Jalme 2005), and sites with more abundant resources could serve to attract females (Ucero et al. 2023). This would explain why display sites that have remained occupied show both, higher NDVI and higher density of houbaras than sites that have been abandoned. Such a process has been described for great bustards at the scale of their whole distribution in the Iberian Peninsula (Álvarez-Martínez et al. 2015).
The significant differences shown between initial and final NDVI of occupied and abandoned display sites indicate that those display sites that remain occupied after the 23-years study period, had already higher NDVI values at the beginning of that period. These results suggest that, two decades ago, currently occupied areas had already better habitat conditions than currently abandoned areas, and thus a higher carrying capacity. This also can help explaining the variation in the abundance of other steppe bird species and density of ravens at display sites occupied across the study period. In 1998 there was already a marginally significant trend towards a higher density of steppe birds around houbara display sites that two decades later remain occupied. In 2021, the differences between occupied and abandoned sites are more pronounced, suggesting that the extinction process is related to the local conditions of each site. In only two decades, differences in habitat quality related to food resource availability have increased steppe bird abundance at display sites occupied today, and this higher density of birds would have attracted ravens. All these results support our interpretation that differences in habitat quality between areas have been a major factor determining the permanence or abandonment of houbara territories in Fuerteventura.
The process that has led to the abandonment of breeding sites in areas with poor habitat may have been aggravated by the aridification of the landscape in Fuerteventura in recent years (Martin et al. 2013, 2015; Suárez and Jaén 2016). In fact, a further increase in the duration and severity of droughts in the Canary Islands is expected in the coming decades (De Castro 2005; Carrillo 2023; IPCC 2023), which could accelerate the current extinction of houbaras. Studies on the island of Lanzarote show that one third of houbaras migrate in summer to irrigated farmland, where they find food resources absent in many breeding areas at that time of year, which appears to be critical for survival (Abril-Colón et al. 2022a,b). Rising temperatures and decreasing rainfall in recent years only exacerbate this lack of food resources in summer. In Fuerteventura, many houbaras select as feeding sites the gavias, traditional cultivation fields designed to retain rain and runoff water, which may be considered equivalent to modern irrigation systems (Medina, 1999). However, agriculture on this island has suffered a progressive abandonment since the middle of the last century (90% of the 7456 ha of cultivated fields abandoned between 1960 and 1982; González-Morales 1989). Many abandoned gavias still retain rainwater and show relatively abundant herbaceous and shrub cover (Launaea arborescens), which has probably helped to maintain a few houbara groups during some decades (Emmerson et al. 1989, 1990, 1992; Martín et al. 1997; Hellmich 1998; Medina 1999). Today, however, with the continued lack of maintenance, gavias have started to break down and lost their capacity to retain water, and therefore have less vegetation cover; in other cases, natural herbaceous or shrub vegettation has been removed. This has decreased substantially the available food for houbaras, as reflected in the poor NDVI values recorded in abandoned display sites.
As for human infrastructure, overhead power lines appear to be the main cause of abandonment detected in the analyses. Certain large-sized birds, such as cranes and bustards, are particularly vulnerable to collision with power lines (Janss and Ferrer 2000; Jenkins et al. 2011; Barrientos et al. 2012; Raab et al. 2014; Vadász and Lóránt 2015; Bernardino et al. 2018; Marcelino et al. 2018; Uddin et al. 2021; Silva et al. 2023), as they have lower manoeuvrability in flight and reduced frontal visibility compared to other birds (Martin and Shaw 2010; Martin 2011). In species of the family Otididae, collision with power lines represents the main cause of anthropogenic mortality (Alonso et al. 1994; Silva et al. 2010, 2023; Jenkins et al. 2011; Burnside et al. 2015; Collar et al. 2017; Palacín et al. 2017; Marcelino et al. 2018; Shaw 2018; Marques et al. 2021; Uddin et al. 2021). Power lines have been cited as a major cause of mortality also in the houbara bustard (Lorenzo 1995; Lorenzo et al. 1997; Lorenzo and Ginovés 2007; García del Rey and Rodríguez-Lorenzo 2011; GREFA 2016; Gómez-Catasús et al. 2020), and a recent study with marked birds has confirmed the importance of power line casualties in this species (Alonso et al. 2022b), suggesting that this factor is affecting the demography of the Canarian houbaras, especially in Fuerteventura, where juvenile productivity is very low (Alonso et al. 2022c). There are studies showing how the presence of power lines can lead to the extinction of local populations of other bustard species. A Great Bustard lek with 15 males disappeared in a few years in central Spain due to the high mortality caused by a single power line that crossed the main display area (Alonso et al. 2003), and in Portugal, Little Bustard populations may be displaced by the construction of power lines, which acting in synergy with other habitat degradation factors, can ultimately cause local extinctions (Silva et al. 2010).
In addition to power lines, roads were also retained as a significant factor determining display site abandonment in some of the plausible models developed. Previous authors had already warned of roadkill mortality in Canarian houbaras (Tejera et al. 2018), and our study with marked birds definitely confirmed the importance of this mortality factor (Alonso et al. 2022b). Roadkills added to power line casualties might have well contributed to the disappearance of many display sites over the last years in Fuerteventura, particularly where habitat has been already deteriorated. With regard to buildings and urban nuclei, our study detected the displacement of four display sites from their initial locations in 1998 following the construction of new, or expansion of nearby urban areas. Previous habitat selection studies found that human settlements affect houbara bustard distribution in northern Africa and the Canary archipelago (Le Cuziat et al. 2005; Carrascal et al. 2006, 2008; Hingrat et al. 2008; Chammem et al. 2012; Schuster et al. 2012) and that males select display sites far from this infrastructure (Ucero et al. 2023).
Finally, although there has been a notable decrease in the number of houbara birds over the study period in practically all areas of the island, the three areas showing the lowest declines were the Tindaya plains, the Corralejo Natural Park and Matas Blancas. These three areas already had a protected status at the beginning of the study period (SPA and/or Natural Park; Table S2). Although the conservation authorities probably decided to protect those areas of the island that already had a higher number of houbara bustards and other steppe birds more than two decades ago (see Table S2), our results show that over the study period houbara numbers still benefitted from that protection status. In fact, it has already been demonstrated in numerous studies that the protection of an area contributes to the conservation of its biodiversity (Naughton-Treves et al. 2005; Saout et al, 2013).
In sum, our study shows that the main causes of the houbara bustard decline in Fuerteventura have been poor habitat quality at many areas on the one hand, and the construction of various human infrastructure on the other. In many areas, habitat quality could have deteriorated in recent years by the extreme aridification of the terrain in the eastern Canary Islands because of recent climatic events linked to global change, and especially in Fuerteventura, the most arid of the whole archipelago. This has undoubtedly contributed to severely reduce breeding productivity in the island. As for the effect of infrastructures, the negative effects of power lines stand out, increasing adult mortality in an already decimated houbara bustard population.
Conservation implications
The houbara bustard is currently on the brink of extinction in Fuerteventura. Any further decline is extremely dangerous, as the current population is close to the minimum viability size. In the neighbour island of Lanzarote, where houbara densities are higher (Alonso et al. 2020), display sites abandoned due to the death of a male are immediately occupied by another male (Alonso et al. 2022a), however this is not the case in Fuerteventura where numbers are already too low. To ensure houbara survival, the following conservation measures aimed at improving habitat quality and reducing anthropogenic mortality should be urgently implemented. First, an appropriate cover of Launaea arborescens shrubland needs to be restored in many areas of the island where the breeding habitat has been degraded over many decades. Concurrently, many other areas, at serious risk of degradation, need to be maintained in order to ensure a sufficient habitat quality for houbara bustards to breed successfully. Second, the damaged gavias should be restored, and a supply of alfalfa plots should be considered in some areas to increase food availability especially in summer, the most critical season for the survival of Canarian houbaras. Third, the construction of new power lines should be avoided, and at least the most dangerous existing power and telephone lines should be buried to stop collision fatalities. Planning of new energy infrastructure should take into account the home range and flight movements of houbara bustards, both inside and outside SPAs, as recommended in other bustards species (Palacín et al. 2023). Fourth, traffic speed limitations should be implemented at some sectors of roads crossing the most important breeding aggregations of the species in the island. Fifth, the creation of new roads and tracks should be limited and restrictions of vehicle traffic on certain tracks during the breeding season should be considered. Finally, the proliferation of scattered buildings in areas with houbaras should be prohibited and urban sprawl controlled. This would prevent further habitat loss and degradation through the construction of infrastructure and minimise disturbance to the birds during the breeding period. As an additional tool to control the effectiveness of these measures, surveillance should be increased in all houbara regions of the island, especially in protected areas, and surveys of the breeding population and juvenile productivity should be carried out each year to monitor the demographic trends of this endangered population.