Distribution and habitat of the Golden Eagle (Aquila chrysaetos) in Sonora, Mexico, 1892-2019

Distribución y hábitat del águila real (Aquila chrysaetos) en Sonora, Mexico, 1892-2019

GOEA were broadly distributed across the Sky Islands region of northeast Sonora, in areas near the northern terminus of the Sierra Madre Occidental in eastern Sonora, and in portions of the borderlands of north-central and northwestern Sonora (Fig. 2). Fewer observations were from central Sonora where they were concentrated in 2 mountain ranges (Aguaje, Libre) near the central coast. Very few observations were from southern Sonora with none on the southwestern coastal plain. Most observations were from the vast Yaqui watershed (33.3%) with some in the Concepción, Sonoyta, Sonora, and Gila watersheds (12.0-13.9%), and fewer from 4 other watersheds and coastal regions (Fig. 3). In the Yaqui watershed, the majority of observations were from the largely temperate Bavispe sub-basin (65.7%) compared to the more sub-tropical Moctezuma (14.3%), lower Yaqui (11.4%), and Aros (8.6%) sub-basins. Observations ranged in elevation from 2 to 2,450 m asl, averaged 1,164 ± 68 (± SE) m overall, and peaked between 1,250 and 1,500 m. There were also numerous observations along the coast and from low deserts below 250 m, and around tall peaks > 2,250 m (Fig. 1).

Most observations were from grassland and shrubland vegetation formations (34.7%), with somewhat fewer in temperate woodland and forest (31.6%) and desert-scrub (30.6%). Among communities, most observations were from semi-desert or plains grassland (34.3%), Madrean evergreen woodland and forest (31.3%), and Sonoran desert-scrub (28.3%). Within the Sonoran Desert, most records were from in the Lower Colorado River Valley subdivision with fewer in the Central Gulf Coast (21.4%), Plains of Sonora (17.9%), and Arizona Upland (10.7%) subdivisions. Observations in Chihuahuan desert-scrub, tropical deciduous forest, and foothills thorn-scrub were rare (2.0%; Fig. 3).

 

Few observations were from the 1890s (3.9%), 1930s (1.9%), 50s (2.9%), and 70s (2.9%), with progressively more from the 80s (6.8%), 90s (13.6%), and 2000-09 (30.1%), and 2010-19 (37.9%). Despite being less numerous, observations from the historical period (n = 30) were distributed broadly (Fig. 4). In comparison, recent observations spanned all general areas of historical occurrence with some exceptions. These exceptions were all in central Sonora and included 2 in the central plains, 1 in the Sierra de Aconchi, and 1 on Isla Tiburón. Recent observations also included many new regions and mountain ranges such as the Sierra el Aguaje in west-central Sonora, various desert mountain ranges in north-central and northwest Sonora, numerous Sky Island mountain ranges in the northeast, near Yécora in east-central Sonora, and near Álamos in the extreme southeast.

Discussion

We documented Golden Eagles (GOEA) across much of northern Sonora and amalgamated over 5 times the number of observations reported by past accounts of the species in Sonora. Our results indicate GOEA are more broadly distributed in the Sky Islands region of north-central and northeast Sonora than previously known.

Moreover, we described the first observations of this species in and around numerous desert mountain ranges in northwest Sonora as far west as the hyper-arid Pinacate Biosphere Reserve, and more locally in portions of coastal west-central Sonora. Presence of nests and pairs in some of these areas in western Sonora combined with observations of adults throughout the breeding season, suggest GOEA breed in these arid environments. Such patterns contrast with past distributional accounts in western Sonora (Howell & Webb, 1995; Russell & Monson, 1998), but conform to observed patterns in adjacent Arizona (Corman & Wise-Gervais, 2005) and the Baja California peninsula in Mexico (de León-Girón et al., 2016). As in the northern Sonoran Desert (Millsap, 1981), however, we suspect GOEA do not nest every year in the more arid portions of their range in Sonora where nesting may depend on sufficient levels of annual primary productivity.

Assuming frequency of observation is positively associated with current densities, our results suggest GOEA are more abundant at moderate to high elevations in the Sky Islands region than in the western deserts. Such patterns may be driven by more variable and less predictable prey availability in more arid regions, and by limited nest-site availability in some areas where mountains with sheltered cliff faces are absent or rare. Although there are few estimates of home range size in arid southwestern North America, available data and known nearest-neighbor distances among nests suggest much lower densities in arid regions (Braham et al., 2015; Millsap, 1981) than in more mesic northern or high-elevation regions (Marzluff et al., 1997; Kochert et al., 2002), which is also suggested by our results and those from Arizona (Corman & Wise-Gervais, 2005).

In addition to observations across much of northern Sonora and more locally in central Sonora, we also found evidence of recent occupancy in the extreme southeast near Álamos. In one instance, this included an adult observed in late April, which is typically after migrants and winter residents have departed for their northern breeding grounds. Data reported here and elsewhere (Kochert et al., 2002; Rodríguez-Estrella, 2002), however, indicate GOEA typically avoid densely forested environments with structure similar to that of tropical deciduous forest, which dominate lowlands and foothills in southeastern Sonora. Increasing coverage of man-made vegetation clearings in tropical deciduous forest (Calderón-Aguilera et al., 2012) combined with extreme freezing events that disturbed vast areas of sub-tropical vegetation in February 2011 and 2013 (Bojórquez et al., 2019), may have augmented habitat area in this region beyond that already present locally in open pre-montane woodlands and shrublands at moderate elevations. More study in southeast Sonora is needed to better understand the status of GOEA in this region.

Populations of GOEA across the western USA seem to be largely stable but there are some suggestions of small declines in southwestern regions (Millsap et al., 2013; Nielson et al., 2014). To the south in Mexico where resident populations are smaller, there is little evidence of recent population declines but data scarcity and lack of systematic monitoring limit our understanding of population trends (de León-Girón et al., 2016; Nocedal & Zúñiga-Fuentes, 2012). In Sonora, we found evidence of recent occupation of nearly all general regions of historical occurrence suggesting, at least at broad scales, that distribution has been largely stable. Although such patterns suggest high levels of persistence and population stability, more rigorous monitoring at smaller scales is needed to understand population dynamics. Areas of historical occurrence where we found no evidence of recent occupancy have been little surveyed in recent decades (e.g., northern Plains of Sonora), and GOEA may still occur in these and other regions. Presence of a large number of new localities and regions of occupation that we report is likely due to limited past survey effort combined with extremely low natural densities of this species in arid regions of southwestern North America (Braham et al., 2015; Millsap, 1981).

Throughout their range, GOEA use a wide variety of open and semi-open environments but generally avoid extensive areas of forest and highly modified environments such as agricultural and urban areas (Johnsgard, 1990; Kochert et al., 2002; Marzluff et al., 1997). Such environments provide habitat for leporids, sciurids, and other preferred prey, and sufficient open space to foster hunting (Watson, 2010). Given the natural rarity of safe nesting sites in many of these open environments, GOEA are frequently observed near cliffs and rocky outcrops even those embedded in wooded or forested landscapes (Kochert et al., 2002; Marzluff et al., 1997; Watson, 2010). Data we report for Sonora match these patterns of broad-scale habitat use. For example, we found GOEA were more frequently observed in semi-desert and Plains grasslands despite relatively low spatial coverage of these vegetation communities on the landscape (Brown, 1982). Secondarily, GOEA were also frequently observed in Madrean evergreen woodland and forest, but at these higher elevations individuals were often detected at either relatively low (e.g., 1,000-1,500 m) or high elevations around tall peaks. Such areas typically support more open oak woodland and savannah where grasses and scattered shrubs predominate, which best foster hunting, and rocky outcrops and cliffs, which foster nesting. Finally, relatively high frequency of observation in Sonoran desert-scrub was weighted heavily toward more open vegetation of the Lower Colorado River Valley subdivision despite higher aridity, and least toward denser more mesic thorn-scrub like Arizona Uplands. In general, Sonora supports vast expanses of relatively open arid and semi-arid environments that in many cases seem to function as habitat for this threatened species in Mexico.

In Sonora, GOEA are present throughout the year with clear peaks in frequency of observation during both the wintering and breeding seasons. As in neighboring Baja California, Mexico (Rodríguez-Estrella et al., 1991), observations from Sonora suggest presence of both a non-resident migratory population in fall and winter, and a resident breeding population that nests in late winter, spring, and early summer. Presence of each of these population segments during overlapping time periods complicated our evaluations of status and breeding distribution somewhat. In many cases, individuals we presumed were winter residents or migrants may have been resident breeders, especially those detected in February in the western deserts where nesting begins earlier (ADF personal observation; Corman & Wise-Gervais, 2005; Millsap, 1981). Similarly, some birds we presumed were breeders may have been non-breeders, especially immature individuals. Long-distance migration by juvenile GOEA can entail progressively greater movements from natal areas during hatch years, and broad exploratory movements on wintering grounds where individuals may remain for more than 12 months before moving north to breed (Murphy et al., 2017; McIntyre et al., 2008; Soutullo et al., 2006). These immature individuals may use large areas of Sonora that overlap the home ranges of 1 or more breeding pairs before they move north to breed.

Developing optimal conservation and management strategies for GOEA in Sonora necessitates detailed knowledge of their ecology and primary threats. With regard to habitat use, our results suggest conservation strategies for GOEA in Sonora should focus specifically on protecting and restoring native grassland, open temperate woodland, shrubland, and desert-scrub, and cliffs and other suitable nest structures within these and other nearby vegetation communities. Given high frequency of occurrence and low landscape coverage, native semi-desert and Plains grasslands seem especially important to this species in Sonora. Grassland communities and associated bird species, however, are highly threatened in North America with alarming patterns of degradation and loss in northwest Mexico (Pool et al., 2014) and in the adjacent USA and Canada (Rosenberg et al., 2019) in recent decades. To address such threats, active measures to conserve and restore grasslands, reduce shrub encroachment, and discourage overgrazing and persecution of important prey species such as prairie dogs (Cynomys spp.) are needed.

In addition to habitat loss and degradation, a broad suite of direct threats to GOAE exist in North America (Katzner, Smith et al. 2012; Kochert & Steenhof, 2002), and many of these threats are highly relevant in Sonora and elsewhere in northern Mexico. Carcasses laced with poisons targeting animals that are perceived to be predators of livestock are serious threats to wild felids, bears, and other native mammals in northwest Mexico (Delfín-Alfonso et al., 2012). Such poisons are known to kill GOEA in the western USA (Kochert & Steenhof, 2002), constitute major threats to GOEA in Sonora, and must be controlled and regulated before the benefits of habitat conservation and restoration can be fully realized. Similarly, incidental captures in leg-hold traps and snares set for mammals can be a significant source of mortality, as can lead poisoning from ingestion of ammunition in carrion in regions where hunting is common (Katzner, Smith et al., 2012; Kramer & Redig, 1997). Electrocution from power poles is a major cause of raptor mortality in the western USA, especially for large species such as GOEA, which are more likely to make contact with energized wires and components (Harness & Wilson, 2001; Kochert & Steenhof, 2002). Efforts to retrofit power poles by increasing separation among components, insulating components, and adding materials to discourage perching on poles are essential to mitigate these threats, but must be installed carefully (Dwyer et al., 2017). Industrial-scale production of wind energy also poses significant threats to GOEA in western North America (Kochert & Steenhof, 2002). Although such facilities are not yet present in Sonora (Comisión de Energía del Estado de Sonora, 2019), careful planning and design combined with detailed information on travel routes and flight behavior of migratory and resident eagles (e.g., Bedrosian et al., 2018; Katzner, Brandes et al., 2012) should help mitigate these threats in the future. Finally, educational programs that highlight the importance of raptors in controlling small mammal populations and that correct the perception that eagles pose major threats to livestock should be developed to target landowners, cowboys, youth, and other groups. Tailoring conservation, mitigation, and educational measures to local contexts, cultures, and threats are essential for conservation of these majestic birds of prey.

Acknowledgements

To the many landowners and managers that facilitated field work including M. Mackenzie, J. L. Romero for helping in the Sierra Huachinera, Consejo Nacional de Ciencia y Tecnología for granting L. Armenta’s doctoral studies, G. Carreon of Naturalia and M. Gómez Ramírez of Northern Jaguar Project for pictures from the Northern Jaguar Reserve, and Northern Jaguar Project and Asociación Conservación del Norte for access and support of monitoring on the Northern Jaguar Reserve. We also thank Victor Hugo Cabrera and José Roberto García Martínez for pictures from Mesa Tres Ríos, S. Winckler for a photo from near Álamos, and Janitzio Égido-Villarreal for support. Finally, ADF thanks D. Ellis for sharing his insights into GOEA ecology in the region.

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