Figure 1 - uploaded by Brett Furnas
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Depiction of the study area in California (Great Valley) and the areas within the known range but outside the study area. Taken as a whole, this was the known Swainson's Hawk breeding distribution in California at the initiation of this 2005 and 2006 survey effort.
Contexts in source publication
Context 1
... Area. The study area encompassed the known distribution of breeding Swainson's Hawks in the Great Valley, as determined from prior data and expert opinion, generally corresponding to the low-elevation areas along the eastern and western edges of the Great Valley, north to Red Bluff and south to the Tehachapi Range (Fig. 1). We developed the study area based on the results of historic and recent studies, all known nesting observations recorded in the CDFW database, and consultation with Swainson's Hawk experts from the CDFW and Technical Advisory ...
Context 2
... Area. The study area encompassed the known distribution of breeding Swainson's Hawks in the Great Valley, as determined from prior data and expert opinion, generally corresponding to the low-elevation areas along the eastern and western edges of the Great Valley, north to Red Bluff and south to the Tehachapi Range (Fig. 1). We developed the study area based on the results of historic and recent studies, all known nesting observations recorded in the CDFW database, and consultation with Swainson's Hawk experts from the CDFW and Technical Advisory ...
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Citations
... A few dark morphs surviving the bottleneck by chance could explain why we see relatively more dark morph Swainson's Hawks in western North America compared to other buteo species (Wheeler et al. 2003) despite sympatric breeding. In addition, much of California's breeding population may have some genetic isolation from other Swainson's Hawk populations likely as a result of low natal dispersal (Houston 2005, Briggs et al. 2012) and high breeding site fidelity (Woodbridge et al. 1995), potentially resulting in differences in migratory behavior and morphology (Hull et al. 2007, Airola et al. 2019, Battistone et al. 2019). This isolation could provide a test of stability in the polymorphism because the population might be relatively unaffected by introgression of genes from surrounding populations. ...
... (R Core Team 2021), and used AIC to evaluate model performance (Anderson and Burnham 2004). Clines in morph distribution across latitude or longitude could skew results because of range shifts between present and historical populations, specifically smaller populations of breeding Swainson's Hawks in southern California (Bloom 1980, Battistone et al. 2019). Specifically, we used quadratic terms of latitude, longitude, and an interaction with a binary variable (''time'': current vs. historical) to examine differences in any potential clines between the periods using melanin score as the response variable. ...
... The limited change in polymorphism distribution across time lends support to the idea that environmental factors may be maintaining this polymorphism (Amar et al. 2019). Therefore, it seems less likely that the high percentage of dark morphs observed in California is the result of the population bottleneck and subsequent partial recovery of western Swainson's Hawks (Battistone et al. 2019). However, the existence of a stable polymorphism suggests there may either be differential selection pressures between morphs among different years or habitats within or around California. ...
Raptor polymorphisms have played an important role in understanding how evolutionary forces within and between populations operate. However, within a population little is known about the history of the polymorphism, the effects of any population bottlenecks, and the overall stability of the polymorphism. We investigated the stability of a melanin-based color polymorphism in Swainson's Hawks (Buteo swainsoni) in California over a 100-year period. In the mid-20th century Swainson's Hawks in California declined by 90%. Therefore, we examined the morphs of breeding individuals collected before 1950 and compared them to images from eBird taken between 2008–2019 and from a Google Images search, because a disproportionate survival of rare alleles after a population bottleneck could lead to changes in current clines. Between the two periods, we did not find differences in plumage morphs, nor did we find evidence of latitude or longitudinal clines over this relatively small spatial scale. Thus, despite a large population decline, this polymorphism has remained consistent over time. Our results suggest that the relatively high occurrence of dark morphs in this population is not simply a result of a bottleneck.
... In California, they nest in valleys, lowlands, and foothills from sea level on the coast to 2,100 m elevation in the Sierra Nevada Mountains (Bloom 1980). On their breeding range the species prefers native grasslands, riparian forests, and diverse agricultural landscapes, avoids orchards and vineyards, and requires non-agricultural trees for nesting (Woodbridge 1991;Battistone et al. 2019). Early accounts and museum records suggest they were widely distributed and abundant in the state before the modern era (Sharp 1902), but declines were observed by the middle of the last century (Grinnell and Miller 1944). ...
... Habitat loss from urban encroachment (e.g., residential, industrial) and certain forms of agriculture were probable causes of the species' decline in some areas of California (Bloom 1980). Recent conservation concern for the species in the state has focused on urban and energy development and orchard and vineyard cultivation which are not considered suitable for nesting (CDFW 2016;Battistone et al. 2019;CDC 2019). Great Basin juniper invasions, thought to result from grazing and fire suppression (Burkhardt and Tisdale 1976), likely reduced nesting and foraging habitat in northeast California (Bloom 1980). ...
... A variety of Swainson's Hawk surveys have occurred throughout California over the past two decades (Briggs et al. 2011;Estep and Dinsdale 2012;Gifford et al. 2012), but they were mostly localized within specific areas of the state, or they were not focused on population estimation. To remedy the lack of robust, comprehensive population estimates, the California Department of Fish and Wildlife (CDFW) coordinated breeding surveys throughout much of the state during 2005(Battistone et al. 2019) and again in 2016 and 2018. Surveys from these four years included measures of detection probability that allow a robust comparison of abundances across years (Pollock et al. 2002). ...
By 1979 Swainson’s Hawks (Buteo swainsoni) had declined to as low as 375 breeding pairs throughout their summer range in California. Shortly thereafter the species was listed as threatened in the state. To evaluate the hawk’s population trend since then, we analyzed data from 1,038 locations surveyed throughout California in either 2005, 2006, 2016, or 2018. We estimated a total statewide population of 18,810 breeding pairs (95% CI: 11,353–37,228) in 2018, and found that alfalfa (Medicago sativa, lucerne) cultivation, agricultural crop diversity, and the occurrence of non-agricultural trees for nesting were positively associated with hawk density. We also concluded that California’s Swainson’s Hawk summering population grew rapidly between 2005 and 2018 at a rate of 13.9% per year (95% CI: 7.8–19.2%). Despite strong evidence that the species has rebounded overall in California, Swainson’s Hawks remain largely extirpated from Southern California where they were historically common. Further, we note that the increase in Swainson’s Hawks has been coincident with expanded orchard and vineyard cultivation which is not considered suitable for nesting. Therefore, we recommend more frequent, improved surveys to monitor the stability of the species’ potential recovery and to better understand the causes. Our results are consistent with increasing raptor populations in North America and Europe that contrast with overall global declines especially in the tropics.
... Several regional surveys conducted in the Central Valley that identified over 1,600 Swainson's Hawk nest sites since 1987 from all regions of the Central Valley (Estep 1989(Estep , 2007(Estep , 2017(Estep , 2020Jones & Stokes Associates 1990;Estep and Dinsdale 2012;Gifford et al. 2012;ICF 2020) reported no use of utility or other artificial structures. Lack of pole or tower use reported in these studies suggests that use of human structures is a recent phenomenon in California that may be driven in part by the increasing Swainson's Hawk nesting population (Battistone et al. 2019, Furnas et al. 2022). ...
