Population and Distribution of Swainson’s Hawks (Buteo swainsoni) in California’s Great Valley: A Framework for Long-Term Monitoring

Battistone, Carie; Furnas, Brett J.; Anderson, Richard L.; Dinsdale, Julie L.; Cripe, Kristi M.; Estep, James A.; Chun, Calvin S. Y.; Torres, Steven G.

doi:10.3356/jrr-18-34

Cited by 6 publications

(3 citation statements)

References 25 publications

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“…In California Swainson's Hawks appear to be diverging in behavior from populations elsewhere (Airola et al 2019, Battistone et al 2019. These be-havioral changes coincided with significant land-use change in the primary breeding range, which could have driven the species' adaptation to agriculture and human proximity.…”

Section: Discussionmentioning

confidence: 98%

“…In 1983, the species was accorded "threatened" status under the California Endangered Species Act (California Natural Diversity Database 2020). Since the early 2000s, the number of Swainson's Hawks breeding in California has increased, especially in the middle latitudes of the Central Valley where 95% of the state's population currently nests (Battistone et al 2019) (Phillips et al 2014;Allen Fish pers. comm.…”

Section: Discussionmentioning

confidence: 99%

“…Possible reasons include the hawk population in the Central Valley reaching carrying capacity, detrimental land-use changes, or both. Since 1979, the California Department of Fish and Wildlife has conducted three statewide surveys of this species (CDFW 2016), the most recent in 2005 and 2006 (Battistone et al 2019). Together they indicate that the Central Valley population has grown, but differences in the surveys' methods and intensity make establishing a trend difficult (CDFW 2016).…”

Section: Discussionmentioning

confidence: 99%

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2022

WBirds

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In the mid-20th century California's breeding population of Swainson's Hawk (Buteo swainsoni) decreased precipitously, and its range contracted. But for reasons not yet clear, since the 1990s the population has increased and the birds have begun to recolonize areas from which they had long been extirpated. Among those areas are the Coast Ranges of central California, in Santa Clara and San Benito counties. In this issue of Western Birds, Stephanie Klein, Larry Baer, and Ryan A. Phillips report their monitoring of up to five pairs nesting in this area in 2019 and 2020. Notably, they found the hawks foraging in farmland even more than in grassland, while avoiding urban development. Merlin Parental Behavior and Chick Development as Revealed by Webcam

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Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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2022

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Rapid growth of the Swainson’s Hawk population in California since 2005

Furnas

Wright

Tennant

et al. 2022

Ornithological Applications

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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.

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A Population Bottleneck Did Not Affect Polymorphism Rates in California Swainson's Hawks

Briggs

Wommack

Sawtelle

et al. 2023

Journal of Raptor Research

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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.

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Population and Distribution of Swainson’s Hawks (Buteo swainsoni) in California’s Great Valley: A Framework for Long-Term Monitoring

Cited by 6 publications

References 25 publications

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Rapid growth of the Swainson’s Hawk population in California since 2005

A Population Bottleneck Did Not Affect Polymorphism Rates in California Swainson's Hawks

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