Federal: Birds of Conservation Concern (BCC) for U.S. Fish and Wildlife Service (USFWS) Region 1; protected under the Migratory Bird Treaty Act.

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1 Legal Status State: Threatened Federal: Birds of Conservation Concern (BCC) for U.S. Fish and Wildlife Service (USFWS) Region 1; protected under the Migratory Bird Treaty Act. Critical Habitat: No critical habitat has been designated for this species. Recovery Planning: There is no existing recovery plan for this species. Notes: No changes to status proposed or anticipated during the permit term. Taxonomy The Swainson s hawk is monotypic and there are no currently accepted subspecies. The Swainson s hawk is most closely related to the Galapagos hawk (B. galapagoensis) and the Hawaiian Hawk (B. solitaries) (Bechard et al. 2010). Distribution General Historically, Swainson s hawks nested in the San Joaquin Valley and populations were once estimated at 17,000 breeding pairs as recently as the early 1900s but surveys conducted during the 1970s estimated that only 110 breeding pairs of Swainson s hawks remained throughout the entire Central Valley (Sousa 2010). Breeding populations in California currently occur in four locations, the Central Valley, the Great Basin, Owens Valley, and Antelope Valley. Approximately 95% of Swainson s hawks in California are found in the Central Valley (an inventory prepared by Anderson et al. [2007], estimated 1,912 pairs in 2005 and 2,251 pairs in 2006). The majority of these birds are found in the Central Valley between approximately Butte County on the north and Merced County on the south, where foraging and nesting habitat conditions are optimal. More than 60% of the statewide population occurs within Yolo, Sacramento, Solano, and San Joaquin Counties (Anderson et al. 2007). 1

2 Distribution and Occurrences within the Study Area Historical Swainson s Hawk Based on a search of the California Natural Diversity Database (CNDDB), there are no historical Swainson s hawk occurrences (prior to 1990) within the permit area or within the larger study area (California Department of Fish and Game 2012). Recent There is one recent California Natural Diversity Database (CNDDB) occurrence (from 1994) for two adults observed along the north side of the Kern River 5.5 miles northeast of the junction of Interstate 5 and Highway 119, west of Bakersfield within the Permit Area. The landowner associated with this occurrence is unknown. There are four additional recent CNDDB occurrences reported within the study area, three from 1994 and one from 2004 (California Department of Fish and Game 2012). Natural History Habitat Requirements Breeding Swainson s hawks typically nest in scattered trees within grassland, shrubland, or agricultural landscapes (e.g., along stream courses or in open woodlands). Today, in California s Central Valley, nests are typically at the edges of narrow bands of riparian vegetation, in isolated oak woodland, and in lone trees, roadside trees, or farmyard trees, as well as in adjacent residential areas. When coexisting with redtailed hawks (Buteo jamaicensis), Swainson s hawks use smaller trees in smaller clumps than do red-tailed hawks (Bechard et al. 2010). Foraging Historically and in existing native habitat, Swainson s hawk forage in open stands of grass-dominated vegetation, sparse shrublands, and small, open woodlands. In many parts of range today, the species has adapted well to foraging in agricultural areas (especially in alfalfa), but cannot forage in most perennial crops or in annual crops that grow higher than native grasses, where prey are more difficult to find. In the Central Valley, Swainson s hawk forage in row, grain, and hay crop agriculture, particularly during and after harvest, when prey are both numerous and conspicuous. Swainson s hawks also forage in areas with cultivation activities that expose prey (e.g., flood irrigation, primarily in alfalfa fields; burning; and disking). In northeastern California, 42.5% of habitat within foraging radius of 12 radio-marked individuals was in active agriculture (Bechard et al. 2010). 2

3 Swainson s hawks are known to forage within a 10-mile radius of nest sites, suggesting that the presence of suitable foraging habitat within the vicinity of nesting habitat is essential to their reproductive success (California Department of Fish and Game 1994). Main foods taken on breeding grounds are vertebrates (mammals, birds, and reptiles). Major rodent prey during breeding season include ground squirrels, pocket gophers, voles (Microtus spp.), and deer mice (Peromyscus spp.). On wintering grounds, apparently eats exclusively insects such as grasshoppers (Dichroplus spp.), dragonflies (Aeshna bonariensi), butterflies and moths (Lepidoptera), and leaf beetles (Coleoptera: Chrysomelidae) found abundantly in alfalfa fields and crops such as sunflowers and corn (Bechard et al. 2010). Table 1. Habitat Associations for Swainson s Hawk Land Cover Type Land Cover Use Habitat Parameters Supporting Information Agriculture Foraging and nesting Adapted to foraging in agricultural fields, but not in orchards and vineyards. Nests in isolated trees. Direct observations Riparian woodland Nesting Nest in riparian trees along major drainages as well as agriculture canals. Direct observations Grasslands Foraging and nesting Forages in open landscapes with low and/or widely spaced vegetation. Nests in isolated trees Direct observations Saltbush scrub and Valley sink scrub Foraging and nesting Forages in open landscapes with low and/or widely spaced vegetation. Nests in isolated trees Direct observations Urban Foraging and nesting Forages in open greenspaces in urban areas, light-density oil fields, and vacant lots. Nests in isolated trees. Direct observations Reproduction Swainson s hawks arrive on the breeding grounds in March April (March in the Central Valley, California) (Table 2) and begin a week-long nest building phase 1 to 2 weeks after arrival. The egg-laying through fledging period lasts on average 73 days per nest, but can last 110 days. Adjacent pairs can be out of sync by 25 days. Average clutch size is 2 or 3 eggs and an average of about 2 young are fledged per successful nest, although this value can drop to an average of less than 1 during years of low prey availability (i.e., not all nests are successful). Young generally fledge mid-july to mid-august at an average age of 43 days (Bechard et al. 2010). 3

4 Table 2. Key Seasonal Periods for Swainson s Hawks Jan Feb Mar April May June July Aug Sep Oct Nov Dec Breeding Migration Wintering Source: Bechard et al Note: Central Valley (California) Swainson s hawks arrive a month earlier on breeding grounds than other populations, possibly because they winter in central Mexico rather than Argentina. Movement Spatial behaviors by Swainson s hawk include migration patterns, breeding home range use, and natal dispersal. Migratory movements occur annually between North American breeding grounds and wintering areas primarily located in South America, although some Swainson s hawks use wintering grounds in California and Mexico. Immature birds and postbreeding adults begin forming migration flocks in August and September, and begin the fall migration in September. Birds migrating to South America leave North America by October and arrive in Argentina in November. The return migration begins late-february and early March in Argentina, with birds arriving in California from early March (Central Valley) through April (other California populations). As reported in Bechard et al. (2010), Fuller et al. tracked 27 Swainson s hawks on their 1996 and 1997 southbound migrations and recorded a mean cumulative travel distance of over 13,500 km (8,370 miles) (Bechard et al. 2010). Movements within home ranges include movement between nesting territories and surrounding foraging habitats. Home range sizes vary greatly among breeding adults though have been observed to span as far as 30 km from a given nest site. Generally, larger home ranges are found in areas with crop types unsuitable for foraging (mature grains, row crops, orchards, and vineyards) and the smallest home ranges are reported at nest sites near alfalfa, fallow fields, and dry pasture. Additionally, home ranges have been found to become larger seasonally, when crops mature and prey are able to more adequately hide from predators (Bechard et al. 2010). Natal dispersal also varies greatly among populations. Central California hawks disperse only a few kilometers (mean of 3.5 km [Estep 1989]), while northeastern California hawks disperse farther (mean of 9 km). But in greater contrast, juvenile Swainson s hawks in Saskatchewan apparently disperse to distances exceeding 310 km (Bechard et al. 2010). 4

5 Table 3. Movement Distances for Swainson s Hawks Type Distance/Area Location of Study Citation Home Range 69 8,718 ha Washington, Oregon Bechard et al Dispersal Range km California Bechard et al Migration Mean of 13,504 km southward, 11,592 km northward United States Bechard et al Note: Home range depends on habitat type. ha = hectare. km = kilometer. Ecological Relationships Predator prey relationships are critical for Swainson s hawk. Reduction of rodent populations due to conversion of native grassland to cropland has resulted in declines of Swainson s hawks in some locations in North America, especially Central California (Risebrough et al. 1989). Also, because of their dependence on insect prey, especially grasshoppers on the wintering grounds, Swainson s hawks are highly susceptible to secondary poisoning from insecticides (Bechard et al. 2010). Swainson s hawks occasionally lose nestlings or fledglings to great horned owl (Bubo virginianus) predation, and Swainson s hawks themselves have preyed on western burrowing owl (Athene cunicularia) fledglings. Interspecific competition and territoriality occurs between Swainson s hawk and sympatric buteos (e.g., red-tailed hawks [Buteo jamaicensis]) over control of nest sites, although Swainson s hawks appear to dominate in most such encounters (Bechard et al. 2010). Population Status and Trends Global: Secure (NatureServe 2011) State: Declining (California Department of Fish and Game 2005) Within the Study Area: Same as State In California the Swainson s hawk is vulnerable to extirpation due to its very restricted range (primarily the Central Valley), few populations, steep population declines, and loss of habitat. By 1980, it was estimated that the California Swainson s hawk population had declined 90% since 1900 (Bechard et al. 2010). Based on inventories conducted between 1979 and 2006, the number of estimated pairs has increased by as much as 81% (from 400 pairs in 1979 to 2,081 pairs in 2006). The 2006 estimated number of pairs was derived from an inventory of Swainson s hawk, initiated by the California Department of Fish and Game (DFG), that surveyed for breeding pairs in California in 2005 and The estimate represents the 5

6 average observed between the 2005 and 2006 results (95% confidence interval of 1,770 to 2,393 pairs). Based on this inventory, approximately 94% of the breeding pairs now occur in the Central Valley (Anderson et al. 2007). As discussed above under the section titled Distribution and Occurrence within the Permit Area, there is only one reported occurrence for this species in the Permit Area and four additional occurrences within the larger study area. Threats and Environmental Stressors The decline of Swainson s hawks in California has been attributed to riparian habitat loss from agricultural activities in the Central Valley, urbanization in the coastal valleys, and a contracting range of Joshua trees and riparian habitats in the Mojave Desert. It was estimated that by the mid-1980s, approximately 93% of riparian habitat in the San Joaquin Valley and 73% of riparian habitat in the Sacramento Valley had been lost since the 1850s. Chronic and acute pesticide poisoning also affects the Swainson s hawk. Pesticide use on South American wintering grounds threatens all North American populations. South American birds have died from ingesting pesticides targeting grasshoppers. In 1996, an estimated 4,100 Swainson s hawks died from acute pesticide poisoning in Argentina (Bechard et al. 2010). Wildfires, declining water tables, and flood control practices also continue to threaten riparian and woodland nesting habitat in California. Off-road vehicle activity and shooting can disrupt nesting, although the latter is not as important a factor as it once was. Intraspecific competition or aggression with other raptors and common ravens (Corvus corax) has been suggested as a stressor elsewhere in the western United States. Additionally, in a study that looked at survivorship of 538 banded Swainson s hawks, 26% of these birds were killed from collisions with anthropogenic structures (cars, trains, fences, powerlines, and other unidentified sources) (Bechard et al. 2010). Conservation and Management Activities A 1994 DFG staff report recommended mitigating for impacts to Swainson s hawk foraging habitat within a 10-mile radius of an active nest site; the 10-mile radius reflects the flight distance between an active nest and foraging habitat (California Department of Fish and Game 1994). Though this guidance is more than 10 years old, it is still used as a reference for creating appropriate mitigation plans for this species. In general, proposed conservation measures, including habitat conservation plans, usually focus on retention of some portion of existing foraging and nesting habitats while allowing other areas to be converted to non-habitat uses (Bechard et al. 2010). Other important conservation considerations are the avoidance of disturbance of breeding pairs and destruction of nest trees. The Swainson s Hawk Technical 6

7 Advisory Committee (2000) developed a set of survey recommendations to maximize the potential for locating nesting Swainson s hawks, and thus reducing the potential for nest failures as a result of project activities/disturbances. The combination of conducting appropriate surveys, analyzing risk, and monitoring nests has been determined to be very effective in reducing the potential for projectinduced nest failures (Swainson s Hawk Technical Advisory Committee 2000). Due to past occurrences (in the 1990 s) of large-scale mortalities of Swainson s hawks reportedly attributed to the use of insecticides, alternative, less toxic pesticides and grasshopper baits should be tested, though research in this area has not currently been formally undertaken (Bechard et al. 2010). Data Characterization The major data gap with respect to this species is the minimal data available on the status of previously discovered nests. Management and Monitoring Considerations Management and monitoring considerations for habitat include maintaining suitable nesting habitat and reliable food sources within at least 10 miles of the these areas. Other management and monitoring considerations for conserving this species are avoidance of impacts on breeding pairs and active nests and monitoring the success of breeding pairs. As described above in Conservation and Management Activities, DFG (1994) and the Swainson s Hawk Technical Advisory Committee (2000) have developed methods for surveying suitable habitats for nests and developing mitigation plans for unavoidable impacts on habitat. These resources continue to be used to effectively avoid and minimize impacts on the species. As conversion of agricultural areas to commercial and residential development continues, impacts on Swainson s hawk populations should be monitored to determine population trends. Predicted Species Distribution in the Study Area Model Description Foraging Habitat Swainson's Hawks occupy large home ranges that are primarily composed of foraging habitats. The amount and intensity of land uses within home ranges are the primary factors determining suitable foraging habitat. Under natural conditions, 7

8 Swainson's Hawks likely foraged in upland grassland and shrub habitats and seasonally flooded perennial grasslands. These habitats are largely extirpated from the Central Valley today, replaced by annual grasslands with low prey populations, and agricultural crops. Swainson's hawks now rely heavily on areas in agricultural production for foraging habitat, with frequent shifts in agricultural practices and habitat quality. However, various crop types support different levels of prey abundance, and that the timing of tilling and harvest activities strongly affects the availability of prey within each crop type. Fallow fields, dry land pasture, and irrigated pasture are also important foraging habitats. Vineyards and mature orchards support low prey populations and have excessive vegetation density and are not used as foraging habitat (Woodbridge 1998). Model Results Figure D-3 shows the modeled foraging habitat for Swainson s hawk within the Plan Area and the Study Area. Swainson s hawks typically nest in scattered trees within grassland, shrubland, or agricultural landscapes (e.g., along stream courses or in open woodlands). Today, in California s Central Valley, nests are typically at the edges of narrow bands of riparian vegetation, in isolated oak woodland, and in lone trees, roadside trees, or farmyard trees, as well as in adjacent residential areas. Because isolated trees could not be delineated at the scale of the model figures, nesting habitat was not modeled and it is assumed that trees of suitable size that occur throughout the Plan Area and Study Area could provide nesting habitat. Literature Cited Anderson, D., R. Anderson, M. Bradbury, C. Chun, J. Dinsdale, J. Estep, K. Fien, and R. Schlorff Final Report for the California Swainson s Hawk Inventory: P Sacramento, CA: U.C. Davis Wildlife Health Center and California Department of Fish and Game Resource Assessment Program. Bechard, M. J., C. S. Houston, J. H. Sarasola and A. S. England Swainson's Hawk. In A. Poole (ed.), The Birds of North America Online. Ithaca: Cornell Lab of Ornithology. Available: < Accessed: August 15, California Department of Fish and Game Staff Report Regarding Mitigation for Impacts to Swainson s Hawk (Buteo swainsonii) in the Central Valley California. Sacramento, CA The Status of Rare, Threatened, or Endangered Plants and Animals of California, Sacramento, CA. 8

9 California Natural Diversity Database, RareFind 3, Version Updated July Swainson s Hawk. Sacramento, CA. NatureServe NatureServe Explorer: An Online Encyclopedia of Life [web application]. (Version 7.1.) NatureServe, Arlington, Virginia. Available: < Accessed: April 19, Risebrough, Robert W., Ronald W. Schlorff, Peter H. Bloom, and Edward E. Littrell Investigations of the Decline of Swainson s Hawk Populations in California. The Journal of Raptor Research 23(3): Sousa, C. L Monitoring Swainson s Hawks, Buteo swainsoni, and other Treenesting Raptors on Properties within and near the Los Baños Wildlife Area Complex, Prepared for Resource Assessment Program California Department of Fish and Game, Los Baños, CA. Swainson s Hawk Technical Advisory Committee Recommended Timing and Methodology for Swainson s Hawk Nesting Surveys in California s Central Valley. Sacramento, CA. Woodbridge, B Swainson's Hawk. In The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. Accessed March 6,

10 B a k e r s f i e l d C o n s e r v a t i o n P l a n W hi t e R i v e r K:\Projects_1\City_of_Bakersfield\00036_12_BakersfieldHCP\mapdoc\Species_Model_Figures\D_3_swainsons_hawk_figure.mxd hw 2/1/2013 M O N T E R E Y S A N L U I S O B I S P O S a n River / Creek J ua n Cr e e k Major Roadway County Boundary HCP Study Area HCP Permit Area Swainson's Hawk CNDDB Occurrence (Presumed Extant) Foraging Habitat Source: City of Bakersfield 2012; CNDDB Miles UV 58 K I N G S 5 T E M B L O R R A N G E C A L I E N T E R A N G E UV 33 Lost Hills S A N T A B A R B A R A S I E R R A M A D R E R A N G E Cuyama Riv er Buttonwillow UV 43 UV 119 UV 99 Taft P o s o C Wasco V E N T U R A r e e k Delano Shafter Bakersfield S A N E M I G D I O M O U N T A I N S Oildale K E R N T U L A R E K E R N Lamont Arvin P o s o C r e e k UV 58 K er n R i v e r Bear Valley Springs S I E R R A V A D A N E UV 178 T E H A C H A P I M O U N T A I N S Figure D-3 Swainson's Hawk Modeled Habitat