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Bank Swallow (Riparia riparia)
Bank Swallow
(Riparia riparia)
Legal Status
State: Threatened
Federal: Bureau of Land Management
Sensitive
Photo courtesy of Jack
Bartholmai.
Critical Habitat: N/A
Recovery Planning: Recovery Plan: Bank
Swallow (Riparia riparia) (CDFG 1992)
Notes: Listing status not expected to change during permit period.
Taxonomy
The bank swallow (Riparia riparia) has long been recognized as a
distinct species. There are five widely recognized subspecies, but only
one (R. r. riparia) occurs in North America (Clements et al. 2009), and
no information has been presented suggesting that the California
birds are taxonomically distinct. Physical characteristics of the species
are detailed by Garrison (1999).
Distribution
General
The bank swallow is one of the most widely distributed birds in the
world, and is probably the most common of all swallows (Garrison
1998, 1999). California is at the southwestern extent of the species’
main breeding range in North America. Approximately 70% of
California's bank swallow population breeds along the Sacramento
and Feather rivers (Garrison et al. 1989; CDFG 1992). However, the
Sacramento-Feather rivers population continues to decline, with 50%
of the breeding population occurring within only eight colonies at the
conclusion of surveys in 2007 (Garcia et al. 2008).
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Bank Swallow (Riparia riparia)
Distribution and Occurrences within the Plan Area
Historical
Early records of bank swallow in the Desert Renewable Energy
Conservation Plan (DRECP) Area are summarized by Grinnell and
Miller (1944, p. 275). Records include a location near Big Pine on the
Owens River in 1893 (the location is imprecise, but if in the Plan Area,
it is at the northern edge) and a location at the southern end of the
Salton Sea (no date given) in Imperial County. Grinnell and Miller
(1944) add that there was no record of this species “from the
southeastern deserts south of Owens Valley” except as noted above.
More recent, but still historical occurrences (i.e., pre-1990), or
occurrences with unknown observation date, are noted in Figure SPB3. These include a total of 13 records in the following areas: north of
Hesperia, Edwards Air Force Base, east of Barstow along the Mojave
River, and west of Barstow near the town of Lockhart, evidently in
association with wetlands marginal to Harper Dry Lake (Dudek 2011).
Recent
There are 48 recent (i.e., since 1990) non-breeding bank swallow
occurrence records in the Plan Area (Figure SP-B3) (Dudek 2011).
Three of the recent locations are in the Barstow/Hesperia area and
the other location is near the City of Ridgecrest, evidently in
association with wastewater treatment ponds located north of the
city. Recent occurrences include: south and east of the Salton Sea,
north of Hesperia, Edwards Air Force Base, California City, the Baker
area, along Interstate 15 east of Fort Irwin, and north of Ridgecrest, as
well as several in the northern portion of the Plan Area in the Owens
Lake area, near Independence, and at Tinemaha Reservoir (Figure SPB3). These occurrences are located near wetlands and open water
that presumably provide suitable insect prey for food. There are no
breeding records for the Plan Area.
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Bank Swallow (Riparia riparia)
Natural History
Habitat Requirements
Breeding habitat for the bank swallow in California consists exclusively
of vertical banks or bluffs with friable soils suitable for burrow
excavation by the birds (CDFG 1992; Garrison 1998). The bluffs must
be at least 1.0 meter (3 feet) in height to provide protection from
predators. Garrison (1998) indicated that in various studies burrows
are generally at least 1.0 meter (3 feet) above the base of the bank and
0.7 meter (2.3 feet) below the top of the bank, with burrows in the
upper third of the bank. Nesting colonies in California have been
observed to occur along banks with an average height of 3.3 meters
(10.8 feet) (Humphrey and Garrison 1987, cited in Garrison 1998). The
bank must be susceptible to erosion of sufficient intensity to maintain a
near-vertical aspect with exposure of bare soils.
Nesting colonies can vary greatly in size, ranging from 10 to several
thousand burrows (CDFG 1992). Smaller colonies may be abandoned
or relocated after a few years, which has contributed to considerable
uncertainty regarding the species’ precise distribution in California
(Garrison 1998; Garcia et al. 2008). Most colonies reported in
California (e.g., by Garrison et al. 1989; CDFG 1992; Garrison 1998;
Garcia et al. 2008) are associated with streams and thus with riparian
vegetation types, and to a lesser degree, with lacustrine and coastal
habitats (Green 1999). This is largely because flowing water is the
most common agent of erosion that exposes and maintains the
vertical profile of the bank or bluff. However, many nesting sites
reported from outside California occur in road cuts or sand and gravel
mines (Garrison 1998). Further, there are notable exceptions to the
strong association of colonies with streams and riparian vegetation in
California. Garrison (1989) reported colonies in extreme northeastern
California that are located up to 4 kilometers (2.5 miles) from
perennial water. Also, Moffatt et al. (2005) found high reoccupancy
rates in colonies close to grassland habitat in the Sacramento-Feather
rivers area.
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Bank Swallow (Riparia riparia)
Foraging Requirements
Bank swallows forage almost exclusively while in flight. Most insects
taken by bank swallows are terrestrial and not dependent on surface
water (Garrison 1989). Aerial foraging occurs over lakes, streams,
meadows, fields, pastures, bogs, forests, and woodlands (CDFG 1992;
Garrison 1998). During breeding, foraging generally occurs within
200 meters (656 feet) of the colony, but may sometimes occur at
distances as great as 8 to 10 kilometers (5.0 to 6.2 miles) from the
colony (Mead 1979 and Turner 1980, cited in Garrison 1998).
Reproduction
Bank swallows are normally colony nesters and are more colonial than
other swallow species, with colonies having as many as 1,500 nesting
pairs (Table 1) (Garrison 1998). The swallows excavate burrows that
are typically 0.65 meter (2 feet) in length (range: 0.2 to 1.0 meter [0.7
to 3 feet]) in friable exposed banks of sand, gravel, or soil. Burrow
entrances measured in California have been found to be approximately
5.5 centimeters (2.2 inches) in height and 7.2 centimeters (2.8 inches)
in width (Garrison 1998). New burrows are typically excavated each
year, but sometimes existing burrows are reused.
Breeding
X
X
X
X
Migration
X
X
X
X
Other
X
X
X
X
________________
Notes: “Other,” Most bank swallows migrate to South America, but they
occasionally overwinter in Southern California.
Sources: Garrison 1999; Green 1999
Dec
Nov
Oct
Sep
Aug
July
June
May
April
March
Feb
Jan
Table 1. Key Seasonal Periods for Bank Swallow
X
Bank swallows are monogamous, although males often attempt extrapair copulation with fertile females. Pairing occurs either before or
soon after the birds arrive at the colony site. Clutches of between 2
and 7 eggs are laid and are incubated primarily by the female. The
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Bank Swallow (Riparia riparia)
incubation period lasts approximately 14 days and is remarkably
uniform across the range of the species (data presented by Garrison
1998). The young remain in the burrow for approximately 18 to 22
days before fledging. Within 10 days of fledging, juveniles are entirely
independent and typically leave the breeding colony to join flocks of
mixed juveniles and adults. These flocks usually forage in the general
vicinity of the colony and roost for the night in trees or on river bars
(Garrison 1998). Only one brood is raised per year (Garrison 1998).
Spatial Behavior
Bank swallows in California move at three spatial scales associated
with migration, breeding, and non-breeding activity (Table 2).
Bank swallows typically migrate in large flocks, often of several
thousand birds, and that normally include other swallow species such
as barn swallow (Hirundo rustica), cliff swallow (Petrochelidon
pyrrhonota), and tree swallow (Tachycineta bicolor) (Garrison 1999).
Most birds migrate to and from widely dispersed locations between
the extreme southern United States (a few birds overwinter in
Southern California) and northern South America (Garrison 1999).
Non-breeding movement is primarily observed among fledged
juveniles and post-breeding adults during the time between fledging
and migration. In California, this principally occurs during July and
August. During this time, birds typically remain near the nesting
colony but no longer use burrows, instead roosting at night in trees,
on sand bars in rivers, or at other sites relatively safe from predators.
Movements may occur up to several kilometers away from the colony
just prior to migration (Garrison 1989, 1998, 1999).
Movements by breeding pairs are largely confined to foraging within
200 meters (656 feet) of the colony. Courtship and mating behaviors
immediately prior to breeding also occur at or very near the colony.
Bank swallows are not territorial except in defense of the immediate
burrow (Garrison 1998, 1999).
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Bank Swallow (Riparia riparia)
Table 2. Spatial Movements by Bank Swallow
Type
Home
Range
Dispersal
Migration
Distance/Area
Within 200
meters (656 feet)
of colony
Within a few
kilometers of the
colony
100 to 10,000
kilometers (62 to
6,214 miles)
Location of Study
California
(Sacramento-Feather
rivers colonies)
Various
Citation
Garrison 1998
Various
Garrison 1999
Garrison 1998, 1999
Ecological Relationships
The principal ecological relationships relevant to bank swallows in
the Plan Area concern their dependence on suitable banks and bluffs
for nesting habitat, and on a sufficient supply of flying insects to
provide forage. Garrison (1998) determined that nesting habitat for
this species is essentially ephemeral: crumbling vertical banks of sand
or soil are intrinsically short-lived habitats that tend to undergo
substantial erosion at annual or more frequent intervals. As a result,
bank swallows depend on access to a variety of potential colony sites
within any given area, and may only use a fraction of the potential
sites in any breeding season (40% to 60% of the available sites in the
Sacramento River area, as estimated by Garrison [1998]). Bank
swallows thus depend on the persistence of bank erosion processes.
Based on work in the Sacramento River area, patch size is highly
variable, with colonies occurring on banks ranging in length from 10
to 2,000 meters (33 to 6,562 feet) and in height from 0.5 to 20 meters
(1.6 to 66 feet), with larger colonies occurring on larger patches and
with a greater tendency for larger colonies than small to persist from
year to year (Garrison 1998).
Bank swallows are subject to predation of eggs and young by snakes,
birds, and mammals that may enter burrows. Of these, snakes seem to
be the principal predators. Fledged birds and adults are subject to
predation by hawks and other birds (Garrison 1998).
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Bank Swallow (Riparia riparia)
Population Status and Trends
Global: Stable (NatureServe 2010)
State: Vulnerable to imperiled
Within Plan Area: Same as state
The bank swallow was once considered a common species in California
(CDFG 1992), but it has disappeared as a breeding bird in Southern
California and has exhibited recent fluctuating population levels in
central California (CDFG 2004). Studies in the 1980s and 1990s of
several Sacramento River colonies, where the largest breeding
populations occur (about 50% of the state total), estimated a high of
12,348 pairs in 1986 to a low of 7,525 pairs in 1991. In 1992, the
number of pairs was estimated to be about 8,550 pairs (CDFG 1992).
Recent analyses using both metapopulation models (Moffatt et al.
2005) and field surveys (Garcia et al. 2008) indicate ongoing decline
of the Sacramento-Feather rivers population, which comprises over
70% of the statewide population.
Threats and Environmental Stressors
Breeding habitat loss due to riprap bank protection of natural stream
banks is the main human-caused threat to the bank swallow in
California (CDFG 2004; Garrison 1998; Garcia et al. 2008). Bank
swallows, however, are relatively insensitive to moderate levels of
human disturbance. Colonies occur, for instance, in road cuts and
along banks beneath agricultural fields. However, bank undercutting
due to boat wakes or fluctuating reservoir levels can cause collapse of
banks used by colonies (Garrison 1998). Garrison (1998) also
identified habitat fragmentation as a concern.
Conservation and Management Activities
Various conservation activities address the bank swallow in the
Sacramento-Feather rivers area (Bank Swallow Technical Advisory
Committee 2011). The state recovery plan for the species (CDFG
1992) notes that the overall goal of the plan is “the maintenance of a
self-sustaining wild population.” The plan also notes that ongoing
river bank protection projects represent the single greatest threat to
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Bank Swallow (Riparia riparia)
bank swallow populations in California. Consequently, the recovery
plan identifies the following measures as necessary to maintain the
continued viability of the species:

Creation of artificial river banks

Long-term protection, enhancement, and maintenance of
natural habitat, particularly along the Sacramento River

Development of set-back levees

Impact avoidance

Creation of habitat preserves.
No ongoing conservation and management activities for bank
swallow, or plans that include this species, have been identified in the
Plan Area.
Data Characterization
The bank swallow is an exceptionally well-studied bird species at a
local scale (e.g., Garcia et al. 2008; Garcia 2009). Studies of the species
in California, especially since the 1970s, have focused almost
exclusively on the main populations in the Sacramento-Feather rivers
area. Very little is known about the distribution of the species
elsewhere in California. Targeted studies would be needed to identify
isolated nesting colonies. Such studies have either not been done or
have not been done recently enough to provide a clear understanding
of the species' distribution within the Plan Area.
Management and Monitoring Considerations
Garrison (1998) describes how efforts to create bank swallow habitat
in California have failed, largely because the sites became eroded and
vegetated after a very few years, and it would have required very high
maintenance expenditures to avoid this outcome. It has become clear
that management for bank swallows requires preserving and
maintaining the geomorphic processes associated with creation of
steep, non-vegetated, short-lived banks that create suitable sites for
colonies, and maintaining such sites in sufficient density and proximity
to allow frequent relocation of colonies. Within the Plan Area, it is also
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Bank Swallow (Riparia riparia)
reasonable to expect that management near suitable breeding habitat
must maintain hydrological conditions conducive to wetland and
riparian settings, which produce large numbers of flying insects.
Predicted Species Distribution in Plan Area
There are 1,053,575 acres of modeled suitable habitat in the Plan
Area. Suitable habitat occurs in the High Desert Plains and Hills,
Mojave Valley-Granite Mountains, Owens Valley, and Silurian ValleyDevil’s Playground ecoregion subsections from 1,300 to 4,300 feet.
Suitable habitat includes areas near perennial streams and rivers,
including the Mojave River and Owens River, perennial lakes and
ponds, playas, swamps/marshes, freshwater emergent wetlands, and
riverine wetlands. Modeled suitable habitat also includes areas with
appropriate riparian vegetation, including Sonoran-Coloradan semidesert wash woodland scrub, Southwestern North American
riparian, flooded and swamp forest/scrubland, and Southwestern
North American introduced riparian scrub. Appendix C includes
specific model parameters and a figure showing the modeled
suitable habitat in the Plan Area.
Literature Cited
Bank Swallow Technical Advisory Committee. 2011. “Bank Swallow
Portal.” Accessed April 22, 2011. http://www.sacramentoriver.org/
bankswallow/index.php.
CDFG (California Department of Fish and Game). 1992. Recovery Plan:
Bank Swallow (Riparia riparia). Nongame Bird and Mammal
Section Report 93.02. Prepared by Nongame Bird and Mammal
Wildlife Management Division. December 1992.
CDFG. 2004. California Rare & Endangered Birds. Sacramento,
California: CDFG.
Clements, J.F., T.S. Schulenberg, M.J. Iliff, B.L. Sullivan, and C.L. Wood.
2009. The Clements Checklist of Birds of the World. Version 6.5.
Edited by F. Gill and D. Donsker. Updated December 2010.
http://www.birds.cornell.edu/clementschecklist/downloadabl
e-clements-checklist.
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Dudek. 2011. “Species Occurrences–Riparia riparia.” DRECP Species
Occurrence Database. Updated November 2011.
Garcia, D. 2009. “Spatial and Temporal Patterns of the Bank Swallow
on the Sacramento River.” MS thesis; California State
University, Chico.
Garcia, D., R. Schlorff, and J. Silveira. 2008. "Bank Swallows on the
Sacramento River, a 10-year Update on Populations and
Conservation Status.” Central Valley Bird Club Bulletin 11(1):1–12.
Garrison, B.A. 1989. Habitat Suitability Index Models: Bank Swallow.
Sacramento, California: U.S. Fish and Wildlife Service, Division
of Ecological Services. September.
Garrison, B.A. 1998. “Bank Swallow (Riparia riparia).” In The Riparian
Bird Conservation Plan: A Strategy for Reversing the Decline of
Riparian-Associated Birds in California. California Partners in
Flight. Accessed April 21, 2011. http://www.prbo.org/
calpif/htmldocs/species/riparian/bank_swallow_acct2.html.
Garrison, B.A. 1999. "Bank Swallow (Riparia riparia).” The Birds of
North America Online. Edited by A. Poole. Ithaca, New York:
Cornell Lab of Ornithology. Accessed April 25, 2011.
http://bna.birds.cornell.edu/bna/species/414/articles/
introduction.
Garrison, B.A., R.W. Schlorff, J.M. Humphrey, S.A. Laymon, and F.J.
Michny. 1989. “Population Trends and Management of the
Bank Swallow (Riparia riparia) on the Sacramento River,
California.” In Proceedings of the California Riparian Systems
Conference: Protection, Management, and Restoration for the
1990s. D. L. Abell (technical coordinator), 267–271. Berkeley,
California: USDA, Forest Service, Pacific SW Forest and Range
Experiment Station. Gen. Tech. Rep. PSW-110.
Green, M. 1999. “Life History Account for the Bank Swallow.” Updated
by CWHR Program Staff, September 1999. California Wildlife
Habitat Relationships System, California Department of Fish
and Game, California Interagency Wildlife Task Group.
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Accessed April 20, 2011. https://nrmsecure.dfg.ca.gov/
FileHandler.ashx?DocumentVersionID=17515.
Grinnell, J., and A.H. Miller. 1944. The Distribution of the Birds of
California. Pacific Coast Avifauna, no. 27. Berkeley, California:
Cooper Ornithological Club.
Moffatt, K.C., E.E. Crone, K.D. Holl, R.W. Schlorff, and B.A. Garrison.
2005. “Importance of Hydrologic and Landscape
Heterogeneity for Restoring Bank Swallow (Riparia riparia)
Colonies along the Sacramento River, California.” Restoration
Ecology 13(2):391–402.
NatureServe. 2010. “Riparia riparia.” NatureServe Explorer: An Online
Encyclopedia of Life. Version 7.1. February 2, 2009. Data last
updated August 2010. Accessed April 20, 2011.
http://www.natureserve.org/explorer/servlet/
NatureServe?searchName=riparia+riparia.
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