California Partners In Flight Riparian Bird Conservation Plan
Tricolored Blackbird(Agelaius tricolor)
Photo by James Gallagher, Sea and Sage Audubon
Prepared by William J. Hamilton III (firstname.lastname@example.org)
Department of Environmental Science and Policy
1 Shields Avenue, Davis, CA 95616
Hamilton, W. J. 2004. Tricolored Blackbird (Agelaius tricolor). In The Riparian Bird Conservation Plan:a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/riparian_v-2.html
There are no identified subspecies. The southern California population should be evaluated for genetic distinctness.
California species of Special Concern.
HISTORICAL BREEDING DISTRIBUTION
For historical references, see Heermann, A. L. 1853, 1859; Baird, S. F. 1870; Bendire, C. E.1895; Dawson, W. L. 1923; Grinnell, J. 1898; Malliard, J. 1914; Neff, J. A. 1937, 1942.
CURRENT BREEDING DISTRIBUTION
The California Department of Fish and Game (CDFG) maintains a biodiversity database for tricolors. This database includes records for breeding and non-breeding tricolors during the breeding season and a winter distribution database. The recent breeding records were compiled by U C Davis and are included in annual reports to USFWS and CDFG. Since 1980, breeding has occurred in 46 California counties (Beedy and Hamilton 1999). With the exception of a few peripheral sites the geographic distribution has not declined perceptively and extension of breeding colonies into Washington and British Columbia are presumably range extensions.
Outside the survey area breeding occurrence is reported in isolated locations in S. Oregon, Washington, and W. Nevada and in Central and W. Baja California Norte south to El Rosario.
Determination of breeding by tricolors is simplified by their colonial behavior and the conspicuousness of both sexes transporting food to nestlings and fledglings. However, fledglings disperse to several km from breeding colonies and continue to be provisioned by parents. Diachronic observations will keep observers from making errors, but amateur observers are seldom sufficiently persistent in their observations of colonies to make reliable estimates of the size, fate and even location of colonies.
AVERAGE TERRITORY SIZE
Often stated to be nonterritorial but both males and females maintain breeding territories held for the duration of a single nesting effort. Territories are compressed and include only nesting space, not foraging areas. The size of territories ranges from less than one to as much as 10 several meters. Unlike most species when tricolors settle at high densities as in flooded willows territories may be vertically stacked.
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Arrival date on breeding grounds:
Mid-March through mid-July, later arrivals presumably individuals that bred elsewhere at more southerly locations earlier in the season (Hamilton 1998). Individuals move north after first nesting efforts. Comparable movements not reported in southern California. As for other Northern Hemisphere passerines the initiation of spring breeding is beginning increasingly earlier.
Breeding season movements:
About 50 days following initial nesting efforts in Sacramento County and San Joaquin Valley, colonies settle in Sacramento Valley during late May through early June (Beedy and Hamilton 1999), presumably following attempted or successful breeding elsewhere earlier.
Departure date from breeding grounds:
Late June through August forms large post breeding roosts in Sacramento Valley.
Spring migration period:
Vacates wintering areas to arrive at breeding locations in Sacramento County and throughout San Joaquin Valley in early March through early April (DeHaven 1975b). In Baja California and S. California, colonies establish April through May.
Fall migration period:
Large foraging flocks occur in S. Solano and Marin Counties by late October (Beedy and Hamilton 1999).
Withdraws from Washington, Oregon and Nevada as well as from Baja California. Also withdraws from coastal regions of Santa Barbara and San Diego Counties and portions of N. California outside of the Central Valley. Wintering populations concentrate in Sacramento-San Joaquin Delta and the central coast, particularly of Monterey, Marin, Sonoma and Santa Cruz counties. Winter distribution summarized by Beedy and Hamilton (1999) including historical references is not in close agreement with a summary of Christmas Bird Count data compiled recently by Kevin Hunting (CDFG). This discrepancy emphasizes the need for extensive study of the distribution and abundance of this species throughout the winter. (See Hunting, CDFG winter distribution map). Winter distribution needs further study.
Wintering tricolors are associated with open rangeland in the Sacramento-San Joaquin Delta and along the central California Coast. Diaries are attractive. During Mid-October flocks to 15,000 frequent dairies on the Point Reyes Peninsula in Marin County. In November large foraging flocks frequent the Sacramento-San Joaquin Delta region.
MIGRATION STOPOVER CHARACTERISTICS
Non-breeding dispersal centers during migration between nesting sites may be mistaken for colony settlement, however no singing or carrying of nest material occurs.
In spring large flocks are associated with dairies and ripening grain heads (milk stage). Flocks as large as 100,000 individuals may gather in March in the general vicinity of potential nesting sites, then fragment to nesting colonies. Loss of any of these fragments leads to resettlement so that it is easy to overestimate how many birds are settling or have settled in an area. For this reason exact estimates of nonbreeding flocks contribute little to estimates of overall abundance of the species.
The migration to the rice districts follows flooding of the rice ground. At
this time before nesting begins roosting is in cattail marshes. At some locations
movement into rice-associated colonies is directly into nesting colonies and
nest building may begin the day males and females arrive.
No specific routes, but demonstrates a specific movement schedule and pattern of areas colonized according to the California ephemeral spring climate, food and water availability. Shows annual site fidelity (Beedy and Hamilton 1997), somewhat the result of adequate site availability or protected habitat resource.
Central place foraging from colonies, typically within a kilometer or two of colonies but not infrequently to six or more km (Hamilton in press).
Tricolors opportunistically respond to insect abundance, thriving on grasshopper and other insect outbreaks. The possibility that tricolors might be grasshopper specialists (Orians 1961; Payne 1969) was challenged by Skorupa et al. (1980). But observations in Kern County in 2001 demonstrate that tricolors may focus on orthopterans during outbreak years and attain resulting high reproductive success (Hamilton, Talluto and Clendennen, pers. obser.).
Spring pre-breeding foraging: Grains associated with dairy feedlots, cracked corn, sprouting rice, ripening oats, milk barley (Skorupa 1980).
Breeding season foraging: Grasshoppers (Orians 1961a, Payne 1969); opportunistic foragers of any abundant insect resource (Beedy and Hamilton 1997) including grasshoppers, beetles [to 61% of all nesting foods in a large study by Crase and DeHaven (1977)], weevils, caddis fly larvae, moth and butterfly larvae, dragonfly larvae, lakeshore midges. Animal matter comprises 91% of the food volume of nestlings and fledglings, 28% adult males, and 56% adult females (Skorupa et al, 1980).
Post breeding and fall foraging: Agricultural cropland with alfalfa and rice, irrigated pasture, lightly grazed grasslands, livestock operations and dairy feedlots.
Winter foraging: Seeds from pasture grassland and weeds, animal matter as available.
Open water within 500 m is a requirement for colony settlement.
Dominant nest substrate species: cattails, bulrushes, Himalaya berry, agricultural silage. A variety of other plant species are used as nesting substrate, all either flooded, spinous or otherwise defended against easy access by mammalian predators. In marshes dense vegetation is preferred but heavily lodged cattails not burned in recent years may preclude settlement. Biennial burning is a preferred management strategy. Tricolors often settle in cattails burned the same season. Tricolors will not settle without access to open water. Strips of emergent vegetation along canals are avoided as nest sites unless they are about 10 or more m wide but in some ponds, especially where associated with Himalayan blackberries and deep water, settlement may be in narrower fetches of cattails. If sites are hard for an observer to reach, the site it is relatively suitable.
of foraging habitat need study. Cessation of livestock grazing enhances attractiveness
but dense grass in following years reduces attractiveness. In rice country the
rice itself is the principal foraging habitat. Some large dragonfly larvae are
taken. There is no systematic study of tricolor economics in rice, a serious
omission because rice is one of the principal habitats of this species.
Cattails, esp. (Typha latifolia), and bulrushes (Scirpus), or (blackberries (Rubus sp.), mustard (Brassica sp.), (nettles (Urtica sp.), thistles (Cirsium and Centaurea spp.) willows (Salix spp.).
HEIGHT OF NEST
0-3 m, typically 0.5-1.5 m
HEIGHT OF NEST PLANT
To 5 or more m, as in oaks, ashes, willows, but typically below canopy in emergent marsh vegetation and copses of blackberries.
PLANT SPECIES CONCEALING THE NEST
Ordinarily there is no special attempt at concealment. Nests are established in the densest substrate vegetation first and progressively outward to peripheral sites that often are sparser (Beedy and Hamilton 1999).
VEGETATION SURROUNDING THE NEST
Colonies nest in similar vegetation but may choose adjacent substrate plants not otherwise utilized unless all sites in the initially settled substrate are utilized. This accounts for nests in ashes (Fraxinus), very twiggy valley oaks (Quercus lobata) and Cephalanthus adjacent to blackberry thickets saturated with nests.
Shallow, open-cup, mud-lined nest of vegetation woven tightly around stem supports and constructed on a created platform in spinous or flooded vegetation. Usually sheltered under overhanging vegetation.
TYPICAL BREEDING DENSITIES
Freshwater marsh nest density ranges from one per square meter to one per six or more square meters. Himalayan blackberry colonies in Glenn County with densities to six nests per square meter (Beedy and Hamilton 1997). Density of nests in silage is comparable to that in cattails or may be far less concentrated. Density in colonies not uniform. Nest settlement is selectively to densest vegetation. As nests fail due to predation, the distance between active nests increases, giving a false impression that a small, dispersed group of birds settled to nest. Tricolors do not abandon nests in the face of overwhelming predation.
Synchronous displays occur at settling colonies (Orians and Christman 1968; Collier 1968). From a management perspective stages in colony development include (1) synchronous en masse flights to prospective foraging areas by colonizing individuals (2) synchronous male song, female nest building and egg-laying with male song persisting, and (3) cessation of most male song following completion of egg laying. When song ends females are incubating and males absent themselves from the colony during the day. At this time colony size is easily underestimated by casual observers, and large colonies can be overlooked. Presence of large, all-male foraging flocks during the breeding season identifies presence of nesting colonies in the vicinity.
Polygynous. 1-4 or more females per male (Collier 1968; Payne 1969, personal observations). Delayed breeding by males but few birds remain unassociated with colonies during the breeding season (Hamilton 1998). First-year females enter spring breeding activities but first-year males appear unsuccessful at breeding although they may display and hold territories (Payne 1969). At colony sites, males initiate nesting by establishing territories in the densest and spiniest areas. Further settlement is peripheral and may include increasingly less suitable (thinner vegetation, shallower water) sites. Males with territories on periphery of colony may not attract females and leave the colony after female settlement is completed (Collier 1968).
1-5, typically 3 or 4. The ratio of 3 to 4 egg clutches varies from mostly one to mostly the other. Five egg clutches are uncommon (< 5%).
DEVELOPMENT AT HATCHING
Both sexes provision. Males are more attentive than redwings but are relatively wary, at least when closely observed by humans.
POST FLEDGING BIOLOGY OF OFFSPRING
Post breeding biology of offspring includes movement from colony to crèche sites up to several km from colonies where parental provisioning continues. Or post fledging provisioning may persist at the natal colony site if it is not disrupted and foraging in the immediate area remains productive.
Males delay breeding to 2 or more years (Payne 1969).
NUMBER OF BROODS
One or two at any particular location. Itinerant breeder. Individuals may spend the entire breeding season (mid-March - mid July) at one location or travel from one breeding location to one or more other locations during the breeding season (mid-March to early July). Some individuals may make several nesting attempts following successive nest losses. Estimates of population size based upon observations of a single nesting attempt per year at some locations overlooks itinerant breeding and the possibility that the observed individuals have bred or will breed again elsewhere (Hamilton 1998).
Cowbird parasitism is uncommon because incubation is initiated when the first egg is laid.
Ordinarily lowland California, < 1000 m
Habitat fragmentation is not a problem if sufficient habitat lies within the commuting range of breeding colonies. A patchwork of habitats can accommodate a breeding colony.
No minimum space to accommodate a colony has been determined. But spaces smaller than 100 ha including foraging habitat comprise only a minor part of all tricolor nesting effort.
Where water levels are being managed, height of nests above water needs to be considered so that water is kept deep but not so deep that flooding of nests occurs. Early nests in growing cattails can be as much as 0.75 m lower than later nests. Because an intruder into a colony will first encounter later nests, the status of early nests may be misjudged. If feasible, a simple water level management strategy is to maintain the level that was there when initial tricolor settlement occurred. A common failure of managed ponds is to allow water levels to fall, providing relatively easy access to mammalian predators. Overflights by crop-dusters or helicopters flushing incubating females when air temperatures are high can result in hatching failure.
is greatest during settlement and egg laying when entrance into colonies should
always be avoided. If necessary, colony observation can be safely done with
minimum disturbance during incubation and early nestling development (to 8 da).
in urban parks and in operational utility areas such as water treatment facilities,
reservoirs and gravel plants suffer negligible losses. Tricolors respond well
to roadside traffic and colonies in Himalaya blackberries at the edge of highways
suffer minimal losses to traffic. Some Caltrans cloverleaf sites may be suitable
for tricolor breeding colonies.
ADJACENT LAND USE
This is a critical habitat feature because tricolors forage away from nesting sites and large colonies require large foraging areas. Most colonies forage within 1.5 km of nest site but foraging up to six km is not uncommon (Crase and DeHaven 1977). Suitable habitat for foraging includes irrigated pasture, dry rangeland and dairy operations providing successive harvest and flooding conditions. Orchards, row crops and vineyards, used occasionally and briefly, do not sustain breeding colonies.
Grau et al. (1987) reported selenium poisoning at Kesterson NWR. Tricolors may be relatively unaffected by poisoning problems because they avoid row crops, vineyards and orchards. Broadcasting of insect baits on rangeland is an uncommon practice in California. There are no reports of mass adult tricolor mortality from any cause since pest eradication efforts ended in the 1930s. The relationship of insect control measures in rice to reproductive success of tricolors foraging in rice fields needs study.
Black crowned night heron (Nycticorax nycticorax) emergent marsh colonies and especially in refuges, coyotes especially in the Central Valley, ravens, and raccoons. Harriers may harry colonies incessantly, imposing a reproductive cost. Staggering losses of all eggs and small chicks at large (> 15,000 nests) colonies to night herons suggests that Central Valley cattail marshes cannot be recommended as tricolor habitat unless predator management is undertaken. But an occasional huge fledging success in cattails as at Delevan NWR in 2000 and 2003 precludes dismissing cattail marsh altogether. Coyotes also impose huge losses on nests in the silage and grain fields and in dry cattail marshes. At some San Joaquin Valley foothill colonies ravens overwhelm colonies.
DEMOGRAPHY AND POPULATION TRENDS
Age and sex
ratios: Cannot be determined from available data. All male flocks during
the breeding season are males waiting for females to complete incubation. In
nonbreeding season some segregation of sexes may occur. While much literature
and reports by casual observers suggest association of tricolors with mixed
species flocks, there may be large, nearly single species flocks in the fall
Survivorship: No information. Absence of these data precludes precise population modeling.
DeHaven et al. (1975) performed a massive experiment to estimate the extent
of philopatry of first year individuals banded as nestlings. They found little
evidence of philopatry but all recoveries (N = 11) were in the Central Valley.
Banding data fail to show any movement between the Central Valley and southern
California, but data (DeHaven and Neff 1973) are too few to resolve the question.
MANAGEMENT ISSUES AND OPTIONS
EXOTIC SPECIES INVASION/ENCROACHMENT
Enhancement of native species (ravens, night herons, coyotes) by incidental or deliberate management is a greater problem than exotics in the case of predators. Great-tailed Grackles (Quiscalus mexicanus) are an increasing threat in southern California. They change the rules for human observer intervention. Any action putting tricolors off nests subjects them to immediate destruction by resident grackles. There is no counterpart threat when grackles are not present as is the case throughout northern California. The role of exotic grass, native grass, several Mediterranean weeds and livestock grazing has not been closely studied. Foraging tricolors are particularly attracted to ephemeral pools when foraging. Their loss subtracts from highly suitable tricolor foraging habitat.
The historic (Neff 1937) and modern (Beedy and Hamilton 1997) record show that most tricolors nest in large colonies. It follows that management and restoration efforts need to deal with the relatively large spatial dimension of large colonies (Hamilton in press). There is a close parallel to the Swainson's Hawk (Buteo swainsonii) problem. This species also requires extensive foraging habitat and relatively small scale nesting habitat (Woodbridge 1998). Both species can forage effectively in a patchwork of foraging habitat, providing an opportunity to manage colonies in relatively highly developed settings as long as suitable and sufficiently extensive foraging habitat is maintained. Tricolors are highly agriculture dependent, favoring productive habitats such as irrigated pasture, maturing grain crops and dairies. Nevertheless, suitable habitats distant from all irrigated agricultural operations do support large colonies, and as managers we need to support efforts to develop and maintain rangeland colonies that do not negatively impact agricultural operations such as haying.
Tricolored Blackbirds are at as high a risk as any of the narrowly endemic North American bird species and are far greater risk than Swainson's Hawks, Burrowing Owls and other relatively widely distributed California species. But because they are a flocking species, and are in some places abundant, they do not command management attention and funding.
Beedy and Hamilton
(1997) summarized management recommendations: (1) avoid reproductive failures
by large colonies throughout the geographic distribution of this species, (2)
to enhance breeding habitat and success on public lands and to encourage commitment
of private lands to tricolor habitat, (3) to enhance public awareness of this
species and (4) to minimize habitat losses. Hamilton (in press) emphasized (1)
monitoring to include measurements of reproductive success, (2) designate adequate
and sufficient habitat for tricolors in HCPs, (3) protect ephemeral habitats,
(4) develop habitat to take advantage of rice as a tricolor nesting habitat
when associated with native marsh vegetation, (5) avoid dairies as a focus for
management and restoration, (6) develop water point sources where their absence
limits tricolor settlement, (7) encourage development of colonies in conspicuous
urban environments where their educational value will be useful, (8) develop
tricolor habitat when reservoirs are designed and constructed, (9) create restoration
colonies, (10) emphasize native plants in restoration efforts and (11) manage
problem species such as ravens, night herons and coyotes whenever possible.
The desire to protect several species with conservation actions sometimes works against the best interests of Tricolored Blackbirds (Hamilton in press). One species, the White-Faced Ibis (Plegadis chihi), is closely associated with some tricolor colonies, protecting tricolors from night heron predation. The current (2003) rapid recovery of ibis populations in the Central Valley enhances the potential of marsh nesting tricolor colonies being damaged by night heron predation.
MONITORING METHODS AND RESEARCH NEEDS
A census as extensive and intensive as those conducted in 1994, 1997 and 2000 needs to be repeated. The details of how those censuses were conducted are contained in the respective reports for those years and in Beedy and Hamilton (1997).
Since many kinds of colonies fail entirely, it is necessary to monitor reproductive success at a representative sample of colonies.
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