Least Bittern
(Ixobrychus exilis)
- DESCRIPTION
- DISTRIBUTION
- HABITAT
- CONSERVATION
- MULTIMEDIA
General Description
The least bittern (Ixobrychus exilis) is the smallest member of the heron family and is one of the most inconspicuous of all marsh birds. It is 11-14.2 in (28-36 cm) long, has a 16.9 in (43 cm) wing span, and weighs 2.8 oz (80 g). It’s very small size and contrasting color pattern are diagnostic field marks. Sexes are similar in size but the plumage differs between male and female. The crown, back, and tail on the male are greenish black, while those of the female are a purple-chestnut. The neck, sides of the body, and underparts are brown and white, with the neck of the female darkly streaked. The wings are chestnut with contrasting and conspicuous pale patches and the wingtips are slate. The bill is thin and yellow; legs and feet are a straw to buffy yellow. Plumage of the juvenile is similar to the female with the crown and back a lighter brown; the chest and throat have a striped appearance.
Legal Status
The least bittern is listed as a species of special concern in Arizona and California. It is listed by the U.S. Fish and Wildlife Service as a migratory nongame bird of management concern. It is not listed by Nevada or Mexico.
Taxonomy
The least bittern was previously classified into eastern (exilis) and western (hesperis) subspecies, but recent data on plumage and morphology do not support this grouping.
Reproduction
In Arizona, resident least bitterns likely begin breeding before migratory populations, with males initiating their cooing calls in March and April. They are a locally common breeder from April through September.
Throughout their entire range, nests are typically built among dense stands of emergent or woody vegetation. The nest platform and canopy is constructed primarily by the male and is made by pulling down and crimping surrounding vegetation; short stems and sticks are added in a spoke-like manner to form the nest. Nests are well concealed, and are usually located adjacent to open water. Nests have been found scattered throughout suitable habitat or concentrated in loose groupings. Typical clutches are 4-5 eggs and the time from laying the first egg to the hatching of the first egg ranges from 19 to 21 days. Both sexes incubate the eggs, the female perhaps more than the male. The chicks are born downy and need to be fed by adults. The young are fed by regurgitation and by the males more than the females. They can forage on their own within 1-2 weeks. The young normally leave the nest permanently by 13-15 days but linger nearby for 1-2 weeks. Approximate age at first flight is 29 days. The least bittern will re-nest and double brood. Newly hatched chicks are covered with a pale buff down, new feathers start to emerge at 8 days, and juvenile plumage is nearly complete at 36 days.
Diet
The least bittern’s major food items are small fish and insects. They have been found to eat freshwater shrimp, crayfish, bluegill, mosquito fish, and threadfin shad. Frogs are also a prey item, as well as insects. Least bitterns may also prey on the eggs and young of yellow-headed blackbirds. The least bittern’s small size, highly compressed trunk, and ability to grasp with its feet enable it to move through dense vegetation. It forages by clinging to emergent vegetation over open water and extending its long neck, wading along the edge of open water, and using small constructed foraging platforms at rich feeding sites. Foraging behaviors used by the least bittern are standing in one place, walking slowly, moving its head back and forth, and flicking its wings to startle prey.
Threats
Least bittern often nest over water and away from shore, making them less vulnerable to land predators. Snapping turtles and red-tailed hawks have been identified as predators of adult least bitterns within its range. Snakes, turtles, crows, raptors, and raccoons have been documented as predators of chicks and eggs. The marsh wren has been suspected of puncturing eggs. Nesting aquatic birds that feed on fish and invertebrates along the lower Colorado River have to potential to bio-accumulate toxic concentrations of selenium in their tissues and eggs. Destruction of wetland habitat is likely the greatest threat to the least bittern nationwide. Lower groundwater levels could reduce the extent or quality of habitat provided by marshes associated with backwaters. Habitat could be removed to maintain channel functions (e.g., dredging desilting basins) in the river and in irrigation ditches. Because the least bittern prefers tall, dense growths of emergent vegetation, burning decadent emergent vegetation to benefit other covered species, such as the Yuma clapper rail, may result in a temporary loss of habitat for the least bittern, as will wildfire episodes.
More Information
Additional information on this species, as well as source documentation, can be found in the species accounts located at this link (PDF). The Conceptual Ecological Model (CEM) can be found here (PDF). Technical Reports on this species can be found here.
Updated December 30, 2016
The breeding range of the least bittern is now from southeastern Canada, through the United States and Mexico to Costa Rica. Breeding has recently been confirmed in central, south central, and southeastern Arizona, as well as southern Nevada. The winter range is primarily south of areas with prolonged winter frosts: along the Atlantic coastal plain from Maryland and Virginia, south to Louisiana and Texas, with peak numbers in southern Florida, the Rio Grande Valley, the lower Colorado River Valley, and Baja California.
The largest populations along the lower Colorado River are in extensive cattail and bulrush marshes, such as at Topock Marsh near Needles, CA and near Imperial Dam, just north of Yuma, AZ. In much of southwestern Arizona at elevations below 1000 ft (305 m), extensive cattail marshes often harbor least bitterns.
The image below shows the distribution range of the least bittern.
In general, the least bittern occupies freshwater and brackish marshes with dense, tall growths of emergent vegetation, interspersed with clumps of woody vegetation and open water. Least bitterns have been found in marshes along rivers, ponds, lake edges, and less frequently, along irrigation and runoff ditches from agricultural areas. In the lower Colorado River Valley, generally marshes that are dominated by dense cattails or bulrushes support large numbers of breeding insectivorous wading birds, including rails and least bitterns. Least bitterns at the Salton Sea are along rivers and wide irrigation ditches, particularly in dense stands of southern and broad-leaved cattails, but some may use common reed or saltcedar if cattails are nearby. Nests have been recorded in a variety of vegetation types and over varying depths of water.
LCR MSCP Conservation Measures
The Habitat Conservation Plan provides conservation measures specific to each species. Listed below are the species specific conservation measures for the least bittern. Click on the arrows to expand the table.
| LEBI1—Create 512 acres of western least bittern habitat |
Create and manage 512 acres of marsh to provide western least bittern habitat (Figure 5-2 in the HCP). This created habitat will also be habitat for the Yuma clapper rail (conservation measure CLRA1). Habitat will be created in patches as large as possible. Smaller patches are likely within the range of habitat patch sizes used by the species for foraging and dispersal, and larger patches may be used for breeding. Western least bittern habitat will be created and maintained as described in Section 5.4.3.3 in the HCP. Marshes created to provide western least bittern habitat will be designed and managed to provide an integrated mosaic of wetland vegetation types, water depths, and open water areas. Priority will be given, when consistent with achieving LCR MSCP goals for other covered species, to establishing habitat near occupied habitat. The largest numbers of western least bitterns in the LCR MSCP planning area are located at Topock Marsh and marshes near Imperial Dam, but they are present in suitable marshes throughout the LCR MSCP planning area. Within this mosaic of marsh conditions, western least bittern habitat will generally be provided by patches of bulrush and cattails interspersed with small patches of open water that maintain water depths no greater than 12 inches. |
| MRM1—Conduct surveys and research to better identify covered and evaluation species habitat requirements |
Conduct surveys and research, as appropriate, to collect information necessary to better define the species habitat requirements and to design and manage fully functioning created covered and evaluation species habitats. This conservation measure applies to those species for which comparable measures are not subsumed under species-specific conservation measures (Section 5.7 in the HCP). They are not applicable to species for which habitat would not be created under the LCR MSCP Conservation Plan, such as the desert tortoise, relict leopard frog, humpback chub, and threecorner milkvetch. |
| MRM2—Monitor and adaptively manage created covered and evaluation species habitats |
Created species habitats will be managed to maintain their functions as species habitat over the term of the LCR MSCP. Created habitat will be monitored and adaptively managed over time to determine the types and frequency of management activities that may be required to maintain created cottonwood-willow, honey mesquite, marsh, and backwater land cover as habitat for covered species. This conservation measure applies to those species for which comparable measures are not subsumed under species-specific conservation measures (Section 5.7 in the HCP). They are not applicable to species for which habitat would not be created under the LCR MSCP Conservation Plan, such as the desert tortoise, relict leopard frog, humpback chub, and threecorner milkvetch. |
| MRM5—Monitor selenium levels in created backwater and marsh land cover types, and study the effect of selenium released as a result of dredging activities |
Conduct monitoring of selenium levels in sediment, water, and/or biota present in LCR MSCP created backwater and marsh land cover types. If monitoring results indicate that management of the LCR MSCP conservation areas increases levels of selenium in created backwaters and marshes or in covered species that use them, the LCR MSCP will undertake research to develop feasible methods to manage the conservation areas in a manner that will eliminate or compensate for the effects of increased selenium levels. If feasible management methods are identified, they will be implemented. This conservation measure will include monitoring the effects of dredging and dredge spoil disposal associated with creating and maintaining backwaters and marshes. If monitoring results indicate that current or future dredging and dredge spoil disposal methods increase selenium levels, the LCR MSCP will only implement methods that will have the least effect on selenium levels. A study will also be conducted to look at the effects of potential releases of selenium from dredging in general. |
| CMM1—Reduce risk of loss of created habitat to wildfire |
Management of LCR MSCP conservation areas will include contributing to and integrating with local, state, and Federal agency fire management plans. Conservation areas will be designed to contain wildfire and facilitate rapid response to suppress fires (e.g., fire management plans will be an element of each conservation area management plan). |
| CMM2—Replace created habitat affected by wildfire |
In the event of created-habitat degradation or loss as a result of wildfire, land management and habitat creation measures to support the reestablishment of native vegetation will be identified and implemented. |
| AMM1—To the extent practicable, avoid and minimize impacts of implementing the LCR MSCP on existing covered species habitats |
To the extent practicable, establishment and management of LCR MSCP–created habitats will avoid removal of existing cottonwood-willow stands, honey mesquite bosques, marsh, and backwaters to avoid and minimize impacts on habitat they provide for covered species. Temporary disturbance of covered species habitats, however, may be associated with habitat creation and subsequent maintenance activities (e.g., controlled burning in marshes and removal of trees to maintain succession objectives). LCR MSCP conservation measures that could result in such temporary disturbances will, to the extent practicable, be designed and implemented to avoid or minimize the potential for disturbance. In addition to implementing AMM3 and AMM4 below, these measures could include conducting preconstruction surveys to determine if covered species are present and, if present, implementing habitat establishment and management activities during periods when the species would be least sensitive to those activities; or redesigning the activities to avoid the need to disturb sensitive habitat use areas; staging construction activities away from sensitive habitat use areas; and implementing BMPs to control erosion when implementing ground disturbing activities. |
| AMM2—Avoid impacts of flow-related covered activities on covered species habitats at Topock Marsh |
Impacts on groundwater levels that support covered species habitat at Topock Marsh will be avoided by maintaining water deliveries for maintenance of water levels and existing conditions. At times, flow-related activities could lower river elevations to levels that could disrupt diversion of water from the river to the marsh. Improvements to intake structures that allow water to continue to be diverted or other measures to maintain the water surface elevation will avoid effects on groundwater elevation. Avoidance of effects could be accomplished with the purchase, installation, and operation of two electric pumps sized to the current inflow at the Topock Marsh diversion inlet. The pumps would most likely need to be operated during summer to make up for the lower flow periods. Implementation of this conservation measure would maintain existing habitat at Topock Marsh for the Yuma clapper rail, southwestern willow flycatcher, Colorado River cotton rat, western least bittern, California black rail, yellow-billed cuckoo, gilded flicker, vermilion flycatcher, Arizona Bell's vireo, and Sonoran yellow warbler. The extent of covered species habitat impacts that will be avoided by maintaining water deliveries to Topock Marsh are presented in Table 4-2 in the HCP. Maintaining water deliveries to Topock Marsh will also maintain razorback sucker and bonytail habitat associated with disconnected backwaters managed for these species. |
| AMM3—To the extent practicable, avoid and minimize disturbance of covered bird species during the breeding season |
To the extent practicable, to avoid and minimize potential impacts on covered bird species, vegetation management activities (e.g., periodic removal of emergent vegetation to maintain canals and drains) associated with implementation of covered activities and the LCR MSCP that could result in disturbance to covered bird species will not be implemented during the breeding season to prevent injury or mortality of eggs and young birds unable to avoid these activities. Table 5-9 in the HCP describes the breeding period for each of the covered species during which, to the extent practicable, vegetation management activities in each species' habitat will be avoided. |
| AMM5—Avoid impacts of operation, maintenance, and replacement of hydroelectric generation and transmission facilities on covered species in the LCR MSCP planning area |
To the extent practicable, before implementing activities associated with OM&R of hydroelectric generation and transmission facilities, measures will be identified and implemented that are necessary to avoid take of covered species where such activities could otherwise result in take. These measures could include conducting surveys to determine if covered species are present and, if so, deferring the implementation of activities to avoid disturbance during the breeding season; redesigning the activities to avoid the need to disturb covered species habitat use areas; staging of equipment outside of covered species habitats; delineating the limits of vegetation control activities to ensure that only the vegetation that needs to be removed to maintain infrastructure is removed; stockpiling and disposing of removed vegetation in a manner that minimizes the risk of fire; and implementing BMPs to control erosion when implementing ground disturbing activities. |
| AMM6—Avoid or minimize impacts on covered species habitats during dredging, bank stabilization activities and other river management activities |
To the extent practicable, before initiating activities involved with river maintenance projects, measures will be identified and implemented that avoid or minimize take of covered species where such activities could otherwise result in take. Such measures could include alternative methods to achieve project goals, timing of activities, pre-activity surveys, and minimizing the area of effect, including offsite direct and indirect effects (e.g., avoiding or minimizing the need to place dredge spoil and discharge lines in covered species habitats; placing dredge spoils in a manner that will not affect covered species habitats). In the event of created-habitat degradation or loss as a result of wildfire, land management and habitat creation measures to support the reestablishment of native vegetation will be identified and implemented. |
Research and Monitoring Activities
LCR MSCP conduct a variety of research and monitoring activities along the LCR encompassing both MSCP and non-MSCP species. For a complete list of all activities, please see the Research and Monitoring Activities web page.
This gallery includes photos of this species. If you require larger photos, please contact our webmaster Michelle Reilly at mreilly@usbr.gov.
