Whooping crane (Grus americana) recovery strategy: chapter 1
- 1.1 Species Assessement Information from COSEWIC
- 1.2 Introduction
- 1.3 Description of the Species
- 1.4 Current Distribution
- 1.5 Current Abundance
- 1.6 Population Trends
- 1.7 Needs of the Whooping Crane
- 1.8 Threats
- 1.9 Information Needs
1.1 Species Assessement Information from COSEWIC
Date of Assessment: November 2000
Common Name: Whooping Crane
Scientific Name: Grus americana
COSEWIC Status: Endangered
Reason for Designation: This globally endangered species occurs in very small numbers in a very restricted breeding range within a portion of Wood Buffalo National Park and adjacent area. Human activity and resource development are potential threats to the population, primarily during migration.
Canadian Occurrence: Alberta, Northwest Territories
COSEWIC Status History: Designated Endangered in April 1978. Status re-examined and confirmed in November 2000. Last assessment based on an update status report.
The Whooping Crane is a flagship species in the North American wildlife conservation movement and symbolizes the struggle for survival that characterizes many endangered species worldwide. It is a large, distinctive, and photogenic bird, popular with the public and the media, and is often used to illustrate endangered species literature.
Once numbering in the several thousands, the Whooping Crane approached the brink of extinction in the 1940s, when only 21 Whooping Cranes remained in the world. Luckily, these large majestic birds were saved from extinction, and by March 2007, the Canadian migratory Whooping Crane population grew to 237 birds. While on the precarious road to recovery, Whooping Cranes remain listed as an endangered species in Canada and the United States.
Historically, population declines were caused by shooting and destruction of nesting habitat in the prairies as a result of agricultural development. Today, the Whooping Crane remains an endangered species because of its low population numbers, slow reproductive potential due to delayed sexual maturity and limited recruitment into the population, a hazardous migration route traversed twice annually, and many human pressures on the wintering grounds. Current threats to the wild cranes include collisions with human made objects such as power lines, shooting, predators, disease, habitat destruction, severe weather, and a loss of two-thirds of the original genetic diversity. Threats to the captive cranes include disease, accidents, and limited genetic diversity.
1.3 Description of the Species
The Whooping Crane is the tallest North American bird, with males approaching 1.5 m when standing erect. Males are generally larger than females, weighing an average of 7.3 kg; captive females average 6.4 kg. Whooping Cranes are sexually monomorphic (Walkinshaw 1973); however, the guard call vocalization and components of the unison call are sexually distinct (Archibald 1975; Carlson 1991), as are some behavioural postures.
Adult plumage is snowy white except for black wing-tips and black and red on the crown and around the bill (Figure 1). The long bill is a dark olive-grey, which becomes lighter during breeding season. Legs and feet are grey-black. Juvenile plumage is a reddish-cinnamon colour. At 120 days of age, white feathers begin to appear on the neck and back of juveniles, and the juvenile plumage is replaced through the winter months. Rusty juvenile plumage remains on the head and upper neck in the winter/spring (Stephenson 1971), with adult plumage becoming complete late in their second summer.
Whooping Cranes are a long-lived species. Current estimates suggest a maximum longevity in the wild of at least 30 years (Mirande et al. 1993). Captive individuals live 35–40 years (Moody 1931; McNulty 1966).
1.4 Current Distribution
Whooping Cranes occur only in North America (Figure 2). They currently exist in the wild at three locations and in captivity at seven sites. The wild populations include (1) a non-migratory flock at and surrounding Kissimmee Prairie, central Florida; (2) a migratory flock established in 2001, located between central Wisconsin and the central Gulf Coast of Florida; and (3) the migratory Aransas–Wood Buffalo Population (AWBP), which nests in Canada and winters in the United States.
The AWBP is the only wild population that occurs in Canada. It breeds in and adjacent to Wood Buffalo National Park and migrates through Alberta, Saskatchewan, and, occasionally, Manitoba, staging in Saskatchewan during the fall. In the United States, the cranes migrate through the central Great Plains and winter on the central Gulf Coast of Texas at Aransas National Wildlife Refuge and vicinity. Subadult cranes have summered at various locations throughout the migration corridor and north of the breeding grounds.
Captive Whooping Cranes are maintained at Patuxent Wildlife Research Center, Laurel, Maryland; International Crane Foundation, Baraboo, Wisconsin; Calgary Zoo, Calgary, Alberta; Audubon Species Survival Center, Belle Chasse, Louisiana; San Antonio Zoo, San Antonio, Texas; New Orleans Zoo, New Orleans, Louisiana; and Lowry Park Zoo, Tampa, Florida.
1.5 Current Abundance
In March 2007, the total wild population was estimated at 344. This included 237 individuals in the only self-sustaining population, the AWBP, 45 captive-raised individuals released in an effort to establish a non-migratory FP, and 62 individuals introduced to the eastern United States that migrate between Wisconsin and Florida.
In March 2007, the total captive population consisted of 145 birds, with annual production from the Calgary Zoo, International Crane Foundation, Patuxent Wildlife Research Center, and San Antonio Zoo. The total population of wild and captive birds was 489.
1.6 Population Trends
The Whooping Crane has a long-term recruitment rate of 13.9%, the highest of any North American crane population (Drewien et al. 1995). The AWBP is increasing at an annual rate of more than 4%. The growth of the AWBP up to the year 2000 seems to have resulted primarily from a decline in the mortality rate rather than an increase in recruitment. Prior to 1970, annual mortality averaged 12.1%, although it has since dropped to 7.7%; overall mortality averages 9.8% per year. During this time, recruitment also declined from the pre-1970 average of 15.9% to 11.3% per year.
It is difficult to predict the future population size given the large variation in annual growth rates for any given year. However, over the next 100 years, it is highly probable that the AWBP will continue to grow, and the probability of extinction of the population is less than 1% (Mirande et al. 1997; Tischendorf 2003), assuming that current environmental conditions do not deteriorate. For a more detailed discussion of population viability assessment, the reader should refer to the International Recovery Plan (Canadian Wildlife Service and U.S. Fish and Wildlife Service 2006).
1.7 Needs of the Whooping Crane
1.7.1 Breeding Habitat
Whooping Cranes formerly bred in isolated marshes on the prairies and in aspen parkland. There are six primary nesting areas within and adjacent to Wood Buffalo National Park; between the headwaters of the Nyarling, Sass, Klewi and Little Buffalo rivers (Figure 3). The area is poorly drained and interspersed with numerous potholes. Wetlands vary considerably in size, shape, and depth, and most possess soft marl bottoms (Timoney et al. 1997). The wetlands are separated by narrow ridges, which support an overstorey of white spruce (Picea glauca), black spruce (P. mariana), tamarack (Larix laricina), and willows (Salix spp.) and an understorey of dwarf birch (Betula glandulosa), Labrador tea (Ledum groenlandicum), bearberry (Arctostaphylos uva-ursi), and several species of lichen, underlain by sphagnum moss (Novakowski 1966). Bulrush (Scirpus validus1) is the dominant emergent in the potholes used for nesting, although cattail (Typha sp.), sedge (Carex aquatilis), musk-grass (Chara sp.), and other aquatic plants are common (Allen 1956; Novakowski 1965, 1966; Kuyt 1976a, 1976b, 1981). Nest sites are located primarily in shallow diatom ponds that contain bulrush (Timoney 1999).
1.7.2 Migration Habitat
Whooping Cranes use a variety of habitats during migration (Howe 1987, 1989; Lingle 1987; Lingle et al. 1991; Johns et al. 1997). The majority of roosting wetlands are less than 4 ha (75%) and within 1 km of a suitable feeding site (Johns et al. 1997). More than 40% of the roosting wetlands are smaller than 0.5 ha (Johns et al. 1997). Cropland accounts for 70% of the feeding sites of non-family birds, whereas wetlands account for 67% of the feeding sites of family groups (Howe 1987).
The most suitable stopover habitat appears to be wetland mosaics (Johns et al. 1997; Richert et al. 2000). Whooping Cranes used primarily shallow, seasonally and semipermanently flooded palustrine wetlands for roosting and various cropland sites and emergent wetlands for feeding (Johns et al. 1997; Austin and Richert 2001). Whooping Cranes also use riverine habitats during migration through Saskatchewan and Nebraska, where they roost on submerged sandbars in wide unobstructed channels, isolated from human disturbance (Armbruster 1990; B. Johns pers. comm.).
Spring migration typically begins between March 25 and April 15, with the last birds usually leaving the wintering grounds by May 1. Spring migration is completed in 2–4 weeks on average.
Autumn migration normally begins in mid-September, with most birds arriving on the wintering grounds between late October and mid-November. Their first stop often occurs in northeastern Alberta or northwestern Saskatchewan. Most of the cranes remain for 2–4 weeks in a region between Meadow Lake, Swift Current, Estevan, and the Quill Lakes, Saskatchewan, where they feed on waste grain in barley and wheat stubble fields and roost in the many wetlands (Johns 1992). Some of these wetlands include Midnight Lake, Witchekan Lake, Blaine Lakes, Radisson Lake, Buffer Lake, Muskiki Lake, Quill Lakes, Kutawagan Lake, Luck Lake, Creelman Marsh, and wetlands near Tribune and Bromhead. Riverine areas include the South Saskatchewan River and its sandbars between Outlook and Saskatoon and the North Saskatchewan River between the Maymont and Petrofka bridges.
1.7.3 Wintering Habitat
About 9000 ha of salt flats on the Aransas National Wildlife Refuge and adjacent islands comprise the principal wintering grounds for the AWBP breeding population. Marshes are dominated by saltgrass (Distichlis spicata), saltwort (Batis maritima), smooth cordgrass (Spartina alterniflora), glasswort (Salicornia sp.), and sea ox-eye (Borrichia frutescens). Inland margins of the flats are dominated by gulf cordgrass (Spartina spartinae). Interior portions of the refuge are gently rolling and sandy and are characterized by oak brush, grassland, swales, and ponds. Typical plants include live oak (Quercus virginiana), redbay (Persea borbonia), and bluestem (Andropogon sp.) (Stevenson and Griffith 1946; Allen 1952; Labuda and Butts 1979).
Whooping Cranes are omnivorous (Walkinshaw 1973), and summer foods include large nymphal or larval forms of insects (Anisoptera: Aeshna spp. and Libellula spp.; and Dytiscidae: Graphoderus occidentalis, Acilius semisulcatus, Rhantus binotatus, and Dytiscus alaskanus), snails (Probythinella lacustris), minnows (Culea inconstans, Phoxinus eos, Margariscus margarita, Phoxinus neogaeus, and Pimephales promelas), seeds (Potamogeton and Myriophyllum), frogs (Rana sylvatica, Acris crepitans, andpossibly Bufo hemiophrys), and rodents (Clethrionomys rutilis and several others) (Allen 1956; Novakowski 1966; Bergeson et al. 2001a; Bergeson 2004). Foods utilized during migration are poorly documented but include frogs, fish, plant tubers, crayfish, insects, and agricultural grains. The greatest proportion of feeding time during migration is spent in harvested grain fields (Johns et al. 1997). The crane's winter diet consists predominantly of animal foods, especially blue crabs (Callinectes sapidus), clams (Tagelus plebius, Ensis minor, Rangia cuneata, Cyrtopleura costata, Phacoides pectinata, Macoma constricta), and the fruit of wolfberry (Lycium carolinianum) (Allen 1952; Uhler and Locke 1970; Blankinship 1976, 1987; Hunt and Slack 1987; Chavez-Ramirez 1996).
1.8.1 Habitat Loss and Degradation
Human population growth in North America has resulted in alteration and destruction of Whooping Crane habitat. Conversion of wetlands and prairie for hay and grain production made much of the historical nesting habitat unsuitable for Whooping Cranes. Disruptive and destructive practices included draining, fencing, cultivation, and the human activities associated with these actions. Settlement of the midcontinent and coastal prairies and associated disturbance, in addition to alteration of habitat, may have interfered with continued use of prairie and wetlands by breeding Whooping Cranes. The extensive drainage of wetlands in the prairie pothole region of Canada and the United States also resulted in a tremendous loss of migration habitat available to Whooping Cranes. In the United States, upstream reservoir construction and water diversions for agriculture and human use have reduced inflows into coastal waters used by Whooping Cranes.
1.8.2 Loss of Genetic Diversity
As a consequence of the 1941 population bottleneck, the current population is derived from an estimated six to eight founders, with a loss of 66% of all genetic material (Mirande et al. 1993; Glenn et al. 1999). The continued loss of genetic material may lead to inbreeding depression and declining productivity (Jimenez et al. 1994; Frankham 1995; Lacy 1997; Brook et al. 2002; Woodworth et al. 2002). A detailed discussion of genetic issues is presented in the International Recovery Plan.
Whooping Cranes are sensitive to disturbance on both breeding and wintering grounds. Some disturbances, such as egg transfer and banding programs, are necessary to implement recovery and are tolerable for short intervals. However, unnecessary disturbances should be avoided, as they may cause birds to leave an area. There is no public access to most of the Whooping Crane nesting habitat; however, access to portions of the Whooping Crane winter habitat is available. Cranes are somewhat tolerant of people in carefully operated boats and land vehicles (Mabie et al. 1989); this is evidenced by the lack of concern that cranes show for barges that travel along the Gulf Intracoastal Waterway. Airboats, low-altitude aircraft, and especially helicopters are more disturbing, and cranes are particularly sensitive to humans on foot (Lewis and Slack 1992; T.E. Lewis pers. comm.; B. Johns pers. comm.). Crane displacement results in short-term or long-term loss of habitat use and social disruption of the flock, limits the ability to obtain food resources, and thus impacts fitness (T.E. Lewis pers. comm.).
1.8.4 Collisions with Power Lines
Collisions with power lines are a substantial cause of Whooping Crane mortality in migration (Brown et al. 1987; Lewis et al. 1992). Collisions with power lines are known to have accounted for the deaths or serious injury of at least 36 Whooping Cranes since 1956. Guy wires associated with telecommunication towers (radio, television, cellular, and microwave) are another threat. Tests of line marking devices, using Sandhill Cranes (Grus canadensis) as surrogate research species, have identified techniques effective in reducing collisions by up to 61% (Morkill 1990; Morkill and Anderson 1991, 1993; Brown and Drewien 1995). Techniques currently recommended include marking lines in areas frequently used by cranes and avoiding placement of new line corridors around wetlands or other crane use areas.
1.8.5 Chemical Spills
The only self-sustaining wild Whooping Crane population remains vulnerable to the potential of contaminant spills. The risk on the breeding range or during migration is minimal; however, the greatest concern is in the Gulf Intracoastal Waterway on the Texas coast. Numerous oil and gas wells and connecting pipelines are located in bay and upland sites near the cranes' winter habitat, and many barges carrying dangerous, toxic chemicals travel the Gulf Intracoastal Waterway daily through Whooping Crane winter habitat. A spill or leak of these substances could contaminate or kill the cranes' food supply or poison the cranes (Robertson et al. 1993).
1.8.6 Disease and Parasites
Little is known about the importance of diseases or parasites as mortality factors for wild Whooping Cranes. Loss of wetlands has concentrated birds, thereby increasing the risk of disease transmission. Although wild Whooping Cranes are presumably susceptible to a variety of infectious and toxicological diseases (including tuberculosis), evidence of disease-related mortality is infrequently documented.
Adult Whooping Cranes are generally not susceptible to predation unless they are weakened by disease or injury or when flightless during feather moult. Eggs and chicks, however, are susceptible to predation (Bergeson et al. 2001b). Potential predators in Wood Buffalo National Park include Black Bear (Ursus americanus), Wolverine (Gulo gulo luscus), Gray Wolf (Canis lupus), Red Fox (Vulpes fulva), Mink (Mustela vison), Lynx (Lynx canadensis), and Common Raven (Corvus corax). The overall impact of predation on AWBP recruitment remains uncertain, but predation may be a factor in the 10-year cycle in Whooping Crane recruitment (Boyce et al. 2005).
Bobcats (Lynx rufus) and American Alligators (Alligator mississippiensis) are significant predators to reintroduced Whooping Cranes in Florida. Predation rates are significant in Florida but appear to be very low in Texas, where wild cranes spend more time in coastal wetlands.
Hunting was one of the primary historical reasons for the Whooping Crane's decline. Enactment of protective legislation coincided with a decline in human-caused mortality. Although hunting Whooping Cranes is no longer legal, occasional shootings occur (Lewis et al. 1992).
1.8.9 Other Threats and Limiting Factors
Other potential threats, such as collisions with aircraft, pesticides, red tide, severe weather, climate change, life history, and food availability/sibling aggression, have been addressed in the International Recovery Plan. While these are considered to be potential limiting factors, none appears to be a primary threat to the species' survival.
1.9 Information Needs
1.9.1 Survey Requirements
- Continue aerial population surveys on nesting and wintering areas.
- Identify unoccupied potential nesting and wintering habitat to identify limitations on population growth.
- Continue water level surveys in Wood Buffalo National Park to fully understand ecological changes and their impact on population trends.
- Conduct prey abundance surveys in Wood Buffalo National Park to fully understand ecological changes and their impact on population trends.
- Continue surveys of coastal water salinity levels, freshwater inflows, and crane food resources at Aransas to identify ecological trends.
- Monitor migration activities to ensure the safety of the cranes, and evaluate changes in conditions faced by migrating birds.
1.9.2 Biological/Ecological Research Requirements
- Continue to identify and address causes of mortality in wild and captive cranes.
- Perform frequent monitoring (including radio-tracking or satellite tracking) of the birds to detect losses and causes of mortality.
- Acquire further understanding of migration stopover habitat to refine the effectiveness of habitat augmentation and management on the Platte River and elsewhere.
- Refine methods to create marsh wintering habitat with dredged sediments to ensure long-term benefits to Whooping Cranes.
- Refine techniques for disease prevention in captive birds (including West Nile vaccination and effective tuberculosis test), pairing and promotion of early breeding, genetic management, nutrition of captive birds, and behavioural training to promote wildness in birds destined for release.
- Refine reintroduction techniques for establishing a second migratory population to promote appropriate migratory behaviour and survival.
- Develop faecal corticosterone test to compare levels of stress associated with various management techniques in captivity.
1.9.3 Threat Clarification Research Requirements
Research needed to clarify threats was previously identified in the “Threats” section (section 1.8). Some examples include research on:
- the impact of reduced freshwater inflows at Aransas;
- causes of mortality in reintroduced populations; and
- techniques to separate family lines, which will preserve and increase the genetic diversity of the flock.
1 Now known as Schoenoplectus tabernaemontani (K.C. Gmel.) Palla
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