Demography and Populations

Age At First Breeding; Intervals Between Breeding

G. c. pratensis banded in Florida form persistent (> 2 wk) dyad relationships at 14 months of age (Nesbitt and Wenner 1987). Initial pair formation among mid-continent cranes occurrs during the third year; by 4 year, 20% pair; 90% to 100% pair before their seventh year (Tacha et al. 1989). Though many first pairs nest and lay fertile eggs, most fail to produce young that survive to attain independence. Cranes in Florida associate with between 3.6 and 6.0 potential mates before attaining breeding status (Nesbitt and Wenner 1987).

Breeding efforts in G. c. pratensis may begin at 2 yr in males and 3 yr in females; mean age of first successful reproduction is 5.2 yr (4.3 yr for G. c. tabida; Nesbitt 1992). G. c. pratensis and eastern tabida are not known to breed successfully before 3 yr of age (Nesbitt and Wenner 1987). In the mid-continent population, successful reproduction may begin at 5 yr of age, but mostly (> 75%) occurs in birds ≥ 8 yr old (Tacha et al. 1989). Once a pair attains reproductive status they usually attempt breeding annually; if nesting conditions are unfavorable pairs may delay or forego nesting that year.

Annual And Lifetime Reproductive Success

Average annual production for any adult (≥ 3 yr old) in G. c. pratensis and the e. populations of tabida is 0.35 young/yr raised to the age of independence (Nesbitt 1992). Annual percentage of juveniles in various populations averages 11.0, range 6.6 to 18.3 (Tacha and Vohs 1984, Tacha et al. 1986, Bishop 1988, Bennett and Bennett 1990, Nesbitt 1992). Lifetime reproduction in G. c. tabida (eastern) and pratensis estimated at 1.86 ± 0.16 SE young for any adult (Nesbitt 1992), and 2.70 ± 0.16 SE young for an established breeder (one that reproduced successfully at least once).

In the e. population of G. c. tabida, 64% of adults reproduce successfully, and 74% of G. c. pratensis succeed; 26% of these adults produce 52% and 62% of the young, respectively (Nesbitt 1992).

In the eastern (unhunted) population of G. c. tabida, annual survival rate (all postjuvenile age classes combined), was 0.874 for males, and 0.858 for females; for pratensis the rate was 0.884 for males and 0.918 for females (S. A. Nesbitt and C. T. Moore, unpubl. data).

Mean life expectancy for any bird that reached the age of independence was 7 yr (S. A. Nesbitt and C. T. Moore, unpubl. data). Maximum age in the wild is 21.6 yr for G. c. pratensis in Florida (SAN), and 19.4 yr for a bird from the mid-continent population (Klimkiewicz and Futcher 1989).

Bacterial, Fungal, And Viral Diseases

Diseases most significant in mortality of adult cranes are avian botulism (Clostridium botulinum) and avian cholera (Pasteurella spp.); avian tuberculosis (Mycobacterium avium) less important but a potential mortality factor (Windingstad 1988). Aspergillosis has occasionally caused deaths but is usually secondary to other factors. Serum samples from over 700 mid-continent Sandhill Cranes showed < 2% had positive indicators of exposure to Salmonella sp. or Type A Influenza, while 11% showed exposure to Newcastle disease (Haley et al. 1984).

No particular diseases or parasites have been associated with the nest. Pre-fledged young have been found debilitated or moribund from infections of coccidia (Eimeria gruis and E. reichenowi; D. J. Forrester pers. comm.).

Exposure

Hail storms, blizzards, and lightning have caused the deaths of a substantial number of birds (90–1,000 at a time; Merrill 1961, Wheeler 1966, Windingstad 1988).

Human Impacts

Large scale mortality of cranes (up to 5,000) has been associated with consumption of mycotoxins in waste peanuts (Windingstad 1988). Aflotoxins and other mycotoxins suggested as the ultimate cause of tumors and other diseases (Allen et al. 1985, Couvillion et al. 1991). Lead poisoning has caused some deaths (Windingstad 1988). Pesticide residues vary, with some individuals having comparatively high levels (Lewis 1974), but pesticides have not been implicated in egg shell thinning (Hoffman 1979) or mortality. Currently pesticides not a significant threat. Collisions with powerlines and fences cause localized mortality, and can be aggravated by snow, fog, or high winds (Lewis 1974, Tacha et al. 1979).

Capture and marking can change behavior and produce mortality. Trapping and marking caused 15% mortality of birds captured in Texas (Tacha et al. 1982). Wheeler and Lewis (1972) reported a mortality of 6.9% associated with capture in Nebraska. Capture with oral tranquilizers produced 3.9% mortality (SAN), which was exacerbated by cold temperatures (Nesbitt 1984). Marking cranes with patagial wing tags can influence post-marking behavior and social status (Tacha 1979). Neck collars can cause mortality (Bennett 1992) but do not otherwise affect behavior (Drewien 1973). Colored plastic leg bands have been used extensively with no effects on behavior (Hoffman 1985); G. c. pratensis in Florida can become habituated to humans and develop a trusting relationship with particular individuals, taking food and becoming quite bold.

Parasites

Twenty-four species of parasites have been associated with G. c. tabida and 22 with pratensis (Forrester el al. 1974, 1975). Significant protozoans are Haemoproteus antigonis, Leucocytozoon grusi, and of Eimeria spp.; several species of trematodes have been found, the most common being Orchipedum jolliei . Most common nematodes are Tetrameres grusi, Strongyloides sp., and Syngamus trachea . Four species of biting lice have been found: Estiopterum brevicephalum, Gruimenopon canadense, Saemundssonia sagulata, and Heleonomus assimilis . Helminth parasites (Iverson et al. 1983) and feather mites (Atyeo and Windingstad 1979) have been reported. Infections with Eimenia sp. occasionally significant (see Breeding: young birds), and cases of disseminated visceral coccidiosis have been reported (Carpenter et al. 1984).

Initial Dispersal From Natal Site

Males more philopatric than females in both migratory and nonmigratory subspecies. Actual values of the relative differences between the sexes and dispersal ranges not reported.

Fidelity To Breeding Site And Winter Home Range

In migratory subspecies, individuals (particularly established pairs) consistently return to the same wintering sites (Tacha et al. 1984) and breeding territories, provided habitat conditions remain suitable (Drewien 1973, Wenner and Nesbitt 1987).

Dispersal From Breeding Site Or Colony

During the post-nesting period, range generally expands among both migratory and nonmigratory subspecies (Drewien 1973, Nesbitt and Williams 1990); this leads into pre-migration staging in migratory subspecies.

Home Range And Territory Size

Nesting territory in Idaho 17 ha, range 10 to 23 ha, n = 5 (Drewien 1973), 85.1 ha, n = 13 in the upper peninsula of Michigan, and 53.5 ha, n = 76, in lower Michigan (Walkinshaw 1973). In central Florida, year-round home ranges average 657 ha (SD = 299 ha, n = 6 prs, Bishop 1988) and 1,366 ha (SE = 393, n = 20, Nesbitt and Williams 1990). Annual home ranges in the Okefenokee Swamp, GA, 93 ha (SE = 26, n = 15, Bennett 1989).

Population estimates and trends have come from direct counts of wintering or migrating birds (see Table 1). Total population estimate for the species is 652,500 to 715,300.

The single, most important factor regulating Sandhill Crane populations is habitat availability. Nesting effort and success, as well as survival of young, correlate directly with the amount and quality of nesting habitat. Mortality associated with hunting also regulates size of the mid-continent population (Sharp and Vogel 1992).