Songbird Protection Coalition
In biology, the definition of recruitment is when juvenile organisms are added to a population, it occurs when individuals survive to be settled and are able to be detected by a human observer. Age-specific survival, status-dependent survival, survivorship to breeding-age, and breeding-range all must be considered to determine recruitment rates in migratory bird populations.
Sandhill cranes evolved a strategy to live long (20+ years) and have a very low recruitment rate of young into the population. Their biology favors a low survival of juvenile colts and a high survival rate of adult breeding birds (Tacha et al. 1992, Drewien et al. 1995, Drewien et al. 2008)...and this slow rate of recruitment has resulted in a very long and slow recovery from near extirpation (i.e. extinction) in Michigan.
In addition to disease, hunting, hailstorms, lightening, poisoning by the ingestion of lead ammunition and tackle and other toxins, predation, avian tuberculosis, and collisions with power lines; several biological factors can determine low recruitment rates in cranes.
The sandhill crane is slow to reach maturity and reproduce for the first time. Michigan is part of the Eastern Population (EP) of sandhill cranes (which demographically include include the Great Lakes Region of Wisconsin, Michigan, Minnesota and Ontario). For the EP of sandhill cranes, the average age for first territory defense and successful reproduction was 4.5 years old for females and 5.1 years for males (Nesbitt 1992).
For breeding pairs, cranes that successfully fledged young for the first time ranged from age 3 years to 10 years old. Females were productive at an earlier age with an average at first year of successfully fledging young being 4.75 years, while for the males it was 7.14 years. And young are known to remain dependent and accompany their parents south as a family unit, returning together in the spring to their "natal site of origin" or place of birth at 10 to 12 months of age.
Age ratios in fall surveys of sandhill cranes have been used for many years as an indication of recruitment across a population. During fall recruitment counts, the number of adults and juveniles in a flock are counted and these data are used a measure of recruitment, however the published ratios are often different and simply not comparable due to inconsistencies and therefor attempts to standardize data were compiled (Drewien et al. 1995). Recruitment ranged from 5 to 14% for several different sandhill crane populations. Among greater sandhill cranes the non-hunting Eastern Population (EP) had the highest average recruitment among several populations at 12% as averaged from several EP staging areas. Drewien et al. (1995) suggested that recruitment rates of 5-10% are necessary for population maintenance at current levels.
Populations studies - while notoriously inconsistent - also indicate the EP of sandhill cranes is stabilizing and leveling out at present numbers, due to wetland habitats reaching self-regulating natural carrying capacities within prime breeding habitats. Michigan is at the northern most breeding range and our recruitment rate falls within the necessary percentage to simply maintain the current population status.
Sources including but are not limited to: United States Fish and Wildlife Service, International Crane Foundation, Ad Hoc Eastern Population Sandhill Crane Committee.