AGE RATIOS 



TABLE 15-5 Relation betwe 

 In birds [after Lack 1951). 



Drtality and longe 



beetle, Triboliiim niadens, it is 199 days, in the fe- 

 male, 242 days; in the male T. conjusum, 178 days 

 and in the female, 196 days (Park 1945). 



The rate of mortality in many species varies from 

 one age level to another ; thus, a mean death rate has 

 only general significance. In birds, however, the 

 death rate is nearly constant once they become adult, 

 and it is then apparent that it varies inversely with 

 adult longevity (Table 15-5). In adult penguins, peli- 

 cans, shorebirds, gulls, and swifts, the annual mor- 

 tality rate is commonly between 12 and 30 per cent; 

 in herons, hawks, and owls it is about 30 per cent ; in 

 ducks, doves, and song birds it is between 40 and 68 

 per cent, while in gallinaceous birds it is the highest, 

 60 to 80 per cent. These rates for game species in- 

 clude mortalities from hunting (Farner 1955). 



TABLE 15-6 Theoretical age composition of stabilized popula- 

 tions with three different survival rates, assuming that the rate 

 of mortality is the same for each age group. The figures are 

 the percentage or number of animals (Ix) in each age class in 

 a population of 100 (from Nice 1937). 



Age(x) 



Survival rate 



The life-table gives the number or percent- 

 age of individuals in a brood or litter surviving to the 

 next age level. From such data, as well as by oc- 

 casional direct observation, it is possible to determine 

 the age structure of a population at any one time. 

 Table 15-6 gives the percentage of each age class in 

 populations of adults having three different mean sur- 

 vival rates in all age classes. It is at once apparent 

 that the number of age classes in a population is 

 greater when survival rates are high than when they 

 are low. It is also evident that there is less difference 

 between number of individuals in succeeding age 

 classes when survival rate is high than when it is low. 



The exact age of the sexually mature adult is 

 usually at best difficult to determine unless one can 

 band or mark the young when they first appear, or 

 unless there are growth rings, such as in the scales 

 and otoliths of fish and in the shells of clams, or 

 other criteria of age that can be used. Immature ani- 

 mals are often distinguishable from adults (Thomp- 

 son 1958) so that adult-young ratios are usually ob- 

 tainable, and often yield important information. The 

 proportion of immature to adults is highest at the end 

 of the breeding season, and then usually declines un- 

 til the beginning of the next period of reproduction, 

 because of the higher mortality rate of the young 

 compared with that of adults. In the California quail, 

 the ratio of immature to adults in October was 70 :30. 

 During the following months the ratio progressively 

 decreased as follows : November and December, 

 62:38; January, 58:42; February, 56:44; March, 

 54 :46 ; and the breeding season, April to Tnne. 50 :50 

 (Emlen 1940). 



Age ratio is of practical value in wildlife manage- 

 ment (Alexander 1958). A low ratio of immature to 

 adults indicates a poor reproductive season and 

 should caution against excessive take or yield, as the 

 population is declining. The precarious state of the 

 whooping crane is indicated in that the entire popu- 

 lation of the species wintering on the Aransas Wild- 

 life Refuge in Texas from 1949 to 1953 has consisted 

 of only 3 to 4 young birds each year compared to 21 

 to 34 adults annually present. Low ratios of young 

 to adults also occur with overpopulation, but over- 

 population is usually easily detected. Bag limits may 

 ordinarily be increased if the ratio of young remains 

 consistently high. Here again, however, high ratios 

 of immature to adults are characteristic when popula- 

 tions are recovering from catastrophes. When a pop- 

 ulation of rusty lizards in Texas was reduced by 

 drought in 1954, the percentage of one-year-olds 

 changed from 62i in the relatively stabilized popula- 

 tion to 85 in the subsequent expanding population 

 (Blair 1957). When a population is stabilized, the 



216 Ecological processes and dynamics 



