Meeting Abstract
To understand the physiological mechanisms and constraints associated with migration, direct comparisons of the migratory (Z.l. gambelii) and resident (Z.l. nuttalli) races of White-crowned Sparrow were made under experimental common garden conditions. Populations selected for the study reside at the same latitude (38°N) for 7 months of the year. For residents this period includes wintering and breeding while migrants undergo pre-alternate molt and vernal migration. The study was conducted for 5 months starting in December and included 11 resident and 12 migrant males held in individual registration cages under local photoperiod. Development of the breeding stage for residents was detected in January with elevated plasma androgens that later peaked in February and March along with lengthening of the cloacal protuberance. By contrast, pre-alternate molt started in migrants in February followed in March and April with increases in body mass, fat stores, flight muscle hypertrophy and color change. By mid-April migratory restlessness was observed. All are traits attributed to preparations for, expression of, and coincident with spring migration in wild birds. Plasma corticosterone was similar in both races except when migrants were undergoing pre-alternate molt. Under identical photoperiodic conditions, residents respond earlier to photoinduction culminating in breeding that in the field occurs two months before that of migrants. Therefore time constraints require that migrants prepare and complete migration before breeding and physiological conditions associated with molt affect glucocorticoid secretion both illustrating distinct selective pressures on each race at each stage.
