Estimation of Survival from Repeated Sightings of Tagged Galahs

Abstract

In a study of the survival of galahs in the wheat belt of Western Australia, 1481 juveniles were individually wing-tagged, either as nestlings or after netting, and over subsequent years resightings and tag returns were recorded. The estimation of a survival curve from a set of resighting and tag return recordings is described. For each cohort of tagged birds a survival curve is estimated and these estimates are compared. Although the initial survival of nestlings varies between years there is no evidence for year-year variation in survival after the 1st Jan. of life. There is agreement between the estimated survival of nestling tagged and netted birds except during a 6-mo. period of the 2nd yr. A single survival curve is estimated by pooling the estimates from different cohorts. The survival rate increases during the first 18 mo. of life but remains constant thereafter. The serious discrepancy between the low estimated survival and the higher survival necessary to maintain a viable population indicates that a proportion of tagged birds must emigrate from the study area and be replaced by untagged immigrants.