Seasonal Biometric Differences Between Sex and Age Groups of the Graceful Warbler Prinia gracilis at Eilat (Israel)

An abundant resident in Israel, the Graceful Warbler breeds in the northern and central parts of the country and has recently invaded desert areas following their human settlement. Possible seasonal changes in age and sex structure were investigated, as were differences in body measurements in individual sex and age classes as well as changes in the numbers of the Graceful Warbler population in Eilat. No difference was recorded between spring and autumn in the proportion of males to females, nor were significant differences found in the numbers of males and females ringed during the spring and autumn seasons. There was a significant difference in the proportion of juvenile to adult birds trapped in spring and in autumn. Moreover, males had longer wings than females in both seasons. There were no differences in body mass or body condition between sexes in the two seasons. Furthermore, adults had longer wings than first-year birds. Juveniles had longer wings in autumn than in spring, but no differences were recorded in the adults in this respect. In addition, juveniles were in better condition in spring than in autumn; however, there was no difference in body condition of the adults between seasons. The fact that a significant trend was found in the numbers of Graceful Warblers trapped in spring but no such trend in autumn, that a large proportion of recaptured birds were noted in both seasons, and that a high number of individual birds were caught repeatedly during the study period, suggests the existence of a stable or increasing breeding population in Eilat all the year round. In addition, the Bird Sanctuary is like an oasis in the desert environs of Eilat. Hence, the lack of differences among the years in the proportions of males and females between the seasons suggests that it is mainly breeding pairs that occupy the area.