Changes in numbers and biometrics in relation to season in sedentary Yellow-vented Balbul Pycnonotus xanthopygos at Eilat: A ringing study

The Yellow-vented Bulbul Pycnonotus xanthopygos is an abundant resident throughout Israel but no natural history and demographic studies on the species have been published to date. We analyzed the ringing data for eight years at the Bird Sanctuary of the International Birding and Research Centre in Eilat (IBRCE). We compared the changes in biometrics of the individuals that live in the desert region and their annual breeding cycle to that reported for other parts of Israel, as evidenced through the ringing data. During the 15 trapping seasons we caught a total of 1105 Yellow-vented Bulbuls. The largest numbers of Yellow-vented Bulbul ringed was in May in the spring and November in the autumn trapping period. Body mass and body condition index for adult birds were significantly lower in spring than in autumn. We suggest that the reduction in body mass is a result of the reproductive effort and is gradually improved through the summer and in autumn as a result of the lack of stress and abundance of fruits in the Bird Sanctuary. Conversely, juvenile body condition did not differ between seasons and is results of parental care during the spring and summer months and fruit abundance in the autumn. This study illustrates how ringing data from one location over many years can serve to understand the natural history and biological processes of sedentary species.