Breeding Success and its Correlation with Nest-Site Characteristics: A Study of a Griffon Vulture Colony in Gamla, Israel

Gamla Nature Reserve once held the largest colony of nesting and roosting Griffon Vultures (Gyps fulvus) in Israel, with 45 to 57 pairs nesting at the colony during our study years (1998-2002), and up to 140 individuals roosting on the canyon's cliffs. Nevertheless, the fledging success there was very low: only 34% of breeding attempts (nest with eggs laid) resulted in fledged young during our study. Poisoning and hunting were the main causes of mortality, but in addition, a shortage of appropriate nesting places may also have been an important limiting factor. Fledging percentage was correlated with nest-site use and "attractiveness": nest sites with greater fledging percentage also had more breeding attempts and were inhabited earlier in the nesting season. The main physical characteristic that enhanced breeding success was the type of nest site; nests in caves were more successful and were used for more breeding attempts than nests that were exposed from above. The influence of microclimatic conditions on nesting success was emphasized by the differences in the intensity of parental care, particularly activities associated with thermoregulation, between parents at the different types of nest sites. Parents at exposed nests invested substantially more time in thermoregulation (i.e., brooding or shading the young), an investment that was negatively correlated with breeding success.