Comparison of reproductive success between parent-reared and hand-reared northern bald ibis Geronticus eremita in captivity during Proyecto Eremita

Abstract

Modern zoos actively collaborate in the conservation of many endangered species by captive breeding for reintroduction. This paper presents the reproductive success of a captive colony of northern bald ibis Geronticus eremita (NBI) at ZooBotánico de Jerez between 1993 and 2013 (21 years). Between 2004 and 2011 was “Proyecto Eremita”, a study of the best releasing techniques for hand-reared NBI in Cadiz in order to establish a free-range, self-sustained population in the wild. During this period, first-clutch eggs were artificially incubated and chicks were hand reared until fledgling stage, allowing the pair to produce a second or replacement clutch that was parent reared. This paper compares the colony’s reproductive success between the years where one single clutch was reared by the pairs (n=13) and the years of Proyecto Eremita (n=8), with two clutches, the first hand reared and the second parent reared. The reproductive success rate was measured in 300 nests. A total of 268 fledglings reached 2 months old, the age considered here as the reproductive success. Two reproductive variables were significantly higher during the Proyecto Eremita years: mean number of fledglings per nest (1.8 versus 0.3) and overall number of fledglings recorded per year (26.5 versus 4.3). The reproductive success of hand-reared clutches was similar to parent-reared clutches. There was a significant and negative effect of colony size on the percentage of birds paired and on reproductive success. Parent-reared clutches during the Proyecto Eremita showed a higher reproductive success compared to parent-reared clutches outside this period. The data show that a remarkably high percentage of mating pairs outside of the Proyecto Eremita period failed at reproduction, probably due to density-dependent effects. The combination of hand-rearing and parent-rearing methods used in this study was a very effective tool to significantly increase the number of fledglings produced for the reintroduction programme.