Estimating population size in Southern Bald Ibis, Geronticus calvus

Catherine L Henderson1,2, Michael J Cunningham3, Res Altwegg4 and Les G Underhill2

1BirdLife South Africa, P.O. Box 515, Randburg;
2Animal Demography Unit, University of Cape Town
3Department of Genetics, University of Pretoria, Pretoria
4South African National Biodiversity Institute, Kirstenbosch, Cape Town

The current population size and number of breeding pairs of Southern Bald Ibis was estimated using colony size and nest numbers. Colony size was measured by evening counts during the breeding season. The number of breeding pairs was determined from mid-breeding-season nest counts during the day. A total of 362 breeding colony sites were reported or inferred from observations over the past 40 years. Between 2007 and 2011, counts were made at 166 of these sites. Of the 114 colonies with both historical and recent data, 37 were found to be abandoned in recent counts. The detection of colonies is comprehensive but we estimate, from gaps in distribution data, that an additional 10 to 30 colonies remain to be discovered (or captured in the database). We assigned size estimates to the 275 predicted colonies by re-sampling from a smoothed, normalized histogram of counts from 124 recently active colonies. This gives an estimated global population size of 7227 +/- 709 Southern Bald Ibis. Given an average count at breeding sites of 3.961 birds per nest, this suggests around 1825 +/- 180 breeding pairs (3650 breeding adults).
Matched historical and recent counts were available for 28 colonies. Of these, 20 colonies declined in size and 22 declined in numbers of nests recorded. Conversely, six colonies showed slight to moderate increases in size and nest counts. In several locations, new colonies have been established in the vicinity of abandoned sites. In spite of this, on average 16% fewer birds and 24% fewer nests were recorded, with an estimated rate of decline of 10% over the past three generations.

Presentation Topic

Estimating population size in Southern Bald Ibis, Geronticus calvus


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Animal Demography Unit, University of Cape Town, BirdLife South Africa, P.O. Box 515, Randburg


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