Abdim’s Storks Ciconia abdimii were studied in a post-breeding staging area dominated by Sahelian grasslands in southeastern Niger. In late September 2003, a total of 5,763 birds was recorded on systematic transect counts. The estimated density was 5.66 per km2, equivalent to a total of c. 17,000 Abdim’s Stork within the utilized area of 3,000 km2. A high percentage of juveniles in the flocks in late August, when the Nigerien breeding population is still at their nests, indicate a southern origin of the birds. The storks probably make northward post-breeding movements into the Sahel to feed on locally abundant food supplies, first of all Senegalese Grasshopper Oedaleus senegalensis, the dominant grasshopper species in the area. The mean density of Oedaleus in late September 2003 was 11.2 per m2 (6.7 g fresh weight/m2). Storks feeding on grasshoppers made on average 5.6 successful pecks per min during their first h of daily feeding, enabling them to fulfil 95% of their daily energy requirements within one h. In 2005, Oedaleus densities were only 5.1 per m2 (2.8 g fresh weight/m2). Foraging storks made on average 2.5 and 1.9 successful pecks per min during the first and second h of daily feeding, respectively, and even two h of active foraging allowed them to fulfil only 70-75% of their daily needs. As a probable consequence, the storks left the area earlier than in 2003. Flocks of Abdim’s Stork are able to significantly reduce grasshopper densities in frequently used areas, but their impact on Oedaleus numbers within the 3,000 km2 utilized area as a whole is probably negligible. However, the impact of the storks should not be viewed in isolation.
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Vol. 31 • No. 2