COMPETITION BETWEEN BIRDS AND MAMMALS - A COMPARISON OF GIVING-UP DENSITIES BETWEEN CRESTED LARKS AND GERBILS

We combined the concept of mechanisms of co-existence with the approac h of giving-up densities to study inter-taxon competition between seed -eating birds and mammals. We measured feeding behaviour in food patch es to define and study the guild of seed-eating vertebrates occupying sandy habitats at Bir Asluj, Negev Desert, Israel. Despite a large num ber of putatively granivorous rodents and birds at the site, two gerbi l species (Allenby's gerbil, Gerbillus allenbyi, and the greater Egypt ian gerbil, G. pyramidum) dominated nocturnal foraging, and a single b ird species (crested lark, Galerida cristata) contributed all of the d aytime foraging. We used giving-up densities to quantify foraging beha viour and foraging efficiencies. A low giving-up density demonstrates the ability of a forager to profitably harvest food at low abundances and to profitably utilize the foraging opportunities left behind by th e less efficient forager. Gerbils had lower giving-up densities in the bush than open microhabitat, and lower giving-up densities in the sem i-stabilized than stabilized sand habitats. Crested larks showed the o pposite: lower giving-up densities in the open than bush, and on the s tabilized than semi-stabilized sand habitats. Despite these patterns, gerbils had substantially lower giving-up densities than crested larks in both microhabitats, all sand habitats, and during each month. Seve ral mechanisms may permit the crested lark to co-exist with the gerbil s. Larks may be cream skimmers on the high spatial and temporal variab ility in seed abundances. Larks may rely on insects, fruit or smaller seeds. Or, larks may rely on adjacent rocky habitats.