The effect of bill structure on seed selection and handling ability of five species of granivorous birds

Abstract

The relationships between bill morphology and seed characteristics (size and nutrients) with regard to feeding were investigated in five species of bird belonging to three families of granivorous passerines (Estrildidae, Ploceidae, Passeridae): Bronze Mannikin (Spermestes cucullatus), Cape Sparrow (Passer melanurus), Red Bishop (Euplectes orix) and Thick-billed Weaver (Amblyospiza albifrons) and the Bengalese Finch (Lonchura domestica). Birds husked all seeds (Japanese millet, white millet, red manna seed, canary seed, red sorghum and black niger) before ingestion. Monocotyledonous seeds were handled significantly faster than dicotyledenous seeds, and spherical seeds faster than elongate seeds. Handling time also increased with the size of seeds. Birds with medium-sized bills (Cape Sparrows) were most effective in husking most species of seeds. Birds with the smallest bills (Bronze Mannikin and Bengalese Finch) husked the smallest seeds fastest while birds with the largest bills (Thick-billed Weaver) husked the largest seeds fastest. Handling time was affected by size and shape of seeds and bill structure. However, handling time and nutritional intake rate did not determine seed selection. Japanese millet was the preferred seed type for all five bird species although it provided fewer calories per seed and less protein than other seeds. Results suggest that nutrition is less important than morphological traits of seeds: birds generally selected food items that were easiest to handle and could be processed faster, thereby gaining nutrients quickly.