Food selection by skylarks and grazing damage to sugar beet seedlings.

Abstract

1. Skylarks foraging on sugar beet fields in spring fed on beetles, weed seeds and weed seedling cotyledons, but they also grazed crop seedlings sometimes causing economic damage.
2. Skylarks preferred to graze seedling species with high nitrogen and phosphorus concentrations. Sugar beet seedlings growing in fields treated with carbamate pesticide aldicarb were less preferred relative to other seedling species than those in untreated fields.
3. The proportion of beetles and seeds in the birds' diet increased with the abundance of these foods and their tendency to graze seedlings declined with increasing weed seed density.
4. The rate at which individual skylarks grazed sugar beet seedlings declined with increasing weed seed but not weed seedling density. However, bird density tended to increase with weed seedling density. Since the densities of seeds and seedlings were closely correlated, the resulting relationship between damage intensity and weed populations was complex.
5. Changes in skylarks' feeding behaviour with changing food density tended to enhance their rate of energy intake. However, a requirement for a minimum intake of certain nutrients, such as nitrogen and phosphorus, may also have affected the birds' foraging strategy.