Food selection by skylarks and grazing damage to sugar beet seedlings.
Observations were made on sugar beet fields in Cambridgeshire and West Suffolk, UK, in April-May 1977 and 1978 from emergence of seedlings until they were 17-25 days old. Skylarks (Alauda arvensis] foraging on the fields fed on beetles, weed seeds and weed seedling cotyledons, but they also grazed crop seedlings sometimes causing economic damage. Skylarks preferred to take seedlings species with high nitrogen and phosphorus concentrations. Sugar beet seedlings growing in fields treated with the carbamate pesticide aldicarb were less preferred relative to other seedling species than those in untreated fields. The proportion of beetles and seeds in the birds'diet increased with the abundance of these foods, and their tendency to take seedlings declined with increasing density of weed seeds. The rate at which individual skylarks took sugar beet seedlings decreased with increasing density of weed seed but did not increase with increasing density of weed seedlings although bird density tended to increase with density of weed seedlings. Since the densities of seeds and seedlings were closely correlated, the resulting relationship between damage intensity and weed populations was complex. Changes in feeding behaviour with changing food density tended to increase their rate of energy intake. It is suggested that a requirement for a minimum intake of certain nutrients, such as N and P, may also have affected the skylarks' foraging behaviour.