COMPETITION FOR FOOD IN SWANS - AN EXPERIMENTAL TEST OF THE TRUNCATEDPHENOTYPE DISTRIBUTION

1. Ideal free models for unequal competitors predict a truncated compe titor phenotype distribution among patches when relative payoffs of ph enotypes vary across patches. Partial truncation is expected under fie ld conditions. This prediction was tested in a field experiment with a n overwintering population of mute swans (Cygnus olor). 2. Two food pa tches were generated in which adult and subadult swans were expected t o have different relative success. In one patch ('clumped'), pieces of bread were thrown over a small area on the water surface. In the othe r patch, pieces of bread were scattered over a larger area. 3. When on ly one patch was offered at a time (no choice situation), adult swans were more successful than subadult swans in the 'clumped' patch, but w ere similarly successful in the 'scattered' patch. Relative payoffs of adult and subadult swans differed significantly between patches. 4. W hen the two patches were offered simultaneously, black-headed gulls (L arns ridibundus) competed with the swans to a considerable extent in s ome replicates. The gulls appeared to be the poorest competitors. They snatched more bread in the scattered than in the clumped patch. Both classes of swans avoided the scattered patch but not the clumped patch with increasing competition from gulls. Gulls preferred the scattered patch and swans increasingly preferred the clumped patch under gull c ompetition, creating a partially truncated distribution. 5. Without th e four replicates in which the gulls had consumed more than 25% of the bread, the percentage of adult swans that chose the clumped patch was significantly higher than the percentage of subadult swans that chose that patch. This is the first experimental verification of a partiall y truncated phenotype distribution. Subadult swans significantly prefe rred the scattered patch whereas adults tended to prefer the clumped p atch. This distribution was predicted from the 'no choice' experiment, where competition by gulls had been similarly weak.