M. Milinski et al., COMPETITION FOR FOOD IN SWANS - AN EXPERIMENTAL TEST OF THE TRUNCATEDPHENOTYPE DISTRIBUTION, Journal of Animal Ecology, 64(6), 1995, pp. 758-766
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.