Experimental evidence for the maintenance of foraging specializations by frequency-dependent choice in flocks of spice finches

Three hypotheses can account for consistent foraging specializations across individuals: (1) food source variation, (2) phenotypic differences and (3) frequency-dependent choice. We examined which explanation was more relevant for flocks of spice finches (Lonchura punctulata) exploiting environments where food could be obtained using two different skills. The food source variation hypothesis predicts that individuals will specialize when the use of two skills is more costly than specialist foraging. The phenotypic differences hypothesis proposes that individuals differ in their ability to use each foraging skill and specialize on the most profitable one. The pattern of specialization is expected to be stable although the number of individuals that use a given skill depends on the phenotypic composition of the flock. The frequency-dependent choice hypothesis also proposes that individuals specialize on the most profitable skill. However, the profitability of each alternative decreases as the number of foragers that use it increases. The hypothesis thus predicts that an equal number of phenotypically identical foragers would gradually specialize on each skill when initially given two equally profitable alternatives. At equilibrium, individual payoffs should be independent of the pattern of specialization. Individuals in flocks of spice finches adjusted their use of the two skills and two birds in each flock specialized on a different skill. We failed to find a significant relationship between specialization and intake rate at the end of testing. The results are compatible with both a variant of the food source variation hypothesis, where individuals experience the cost of generalist foraging differently, and the frequency-dependent choice hypothesis.