CONICET | Buscador de Institutos y Recursos Humanos

Weevaluated whether seed mass, handling time, handling efficiency andprofitability account for (a) preferences in controlled experiments,and (b) field-diet composition of four bird species of the Montedesert, Argentina. The question of whether birds maximise theirenergy intake rates while feeding on seeds is assessed. We usedfeeding experiments with six native seed species of 0.07 0.75 mg(i.e. the seed-size range consumed in nature), which account for 0.59 0.84 of the field diet of the four birds. We measuredseed-handling times, and used published information on birdpreferences and diets, and on seed chemistry, for furthercalculations. Bird preferences were always positively related to seedmass, and also to seed profitability in the two intermediate-sizedbirds. Diet composition correlated positively with seed mass andnegatively with seed profitability in three species, but some birdsalso showed a flexible behaviour eating the most attractive seedsaccording to their availability. This behaviour is not genuinelyopportunistic because it only focuses on a restricted fraction of thetotal seed species present in the field. Contrary to expectations ofspecies coexistence due to resource partitioning, small and largebirds showed similar feeding efficiencies when eating the smaller andthe larger seeds. The positive association between seed mass andprofitability in several studies suggests that most birds canmaximise their energy reward, on average and in the long term, bypreferring the larger seeds. A combination of potential feedingoptimisation with certain flexibility in the field may characterisethe feeding ecology of desert seed-eating birds.