Plausible causes of seed preferences and diet composition in seed-eating passerines

We evaluated whether seed mass, handling time, handling efficiency and profitability account for (a) preferences in controlled experiments, and (b) field-diet composition of four bird species of the Monte desert, Argentina. The question of whether birds maximise their energy intake rates while feeding on seeds is assessed. We used feeding 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 measured seed-handling times, and used published information on bird preferences and diets, and on seed chemistry, for further calculations. Bird preferences were always positively related to seed mass, and also to seed profitability in the two intermediate-sized birds. Diet composition correlated positively with seed mass and negatively with seed profitability in three species, but some birds also showed a flexible behaviour eating the most attractive seeds according to their availability. This behaviour is not genuinely opportunistic because it only focuses on a restricted fraction of the total seed species present in the field. Contrary to expectations of species coexistence due to resource partitioning, small and large birds showed similar feeding efficiencies when eating the smaller and the larger seeds. The positive association between seed mass and profitability in several studies suggests that most birds can maximise their energy reward, on average and in the long term, by preferring the larger seeds. A combination of potential feeding optimisation with certain flexibility in the field may characterise the feeding ecology of desert seed-eating birds.