Seed hardness together with maximal bite force determines
which part of the available food resources a bird is able to use.
Species with a maximal bite force that is higher than the seed
hardness range are able to eat all seeds. When maximal bite
force falls within the range of seed hardness a bird will also
pick up seeds that are too hard to crack. Time spent handling
seeds that have to be rejected because they are too hard leads
to a decrease in food intake rate. High percentages of rejection
of seeds occur in the field (Grant, 1981; Greig-Smith and
Wilson, 1985). Laboratory experiments with Java sparrows showed that birds do use size cues as an indicator for seed
hardness to avoid picking up seeds that are too hard to crack,
even when the correlation between seed size and hardness is
very low (Van der Meij and Bout, 2000). Selective uptake of
seeds has been reported for other species as well (Hespenheide,
1966; Willson, 1972).