Seed preferences suggest a high vulnerability of the Yellow Cardinal (Gubernatrix cristata) to habitat degradation in Argentina

ABSTRACT The Yellow Cardinal is a globally endangered species endemic of southern South America. Knowledge of its general biology has increased in recent years, but its feeding ecology is less well-known. Assessing the feeding ecology of endangered species imposes ethical dilemmas, and we therefore took advantage of Yellow Cardinals rescued by rangers from illegal trafficking in the Monte desert, Mendoza, Argentina, to experimentally assess their seed preferences by using two different seed sets to test the hypotheses that (1) the birds prefer medium-sized and large seeds of native grass species, which are the seeds that are most reduced by cattle grazing, and (2) birds are not able to handle and consume the large seeds typical of non-native crop species. Choice and non-choice experiments were combined to assess the seed preferences. In the experiments with eight native seeds <1 mg (4 grasses, 4 forbs) the birds preferred the medium-sized and large grass seeds over the forb seeds. In the experiment with eight grass seeds (4 native, 4 commercial; 0.23–33 mg) birds had difficulty in handling the heaviest seeds, with a threshold between 7 and 25 mg. This difficulty might explain the positive association of the Yellow Cardinal with wild savannahs and xerophytic shrubland but not with agricultural fields, since common crops have large seeds (40–350 mg). Given the species affinity for habitats that are often used for livestock ranching, overgrazing should be included as a source of habitat degradation due to the reduction in key food resources.