Influence of seed size on feeding preferences and diet composition of three sympatric harvester ants in the central Monte Desert, Argentina Abstract Selective seed consumption by harvester ants may affect seed abundance and composition and, ultimately, plant communities. We evaluated the influence of seed size on preferences and diet of Pogonomyrmex mendozanus, P. rastratus, and P. inermis in the central Monte Desert, Argentina. In choice experiments with Pappophorum spp. seeds of different sizes, P. mendozanus and P. rastratus preferred large seeds, maximizing energy reward. P. inermis showed a less-marked preference for large seeds, which was probably due to morphological constraints imposed by its small body size. Under natural conditions, none of the three species selected larger Pappophorum spp. seeds probably because of high travel and handling costs. Seeds of intermediate size predominated in the diet of the three species but a slight size match was detected as P. mendozanus carried larger seeds than P. rastratus and this than P. inermis, matching body-size differences. Thus, ants probably maximize energy reward but face morphological restrictions and higher costs when carrying and holding large seeds. While seeds of intermediate size are the most vulnerable ones to ant predation, small seeds are favored, as they are abundant in the soil seed bank and lowly predated.

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Habitat degradation caused by cattle grazing may be a serious threat for seed-eating birds because the availability of beneficial seeds usually diminishes in grazed areas. Ecologically plastic species might, however, circumvent food deprivation via changes in foraging behaviour. We studied the limits of feeding flexibility and factors affecting seed preferences in Zonotrichia capensis, Diuca diuca, and Saltatricula multicolor. We experimentally assessed preferences for seeds of eight grass and eight forb species by using a protocol that combines choice and non-choice trials, and employed a different batch of experiments to evaluate some plausible causes of different feeding flexibility. On average, birds consumed 45-140% more grass than forb seeds, confirming previous results. Z. capensis preferred several grass and forb seeds, and showed maximum feeding flexibility. S. multicolor and, to a lesser extent, D. diuca, were grass specialists that preferred large and medium-sized grass seeds. The size of forb seeds did not affect preferences. Coat thickness of grass seeds did not seriously reduce consumption levels. Birds showed low ability to feed on resources characteristic of degraded environments (i.e. annual grass seeds). Species-specific differences in behavioural flexibility could be used to predict dietary and numerical responses of seed-eating birds to habitat degradation.