Diet switching of seed-eating birds wintering in grazed habitats of the central Monte desert, Argentina

L MARONE; OLMEDO, M; VALDÉS, D. V.; A ZARCO; LOPEZ DE CASENAVE J; POL, R

THE CONDOR

COOPER ORNITHOLOGICAL SOC

Año: 2017

0010-5422

Although bird population declines associated with land degradation are common, the initial response of organisms to rapid human-induced environmental change is usually behavioral. Reductions in seed availability due to cattle grazing may trigger diet switching in seed-eating birds, but empirical examples of such behavior are lacking. We asked whether cattle grazing changes the composition and reduces the size of seed reserves, and whether seed shortage causes diet shifts in four species of seed-eating birds wintering in the central Monte desert, Argentina. We assessed the soil seed bank composition and the granivorous fraction of the birds? diet. Digestive tract or crop contents were obtained by using the flushing method on mist-netted individuals, and the seeds were sorted and assigned to one of three functional groups (small grass seeds, large and medium-sized grass seeds, or forb seeds). Cattle grazing reduced the abundance of the preferred large and medium-sized grass seeds by 60-90%. The grass-seed specialists Many-colored Chaco Finch (Saltatricula multicolor) and Ringed Warbling-Finch (Microspingus torquatus) did not change their diets in grazed areas, but the expanding specialists Rufous-collared Sparrow (Zonotrichia capensis) and Common Diuca-Finch (Diuca diuca) incorporated increased proportions of small grass seeds as well as forb seeds. These results were correctly predicted from species-specific differences in feeding flexibility established previously in cafeteria experiments. Based on species-specific diet composition, the energy reward of seeds by unit mass consumed decreased moderately (5-21%) in the grazed sites for D. diuca, S. multicolor and M. torquatus. Starch content also decreased by 8-21% for S. multicolor and M. torquatus. Although such deficits might be compensated by a slight increase in absolute mass of seeds or alternative food items consumed in degraded lands, the substantial reduction in the availability of grass seeds may reduce the capacity of degraded lands to support specialist granivorous birds.