Selection of foraging sites by desert granivorous birds: vegetation structure, seed availability, species-specific foraging tactics, and spatial scale

MILESI, F.A.; J. LOPEZ DE CASENAVE; V.R. CUETO

AUK

American Ornithological Union

Año: 2008 vol. 125 p. 473 - 484

0004-8038

Abstract: We conducted a bird-centered analysis to determine the foraging microsites of the granivorous birds in the algarrobal habitat of the central Monte Desert, in trying to understand their interrelationship with plants. To evaluate the spatial selection at the microsite scale (1-m radius), related to the heterogeneity of the seed bank, we compared the structural and floristic characteristics of microsites centered where a granivorous bird was detected foraging (= use) with those of 60 microsites randomly located in the same area (= availability). Variables estimated included horizontal cover of perennial plants and litter, vegetation height and vertical complexity, from contacts on a thin pole erected on 10 points on each of four perpendicular transects. We did the same analysis on very similar variables characterizing the patches at a bigger scale (mesosites: 10 m radius). The matrix of random sites vs. standardized variables was subjected to a PCA to reduce the dimensionality of the data set, and the used microsites where then situated on the space generated by the main components by calculating their scores on each axis. Selection was evaluated graphically (scatterplots) and statistically (Kolmogorov-Smirnov tests). Granivorous birds used the whole range of microsites available in this habitat, even when the quantity and composition of seeds available differ greatly between them. Even so, the probability of seeds of being removed is not the same depending on their location. The microsites used for predispersal consumption (standing seeds) have more cover of shrubs, grasses and litter than those randomly located. Those used for postdispersal consumption (soil seed bank) do not differ in any of the important dimensions. So there are no safe microsites for seeds after dispersal. At the mesosite scale, birds did not use those patches with the lowest shrub and litter covers, independently of the kind of consumption. All these patterns found for the whole guild of granivorous birds depend strongly on the species-specific foraging strategies of the birds, since there are species typically involved in predispersal granivory and those removing seeds from the soil. Also, the spatial use changes with the seasons, since the relative importance of each bird species changes due to migratory movements and diet variations, and the main type of seed consumption changes with plant phenology. These results are discussed in relation to the known patterns of spatial heterogeneity and dynamics of the soil seed bank and bird habitat selection at bigger scales.