The effect of anthropization on sedentary landbirds occurring in late summer on a small Mediterranean island (San Pietro, SW Sardinia) has been studied comparing abundance and biomass among different ecological guilds. The two specialized insular guilds (Mediterranean warblers and rocky cliff species) represent about half of the total individual abundance (47%) but only 16% in terms of biomass. Although almost all the surface of the island is covered by Mediterranean scrub (maquis), Mediterranean warblers were not as dominant as the guild of strictly synanthropic species. This latter guild was the most represented in frequency both for abundance and biomass. I hypothesize that (i) here the anthropization could be considered a locally relevant process disrupting the resource flow and lowering the trophic level and (ii) the dominance of synanthropic birds in terms of biomass could be a general phenomenon extended to a large set of Mediterranean islands. The three most abundant synanthropic species, linked to highly disturbed habitats, showed a larger body mass when compared to specialized species as Mediterranean warblers: when applying the Abundance/Biomass Comparison (ABC), an early cumulating biomass curve was observed, partially overlapping with abundance curve. This pattern did not match with the classic predictions for the ABC model (i.e., species with higher biomass are typical of undisturbed assemblage) and could be wrongly interpreted. Therefore, I suggest that the ABC assumptions are not universal but limited only to assemblages where high body mass species coincide to species of a higher trophic level.