Abstract In small Mammals biogeography, available data are up to now by far too scanty for elucidate the distribution of a lot of taxa, especially with regard to the absence from a given area. In this respect, standardized quantitative sampling techniques, like Owl pellets analysis can enable not only to enhance faunistic knowledges, but also to estimate the actual absence probability of a given taxon "m", lacking from the diet of an individual raptor. For the last purpose, the relevant frequencies of "m" in the other ecologically similar sites of the same raptor species diets are averaged ($f_m$ ): the relevant standard error (multiplicated by a coefficient, according to the desired degree of accuracy, in relation of the integral of probabilities) subtracted ($\overline{F}_m - a E$): then, the probability that a single specimen is not pertaining to "m" is obtained ($P_0 = 1 - F_m + a E$); lastly, the desiderate accuracy probability ($P_d$) is chosen. Now, "$N_d$" (the number of individuals of all prey species in a single site needed for obtain, with the desired probability, a specimen at least of "m") is obtained through $$N = \frac{\ln P_d}{\ln P_0}$$ Obviously, every site-diet with more than "N" preyed individuals and without any "i" specimen is considered to be lacking of such taxon. A "usefulness index" for the above purposes is outlined and checked about three raptors. Some exanples about usefulness of the Owl pellet analysis method in biogeography are given, concerning Tyto alba diets in peninsular Italy about: - Sorex minutus, lacking in some quite insulated areas; - Sorex araneus (sensu stricto, after GRAF et al., 1979), present also in lowland areas in Emilia-Romagna; - Crocidura suaveolens and - Suncus etruscus, present also in the southermost part of Calabria (Reggio province); - Talpa caeca, present also in the Antiapennines of Latium (Cimini mounts); - Talpa romana, present also in the middle of Tuscany; - Clethrionomys glareolus, lacking in few small, insulated areas, but present in coastal woodlands; - Microtus (Pitymys) sp., apparently lacking only in some coastal areas of Toscana and Liguria, or in some Etna slopes (Linguaglossa). Riassunto Un metodo standardizzato e quantitativo di raccolta dei dati quale l'esame delle borre di rapaci consente non solo di integrare le conoscenze faunistiche, ma pure di tentare una valutazione della probabilità di assenza di un dato taxon da un'area geografica. In proposito, vengono presentati esempi originali circa Sorex minutus, Sorex araneus, Crocidura suaveolens, Suncus etruscus, Talpa caeca, Microtus (Pitymys) savii e Microtus (Pitymys) multiplex in Italia peninsulare.