4.1 — Number of migrants

Table 4.1 gives an overview of the number of migrants and map coverage for major groups. The designations of these common names were made according to common usage and the organisation of major specialist and conservation groups. The organisation of data files on the CD has been designed according to these categories. Though 'unscientific', these groups proved to be robust and facilitate rapid orientation for non-specialists. Migration status was assessed according to the information sources cited in Table 4.1. 'Partial' coverage means that reliable identification of all migrants within a group was impossible, due to widely scattered or inhomogeneous information sources, or simply major knowledge gaps. This is the case for mammals. 'Complete' designates satisfactory assessment, which was only achieved for non-passerine birds, using the maps and descriptions of migratory movements given by del Hoyo et al. (1992-2001). But even this comprehensive compilation does not cover details such as migration routes, or differences of migration behaviour within a species. The migratory status of fishes has been assigned in cooperation with Fishbase (Froese & Pauly 2000), according to major reviews (McKeown 1984, McDowall 1988), but coverage is incomplete for tropical species.

In short, the answer to the frequently asked question for the number of migratory species can still not be answered precisely; the number of migratory species lies somewhere between 4000 and 6000 (see also Figure 2.1). Besides problems of definitions and insufficient knowledge, taxonomic uncertainties affect the estimate considerably. Taxonomic rank of subspecies/species is a constant matter of debate, not only for birds, but even apes and whales, and the number of tropical invertebrate species can only be estimated.

The smallest taxonomic unit within the database are "populations", as different populations exhibit different migratory behaviour. However, this led to a considerable number of logical inconsistencies, due to unclear or even contradictory definitions of such populations or "stocks". Many subspecies listed by del Hoyo et al. (1992-2001) are considered as species by Sibley & Monroe (1991, 1993). This requires administration of parallel taxonomies, because most maps are based on del Hoyo et al., while species authority files stick to Sibley & Monroe (1991, 1993). An example are the petrels, mapped in Figures A2.56, A2.57). In this case, subspecies are easily identified. However, there are other maps and distribution data which do not differentiate clearly between populations, which is simply due to the fact that "given the current information available [for waterfowl] it is rarely possible to define ideal populations" (Rose & Scott 1997, p. 8). In other words, populations do not always refer to well-defined, "real" biological units, but to scientific opinions. Clear geographic and biological delimitations of populations still require a considerable amount of research. Therefore, taxonomic authority files below the species level are lacking or tentative. Subspecies and population names contained in the GROMS are based on Rice (1998) for marine mammals, del Hoyo et al. (1992-2001) for non-passerine birds, Rose & Scott 1997 for waterbirds, and Hilton-Taylor (2000) for stocks and subspecies differentiated by the IUCN Red List 2000.

In spite of these difficulties, a differentiation of subspecies and populations is necessary for an adequate description of migration behaviour, which often varies on the population level. Figure A2.42 illustrates the diversity of migration patterns within one species, the sandhill crane (Grus canadensis).The respective subspecies exhibit practically all types of migration, from "long-distance" (Greater sandhill crane - Grus canadensis tabida) to "non-migratory", resident populations (e.g. Cuban sandhill crane - Grus canadensis nesiotes).²⁶ A considerable number of songbirds show a "leapfrog pattern" of migration, where populations of higher latitudes "jump" over less migratory populations (Elphick 1995).

This document should be quoted as part of the publication "Riede, K. (2001): The Global Register of Migratory Species ­ Database, GIS Maps and Threat Analysis. Münster (Landwirtschaftsverlag), 400 pp." + CD

 by Klaus Riede