Results < Number of migrants > Threat assessment

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.

Group Migrants (n) Migration
Major source GIS maps Map sources Comments
Bats 125 Partial Various 16 Baker (1978) insufficient for tropical areas
Terrestrial mammals 43 Partial Baker 1978 12 Inst. Appl. Ecol. 1998 insufficient for populations
Seals 37 Partial Baker 1978 30 Ridgway & Harrison 1981a,b knowledge gaps for rarer species
Whales and dolphins 86 Partial Culik 2001 38 Ridgway & Harrison 1985, 1989, 1994, 1999 major knowledge gaps for small whales
BIRDS: 1658
Passeriformes - Songbirds 579 Partial Boehning-Gaese et al. 1998 14 Various Americas, Europe, Africa-Asia; only threatened species for South East Asia and Australasia (Bird Life Int´l 2001)
Waterbirds 283 Complete del Hoyo et al. 1992-2001 76 del Hoyo et al. 1996,
Scott & Rose 1996,
Johnsgard 1983 (cranes)
point data for Eurasian Anatidae (Scott & Rose 1996); flyways from various sources
Plovers, lapwings 159 Complete 34 del Hoyo et al. 1996  



Maps used for GROMS on a global scale, mostly without flyways, knowledge gaps for exact distribution and migration of certain populations and/or subspecies.

Maps at higher scale and high resolution data available, but only for well-monitored regions (North America, Europe, Southern Africa, Australia and parts of Australasia)

Seabirds 295 Complete 173 del Hoyo et al. 1992, 1996  
Raptors 117 Complete 104 del Hoyo et al. 1994  

Pheasants, bustards, sandgrouse, buttonquails

31 Complete 11 del Hoyo et al. 1994, 1996  
Swifts and hummingbirds 48 Complete 13 del Hoyo et al. 1999  

Bee-eaters, rollers, kingfishers

24 Complete 0 del Hoyo et al. 2001  
Cuckoos 42 Complete 1 Curry-Lindahl 1981  
Hoopoes 1 Complete 1 Curry-Lindahl 1981  
Nightjars 28 Complete 0    
Owls 14 Complete 0    
Pigeons 15 Complete 1 Grzimek 1980  
Woodpeckers 10 Complete 0    
Parrots 9 Complete 0    
Turtles, crocodiles 10 Complete   8 Iverson 1992;
Nesting beaches only for Indopacific (UNEP-WCMC 1999), only as printed maps
"PISCES": 874
Fishes 874 Partial FishBase:
Froese & Pauly 1998
19 F.R. Germany 1999
severe knowledge gaps for migration in tropical rivers

not covered

Urquharta 1960 0   severe knowledge gaps for subtropical/tropical insects, crabs and shrimps (Crustacea)

Identification of migrants, GIS map coverage and used information sources for major animal groups. For exact taxonomic definitions of groups and further details, see the specific sections on particular taxa (4.3). Besides true migrants, GROMS includes potential migrants, where migration behaviour is not well known.

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).

25 The migratory butterfly Danaus plexippus has been included, because it is listed on CMS Appendix II.
26 A computer animation of the complex movement patterns is available on the GROMS-CD.
Results < Number of migrants > Threat assessment

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