MAPS: TRIPLE DENSITY TO SHOW ABUNDANCE, MULTI-COLOURED TO SHOW SUB-SPECIES

Maps that show the distribution of each species are an essential and almost universal feature of modern field guides. Northern hemisphere guides often use colours to show the breeding range (usually summer), winter range and where the two overlap, a year-round range. 

In Australia, with no large region of sub-zero winters, more birds are sedentary, or locally nomadic only. Breeding records of many species are sparse, especially in the more remote regions, and for small birds with well hidden nests

In this guide, density of map colour is used to show abundance within the known range, the colour depth corresponding with density of records of the species—the deepest tone is where the species has most often been recorded. Blue is used if there is but one race of the species in Australia, or usually for the nominate race when there are several, the others in brown, yellow and green.

The Atlas of Australian Birds (Royal Australian Ornithologists Union, 1984) is a widely used source of bird distribution data. This book was the result of a 5-year survey of birds by some 3000 members and other ‘atlassers’. As in the fictitious example at left, an open circle indicates, by its size, the number of sightings for each bird species; a solid circle marks where breeding was observed. Another survey is being undertaken between 1998 and 2002. The Atlas of Victorian Birds (Dept of Conservation, Forests, Lands, and RAOU, 1987) similarly shows the distribution of birds within Victoria.


Many Australian birds have several separated or merging populations, perhaps with hybrid forms where they meet. If these populations are sufficiently different, then each may be identified as a separate subspecies or race. On these maps, merging or intergradation of races is shown by the blending together of the colours.

 Opinions differ widely on the number, name and distribution boundaries of the races of many species. Changes to species and races need to be soundly based and widely accepted


FEATURES OF THE MULTI-COLOURED, TRIPLE DENSITY MAPS USED ABOVE

1 MID BLUE: where a species could reliably be expected to occur—where records are not extremely sparse or isolated, nor represent a part of the bird’s range that is, apparently, no longer occupied—are coloured mid blue (or a mid tone of another colour for a subspecies or race). [Note that ‘race’ and ‘subspecies’ are used interchangeably.]

2 PALE BLUE: areas where it is unlikely, but possible, that the bird could be seen are a paler tint. This pale tint shows the species’ occurrence where it has been only very rarely recorded, perhaps being a rare vagrant, or a doubtful sighting, or of long-past occurrence.

3 DEEP BLUE:areas where the species has most often been recorded: these are localities with the greatest number of sightings in the RAOU Atlas and other surveys, and in historical and other records. These may not be where the species actually occurs in greatest numbers, but rather show where most bird observers have been successful in recording the species. Often, as well as having a high population of the birds, the locality is easily accessible. This is where anyone who wants to see the bird is most likely to succeed—especially if time is limited.

 

Distribution data from these and other sources has been used to some extent by most guide and reference book authors, but with various interpretations, so that maps differ substantially from book to book. Some follow very closely the pattern displayed in the atlas type of bird distribution survey. Others are based on diverse references, both recent and historical. Often maps are partially based on the author’s personal experience or research; others again might be based on, for example, museum records.

Some distribution maps include isolated and doubtful sightings far from the main range and fill the gaps between to present a broad interpretation of the range of the species. In parts of the range of that species, there will be very little chance of finding the species—but such areas of the map might not be differentiated from those parts where the bird is common.

In other maps the same bird records may have been interpreted much more conservatively, with many sparse, vagrant or doubtful records being rejected , so giving a map showing a far more restricted range. The bird would almost certainly occur throughout that range, but there is no indication that the species has, if only on rare occasions, or perhaps in early records, been observed far beyond the conservative range shown.

 

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