The simplest atlases have two weaknesses. First, they provide no measure
of abundance. A rare species that is found to breed once in the study period,
which is usually several years, is mapped just the same as one with thousands of
pairs in the grid cell. Second and more problematic is any bias due to difference
of intensity of coverage across grid cells. Part of the study area inevitably will have
more birdwatchers or easier access, so a higher proportion of the breeding birds
will be found and mapped. This means that you cannot be certain of the degree
to which a mapped range is biased by indicating apparently thinner occurrence
in a region that is actually just less well surveyed. This is a particular weakness
when it comes to comparing maps made at different periods of time. People will
surely want to use any particular atlas for this purpose at some time in the future
and it cannot legitimately be done if there is bias in coverage.
Both these weaknesses can be overcome at the same time by using effort
standardization approaches as described above while remaining within the target
grid cells. Different atlases have used different units (see Table 1.3).
Most atlas studies have chosen a study region, divided it into a suitable grid
and then attempted to cover every grid cell to an adequate level. The outputs
are generally presented as maps with one of several different kinds of dot either
reflecting relative abundance or level of proof of breeding. There is no inherent
reason why atlases should not be constructed from a sampling approach aiming
only to visit a selection of grid squares. Sampling could be random, stratified or
systematic, or an appropriate blend. This approach would have the advantage
of considerable saving of effort compared with the traditional approach, which
Atlas studies| 9Table 1.2Categories of evidence of breeding in European and American bird atlases
Observed Species observed in a block during its breeding season, but no
evidence of breeding. Not in suitable nesting habitat May include
birds such as raptors, waders or gulls far from any breeding area. Used
in America but not Europe
Possible breeding Species observed in breeding season in possible nesting habitat
including single records of song
Probable breeding Pair observed or permanent territory presumed from records at least a
week apart in suitable nesting habitat in breeding season. Courtship
and display or agitated behaviorseen or heard. Nest building or brood
patch observed
Confirmed breeding Nest with eggs or young seen or heard including those not accessible.
Recently fledged young or downy young or distraction display. Adult
carrying food for young or fecal sacs. Recently used nest or eggshells
See sources for fuller treatment (Hagemeijer and Blair 1997; Smith 1990).