Limitations of the Atlas Data
Although the data on this CD are the most comprehensive available as of 2000, it is virtually certain that additional breeding species could, with more prolonged or focused effort, be found in a majority of the atlas units. Some species are simply too difficult to detect on a consistent basis. Species distributions also change with time. Thus, atlas data should never be used in lieu of detailed, current surveys of a particular area. Data from this atlas project should not be used to infer presence of absence of species in a unit at other times of the year.

Data are also limited by the fact that in most units, atlasers were unable to visit every habitat present because many habitats were inaccessible due to private ownership, difficult terrain, or distance from roads. Also, during the period of the project, land cover changed dramatically in some of the units, in some cases due to residential development and logging, and in other instances due to wildfires.� We did not attempt to quantify these changes.

Species totals for the 93 hexagons that are only partly in Oregon should not be compared with other hexagons, because the area surveyed was smaller and thus almost inevitably will contain fewer species. Other limitations of the data are noted in the section Data Quality Assurance and as footnotes in other parts of this CD.

Limitations of the Habitat Maps and Habitat Associations Information

This CD describes each species� level of association with potentially 60 habitat (vegetation community) types, and then provides a map of possible habitat of the species at two scales.� In the species accounts on this CD, the levels of association are labeled �strongest,� �strong,� �some,� and �lesser,� which are intended to reflect strength of association of the species with that habitat while nesting or feeding during a breeding season with average annual weather conditions. �Not all levels are assigned to habitats of all species.

These qualitative terms used to describe the levels of association do not correspond to particular levels of statistical significance obtained from any data analysis, nor to any measurements of frequency of nesting or the ability of particular habitat types to sustain populations of a species.� We assigned these relative levels of association to each species by first noting which vegetation types the species is considered to use according to the ONHP, and then examined scores (on a 0-3 scale) assigned to a similar but condensed list of 28 habitat types produced during workshops of Pacific Northwest avian experts and included on the CD in the book, Wildlife-Habitat Relationships in Oregon and Washington (Johnson & O�Neil 2001).� Then, our project coordinator �cross-walked� the information in this book and the ONHP database to the more detailed 60 habitat types we use on this CD, and the project steering committee reviewed and recommended many refinements based on their collective birding experience.� Finally, using digital maps of these 60 types provided by the ONHP (see below), we created a map of habitat possibly used by each species in each hexagon that is within its expected geographic range in Oregon.� Note that these maps do not show where the species was ACTUALLY observed -- only where any habitat type classified as �strongest,� �strong,� or �some� for the species is reported to exist according to the digital maps provided to us by the ONHP.� For this and other reasons we prefer to use the term �possible habitat� rather than �habitat� to describe these maps.�� Limitations of these maps include, but are not limited to, the following:

1. The choice of these particular 60 classes for representing avian habitat associations in Oregon is somewhat arbitrary.� We do not know the degree to which birds perceive habitat the same way humans do, e.g., whether 20 or 80 or 200 vegetation classes would do a better job of representing breeding site selection.� Indeed, many species may select breeding sites based on non-vegetation factors, such as climate and topography, but such factors were not explicitly considered in coming up with these 60 classes.� Rather, these classes represent perhaps the maximum number that could be interpreted by the ONHP staff using the satellite imagery available to them.

2. The satellite imagery on which these classes were based is from the early 1990s, thus preceding this project.� Distribution of some habitats undoubtedly has changed since then.

3.� The units of mapped habitat (polygons) almost always contain some internal mix of vegetation types.� For example, a polygon labeled and mapped only as �Lodgepole Pine Forest and Woodland� may in fact contain some �Wet Montane Meadow,� �Recently Cutover/Burnt Forest,� and �Lava/Pumice,� especially if these are present in contiguous patches smaller than 247 acres.� Internal heterogeneity of the mapped habitat polygons is expected to be greatest in southwest Oregon and in areas recently subjected to resource extraction or development.

4.� The accuracy and consistency of ONHP�s delineations of the vegetation classes has not been tested statewide (Kagan et al. 1999).�