Project Description :: Mapping Approach :: Species Range Maps :: Species Tables

INTERACTIVE MAPS Northern Region | Central Region | Southern Region




The approach taken to mapping colonies may vary depending on regional or local conditions. Mapping of coastal colonies of seabirds typically is fairly straightforward because the islands, offshore rocks, and sea cliffs used for most nest sites are relatively stable both from year to year and over the long term. By contrast, many islands, marshes, and flooded trees at inland sites may be ephemeral in nature and hence their size or availability for nesting can change substantially from year to year or even over the course of a single nesting season.

For this reason, we generally considered the whole of an individual site (e.g., lake, wetland) as the unit for mapping a colony. Colony symbols, and their latitude-longitude coordinates, are typically located at either (a) a central location within the overall site; (b) where most nesting aggregations, or the largest, typically form; or (c) where colonies were found during a particular survey if knowledge of the dynamics of nesting locations for the site as a whole are not well known or if areas of suitable nesting habitat are ephemeral and not discrete in nature (e.g., agricultural lands submerged by natural, versus intentional, flooding).

One exception to mapping the entire "site" as a colony is when cormorant colonies form where clumps of trees along streams are periodically isolated by high water. In such cases, the individual colony area is mapped, as it would not be informative to map an entire stream or river, as it would be for many lakes or marshes

In some cases we named the colony site as a whole but used a different symbol to map subcolonies in discrete areas within the overall site that are generally available for nesting annually. Such exceptions included lakes or wetlands where more than one island (or sets of islands or islets) are regularly available for nesting (e.g., Mono Lake), large lakes where nesting marshes are located in discrete coves or shoreline segments that typically do not dry out during the nesting season (e.g., Eagle Lake), or large lakes where a combination of islands and marshes or flooded trees in discrete areas are available for nesting more or less annually (e.g., Salton Sea). We also mapped subcolonies within well-defined units or impoundments of state or federal wildlife refuges regardless of whether suitable nesting substrate is generally available each year.

We did not try to map colonies of Black Terns in the extensive area of cultivated rice fields in California's Sacramento Valley both because it was not possible to locate all colonies and because colonies do not tend to form in the same fields year after year.

BACK TO Project Description | Species Range Maps | Species Tables

For more information, contact:
Dave Shuford , PRBO Conservation Science, 3820 Cypress Drive #11, Petaluma, CA 94954; email: