By Bill Bousman

Santa Clara County Bird Checklist, 30 Sep 2016

The present checklist, as with past ones, is based on observations of birders, both in the historical record and in records provided to me since 1980. The latter records have been written down in the county notebooks and since 1993, the notebooks have become Word files. These records are selective. First, most observers make their own decisions on what observations they believe are important. Second, in transferring records to the notebooks, I am also selective on what records are included. Because of this process, the majority of records are for uncommon to very rare.

The present checklist, for the most part, shows how common or uncommon a species may be using five different lines to distinguish between (1) common, (2) fairly common, (3) uncommon, (4) rare, and (5) very rare birds. The terms apply to birds seen in Santa Clara county in appropriate habitat, although sometimes it is difficult to be certain how that habitat is defined. For the most part these distinctions are qualitative and based on observer experience. Birds beyond the very rare are typically shown by solid circular symbols. For these species a symbol is usually representative of a single bird. If the bird remains beyond a day, then two symbols are used, representing the first and last days. If the time the bird is present is sufficient, then a very thin dashed line is used to connect the first and last symbols. If a species is seen at a similar time in another year, then the symbol is either raised or lowered to show it is a different bird. As more data is obtained, there comes a point when the symbols are replaced with a dashed line appropriate for a very rare species.

The bar-graph nature of the checklist seeks to show the temporal variation of each species over a typical year’s period. This is most easily done with species that are rare, very rare, or extremely rare as most of our observational data in the notebooks is for these species. For common to uncommon species, it is necessary to use large systematic data sets. These may be from banding operations, organized census data obtained over long periods of time, and single-observer census data. These data sets need to be well balanced over a year’s time, have good resolution, and include many counts. In the best of cases they can provide a good definition of the temporal variation for more common birds, but in some cases, we don’t have a good understanding of how numbers of any one species change over the year.

Analysis of temporal variation tends to be more quantitative than the distinctions from common to rare discussed above. About 200 of our 400+ species have been analyzed and I’ve included a sample of the analysis for the Western Tanager as an Excel spreadsheet: Western Tanager_multitab_cum.xls. An uncommon to very rare species over a year’s period, this analysis provides a good representation for this species in its four roles as spring migrant, summer breeder, fall migrant, and wintering bird. Although this bar-graph checklist attempts a uniform approach to the distribution of birds in the county, it has limitations. A number of species of ducks and other waterbirds are found in summer that show injuries. Such injured birds probably should not be included on the checklist, but there are some occasions that there are no apparent injuries. Another problem is represented by a few species that are highly erratic in their occurrence. They may be found common in one year and absent in another. The analyses I use for these species represents a long-term average, yet there is no such thing as a typical year. Those interested in such variation should look instead at a plot of year-to-year numbers, rather than a simple bar-graph.

Bill Bousman
Menlo Park
5 Feb 2017

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