These graphs show the total number of individuals of
each species recorded on all routes in one county
in each year.
Each species has a
four-letter code
(usually the
first two letters of its first name and the first
two letters of the its second name (for instance,
EABL for Eastern Bluebird).
Some species show trends (either upward or
downward) in their numbers over the period of the
surveys. A straight line on a graph
indicates a trend that is statistically significant
(not likely to be an erroneous conclusion) or
almost statistically significant.
**
A double asterisk in the upper right corner
of a graph indicates a highly significant trend (with a
probability of error less than 1%).
*
A single asterisk in the upper right corner
indicates a significant trend (with a probability
of error less than 5% but greater than 1%).
A line but no asterisk indicates a
possible trend (with a probability of error less
than 10% but greater than 5%).
For more information about the N. C. MBBS, see the
MBBS homepage
For a list of the species (with four-letter codes), see
MBBS birds
Please send suggestions to
Haven Wiley>!

Some details . . .
The lines for trends were calculated with procedure lm{stats}
and the graphs were prepared with plot{graphics} in R 2.9.2
for Mac (GUI 1.29) (The R Foundation for Statistical Computing 2009).
The data are available in a text (ASCII) file as a tab-delimited table which
can be read into a data frame in R at
MBBS
data
and the R script is available in a text (ASCII) file at
MBBS
analysis
Although the volunteers who conducted these surveys did a great job of
covering nearly each route every year, there were a few gaps, when a route
was not surveyed one year. Because the numbers of routes in each
county were not high (8-14 routes per county), these missed routes could
have a noticeable effect on the numbers of each species recorded that year.
To reduce this effect, the missing values were filled with the
average number of each species recorded on that route during years when it
was not missed. This procedure would slightly reduce any trend in the
numbers of each species. The numbers of routes missed during the
survey were small (1 in Orange County, 2 in Chatham County, and 1 in Durham
County, each in one year only).

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There is a graph for every species except the rare ones (those species
encountered on the surveys fewer than 5 times on average each year in each
county). These birds were too scarce to analyze.
For information about reading these graphs, see the sidebar to the left.
Notice the downward
trends in Chimney Swifts,
Red-eyed Vireos, Barn Swallows, and Wood Thrushes.
There are upward trends in Fish Crows in Chatham and Durham Counties.
Among the species holding their own, despite nationwide concerns
about their populations, are Eastern Meadowlark and many (but not all)
warblers and tanagers.
Check the graphs for some surprises!

Orange County 1999-2009
Chatham County
2000-2009
Durham County 2002-2009

For information about how these surveys are conducted, see the
Mini Breeding Bird Survey home page.
In summary, the surveys consist of randomly located routes in each county
(12 in Orange, 14 in Chatham, 8 in Durham, the smallest of the three
counties). Observers stop to count birds every half mile along
randomly located 10-mile routes on secondary roads. Although these
surveys do not detect all the birds in a county, they do provide a
reasonaby dense random sampling of the birds. They thus are a basis
for evaluating trends in numbers of birds. Because the surveys take
place on mornings in May and June, they focus on breeding birds.
Because they are made from roads in the first couple of hours after sunrise,
they detect some species more reliably than others. For instance,
nocturnal birds (owls and whip-poor-wills), soaring birds (most hawks and
vultures), and birds of deep forest are no doubt underestimated.
Nevertheless, even these birds are sampled consistently from year to year.

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