Are Birds Declining, or Not?
Why is this Controversial?
You would think that it would be easy to figure out if a species is
increasing or declining. Sometimes it is- there are some species (such as
Northern Bobwhite or Loggerhead Shrike)
that are so consistently declining that few have the gumption
to question it. Most of the time, though, species are decreasing in some
locations (or time periods), but increasing elsewhere- and people who analyze the survey
have to decide 3 issues:
As noted elsewhere on the web site, all of these items are sometimes
difficult to figure. Also, people often are interested in groups of species,
such as grassland-breeding birds, that they think might all be declining due to
some common cause. Analysis of species groups introduces a further
complication: How do we account for the fact that the quality of
our estimates of population change differ among species, and species
tend to have different ranges, so in different regions that species
in the groups can differ? The overall summary statistic given at the start of
this section (51.5 %) is for 421 species, over the entire survey area, for
a 31 year time period. It provides some information, but clearly hides
a lot of detail of importance to managers.
- Is the decline relevant, or is it just due to chance (after all,
populations are never stable, but undergo changes that are natural)?
Often, we do not know enough about natural population dynamics to
interpret an observed population change. And, the nature of the statistical
analysis indicates that occasional estimates of decline occur due to chance,
so an occasional estimate of decline might just represent the vagaries of
- Is there information associated with the declines that is useful
for management- can we assign a cause to the decline, and then use the
information to stop the species from declining?
- Do the declines outweigh the increases- over all the time and space,
is the total population increasing or declining?
So, our philosopy in this summary has been to provide users with all
the information, at (almost) any scale, with a options for analysis
methods. Using these tools, you should be able to either answer the
questions to your satisfaction, or at least completely confuse yourself.
Here, however, we provide some summary information that lets you
know what we think about population change. There aren't a lot of surprises
here; if you've been paying attention, you have seen some of the
recent work on these topics (e.g.,
1997 , Peterjohn et al. 1996, Sauer et al. 1996, James et al. 1996).
With a few exceptions, researchers generally agree on the large-scale
patterns of population change.
Address and telephone:
Patuxent Wildlife Research Center
Last updated 4/23/97
12100 Beech Forest Road, Suite 4039
Laurel, Maryland 20708-4039 USA