Woodland birds are bird species that depend on native wood-
lands. Unfortunately, woodlands have been widely cleared for
agriculture and urban development, leading to a widespread belief
that woodland birds must be declining.
Many have studied the decline of woodland birds, most
commonly the effect of changing tree cover and fragmentation.
The results of these studies vary. Some ind evidence of decline;
others dispute that a decline is taking place.Similarly, the nature of the relationship between woodland
birds and tree cover and fragmentation varies substantially too.
These differences might be due to regional or scale differences
between studies, but could there also be underlying disagreement
about what actually constitutes a “woodland bird”?
In ecology, there have been sporadic efforts to promote consis-
tency in terminology but little progress. Inconsistent terminology
can lead to a range of problems, including diiculties in inding
relevant studies, redundant investigations and an inability to
synthesise across studies. It can also create problems when commu-
nicating indings to other scientists, policy-makers and the public.
How important is consistent terminology when it comes to
determining the conservation status and trends of a group of birds
loosely referred to as “woodland birds”? To answer this question,
I led an investigation that systematically reviewed the literature and
compiled a set of 38 lists of woodland birds. This allowed us to work
out how consistently each species was classiied as a woodland
bird.
We found that eight species were always classiied as wood-
land birds and 13 species were always classiied as non-woodland
birds. The remaining 144 species were sometimes classiied as
woodland birds and sometimes as non-woodland birds. This
surprised us, as we had expected that only a few, less-understood
species would be classiied inconsistently.We surveyed the authors of the papers we had reviewed and
found that the main reasons researchers classify different species
as woodland birds were:- different aims of research: researchers tailor their list of
“woodland birds” to include species they expect to respond
most strongly to the phenomenon they are interested in; - disagreement about what a woodland is; and
- disagreement about how to determine which birds depend
on woodlands.
What impact does this have? Colleagues have previously
used a subset of species that they considered to be woodland birds
to model the effect of habitat aggregation (the inverse of habitat
fragmentation) on the occurrence of woodland birds. We re-
ran their model, irst using the entire complement of species and
then using different subsets to emulate the effect of being
increasingly selective about which species are considered wood-
land birds.
We found that as we become more selective about which
species are included, the estimated effect of tree cover aggre-
gation increases. Our analysis revealed a systematic bias in results
whereby studies that are less selective about which species are
woodland birds are likely to obtain different results (probably
with lower effect sizes) than those that are very selective about
their classiication. In other words, how you deine woodland
bird species has an important bearing on the results you obtain.
What does this mean? When comparing results from studies
using different classiications it’s impossible to know whether
differences are attributable to data collection, the survey area
or analyses, or whether they are due to differences in classii-
cation. This essentially renders all studies with non-identical lists
incomparable.
This is particularly problematic when trying to understand
woodland bird ecology or predict how they will react to manage-
ment. Only a small subset of research uses identical lists of wood-
land birds, so researchers must choose between including all
available information (which risks confounding results due to
differences in classiication) or only including studies that use the
same list of woodland birds (which risks the exclusion of valu-
able insights from other studies).
So what should we do? Our analysis leads us to suggest that
woodland bird researchers should unite behind a single deini-
tion and list of woodland birds. We also believe that this is an
approach that would be beneicial to ecology as a whole.
It’s unlikely that woodland bird research is the only realm of
ecology where terms are being used inconsistently and clouding
results. If our indings hold true for other terms, we believe that
it’s extremely important to develop a consistent deinitions of
these terms.
Hannah Fraser is a member of the ARC Centre of Excellence for Environmental Decisions
(CEED). She was based at the University of Melbourne for this research.
48 | MAY 2016
ECOLOGIC Hannah FraserWhat’s in a Name?
Inconsistent classification of species introduces
systematic bias to ecological studies.Is the black-faced woodswallow a woodland bird? Its name
suggests it is, yet it is classified as a woodland bird in only
37.5% of lists. Credit: Eric Vanderuys