5. Section Overview—On the Need for Demographic Approaches to Answer Questions

5. Section Overview—On the Need for Demographic
Approaches to Answer Questions
About Conservation and
Management of Migratory Landbirds
Robert J. Cooper
Nadav Nur
In the preceding section, emphasis was placed on monitoring birds over large areas, and the monitoring method
emphasized was the point count. Indeed, point counts are
useful when a large area must be surveyed, or if all that is
required is a simple list of the species present. However, it
is becoming increasingly clear that for many objectives
pertinent to research and management, we must know
something about the demographic parameters of bird populations. By demographic parameters we mean vital rates
such as productivity and survival, and estimates of population increase or decrease obtained from vital rates (Noon and
Sauer 1992). Point counts do not provide these data. Moreover, presence/absence, or abundance, can be a misleading
indicator of habitat quality (Van Horne 1983). For example,
forest fragments occupied by a species may not be suitable
breeding habitat if the individuals detected are mostly
unmated males (Gibbs and Faaborg 1990), or if pairs present
consistently produce no young (i.e., a sink; Pulliam 1988).
Any attempt to relate population viability to landscape
characteristics or to alternative management practices is
also meaningless without some estimate of the vital rates of
the populations in question. That sentiment is echoed by
many of the papers in this section.
Those readers unfamiliar with demographic field work
might initially believe such studies to be beyond their means
in terms of both money and expertise. However: (1) demographic information can often be obtained without nest
finding and monitoring, which are especially labor-intensive; (2) standardized protocols have been worked out and
now exist for several approaches to demographic monitoring; and (3) with all the demographic studies recently initiated, more and more people are gaining the required skills
in nest finding and mist-netting. The British Trust for
Ornithology has demonstrated, for Britain, that demographic
monitoring is feasible and yields valuable insights (Baillie
1990; Peach and others 1991).
There are several general approaches used in demographic
studies. Three such methods—intensive nest monitoring
and netting, nest monitoring alone, and constant-effort
In: Bonney, Rick; Pashley, David N.; Cooper, Robert J.; Niles, Larry,
eds. 2000. Strategies for bird conservation: The Partners in Flight planning process; Proceedings of the 3rd Partners in Flight Workshop; 1995
October 1-5; Cape May, NJ. Proceedings RMRS-P-16. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Robert J. Cooper, Warnell School of Forest Resources, University of
Georgia, Athens, GA 30602. Nadav Nur, Point Reyes Bird Observatory, 4990
Shoreline Highway, Stinson Beach, CA 94970.
USDA Forest Service Proceedings RMRS-P-16. 2000
mist-netting—are represented in this section. A fourth
method, using behavioral cues as an index to reproductive
success, is not covered here but is useful when working with
species whose nests are extremely difficult to find (Vickery
and others 1992).
Approaches ____________________
Intensive Nest Monitoring and Netting
In the intensive approach, investigators work on plots or
study areas of various sizes, attempting to find and monitor
as many nests as possible in the area. In addition, because
it is recognized that many species are capable of producing
>1 brood in a season, adult birds are captured using mist
nets and then uniquely marked with a set of color bands.
Thus, multiple broods can be assigned to the correct female.
Color banding adults also allows annual survival to be
estimated by resighting or recapturing banded birds that
have survived each year (Marshall and others, this proceedings). Examples of this approach represent some of the best
population ecology work published to date on Neotropical
migrant species. They include Nolan (1978), Holmes and
others (1992), Sherry and Holmes (1992), Roth and Johnson
(1993), and others. Most studies are long term (>5 yrs).
Intensive studies, discussed in this section by Sherry and
Holmes, are ideal in many ways because productivity is
measured directly instead of indirectly using mist-netting
or reproductive success. Drawbacks are cost and the need
to focus on a relatively small area (<50 ha). Studies that
compare, say, several different treatments with replicates of
each treatment become extremely expensive and logistically
challenging, usually requiring collaboration among several
partners (Cooper and others, this proceedings).
Nest Monitoring Alone
Clearly, an important component of annual productivity
is nesting success, defined as the likelihood of a nest producing >1 conspecific young. Many studies have therefore
examined just this component of reproductive success. This
approach is best exemplified by the Breeding Bird Inventory
and Research Database (BBIRD) effort. Discussed in this
section by Conway and Martin and by Hejl and Holmes,
BBIRD is a database that includes data on thousands of
nests from numerous locations nationwide. Most of the data
are collected according to a standardized protocol (Martin
and others 1997) on one or more plots between 20 and 50 ha
in size. Data collected by most contributors include nesting
success, habitat measurements taken at and around the
nest as well as at random sites within study plots, and point
counts taken at those same random points. Many but not all
contributors employ BBIRD protocols to study questions
relative to habitat fragmentation or management practices,
among others (e.g., Cooper and others, this proceedings).
For someone initiating a project that includes estimation
of productivity, utilizing BBIRD protocols has several advantages. First, although methods will have to be fine-tuned
for each specific project, a standardized set of variables has
already been established. These include standardized methods for point counts and habitat measurement, which also
yield valuable information. Many aspects of logistics (e.g.,
number and size of plots, crew size and makeup) have also
been worked out. Therefore, the investigator does not have
to start from scratch. Also, much of the BBIRD program
revolves around nest monitoring and measurement. Thus, it
follows the spirit of intensive studies in that productivity of
individual nests is assessed directly. A drawback is that
BBIRD protocols do not include mist-netting and banding
birds, so that while nesting success and productivity of
individual nests are assessed, seasonal productivity is not.
One potential drawback to nest monitoring studies is
that investigators may expend effort to find and monitor as
many nests of as many species as possible, usually on several
plots. This can present a problem in that most species will be
represented by only a few nests which, while they contribute
to the overall database, are too few to make any sort of
population inference in that particular study. Effort expended on those species may detract from effort perhaps
better spent on well-represented species (Hejl and Holmes,
this proceedings). As a result, sample sizes (i.e., number of
nests per species) within plots are often not adequate to
estimate parameters of interest such as nesting success
(Hensler and Nichols 1981; Nur and others 1999). This
problem sometimes can be overcome by focusing effort on the
nests of a few focal bird species and ignoring the rest (Hejl
and Holmes, this proceedings).
Last, conclusions based on nesting success alone can be
misleading because it is only one component of annual
productivity. However, if additional information, such as the
length of the breeding period and the time between nesting
attempts (successful and unsuccessful), is known, then models such as those developed by Pease and Gryzbowski (1995)
can estimate annual productivity. These models also take into
account the fact that brood parasitism precludes renesting as
long as the nest is active, which in some instances can
significantly modify estimates of annual productivity.
Constant-Effort Mist-Netting
Constant-effort mist-netting takes an approach that is
different from the above two. It entails capturing birds in
mist nets using standardized procedures and drawing inferences about productivity and survival from the numbers and
ages of birds captured both among and within years. In the
U.S.A., it is best exemplified by the Monitoring Avian Productivity and Survivorship (MAPS) program, discussed in
this section by DeSante, Rosenberg and DeSante, and Nur
and others. Nests are generally not monitored (although
doing so as a check on productivity estimates is encouraged). Rather, assessment of productivity is based on captures of hatch-year (HY) birds in mist nets. Typical protocols call for >10 12-m nets to be operated throughout the
breeding season, once every 10 days, but in some cases
mist nets are run more often (Nur and Geupel 1993a). The
method assumes that the intraspecific ratio of HY birds to
adult birds is an index to the productivity for that species
in that area (Nur and Geupel 1993a). It therefore bypasses
outcomes of individual nests and instead assesses seasonal
output only.
The method has several advantages. First, it provides
assessment of production of young of the year, integrated
over a potentially large area (the catchment area of the nets).
Second, the productivity index incorporates mortality of
newly fledged young (generally, only independent young are
caught in nets, two or more weeks after fledging; Nur and
Geupel 1993a). And, third, annual survival of adults can be
assessed on the basis of capture-recapture analysis (example in Nur and others in this section).
There are other advantages. First, a smaller crew is
needed. In some locations, a single experienced bander can
operate all the nets. Also, the system lends itself to training
new banders by giving them experience in removing birds
from nets and banding them while the supervisor is nearby.
More than a few ornithologists decided on their career while
under the careful tutelage of an experienced and patient
bander. Also, the system lends itself to public demonstrations of field work with migratory birds, meeting outreach
and education needs along with monitoring and research.
Last, protocols call for point counts and additional habitat
measurements, although these are not as extensive as BBIRD
habitat measurements.
The chief disadvantage of constant-effort mist-netting
involves problems with small numbers of captures in some
habitat types. In particular, HY birds are often captured in
bunches or else are not captured at all. While results are
promising from scrub-shrub habitats, in which mist nets
cover most of the vertical extent of the vegetation (see Nur
and others in this section), similarly detailed data are
lacking from forest habitats. Given the clumped nature of
captures, this is not surprising. This effect can be ameliorated by analyzing data from captures over a larger scale. In
fact, the MAPS program is really not designed to provide
productivity estimates on a small scale (e.g., a 20-ha plot;
DeSante, this proceedings), but rather on a regional scale.
Discussion _____________________
When deciding on a demographic approach, the most
important consideration is the objective of the project
(Ralph and others 1993; Johnson, this proceedings). A
research project will have a different set of objectives than
a monitoring or outreach project. Cost is nearly always a
consideration, too. Each of the approaches summarized
above has advantages and shortcomings; these are discussed in greater detail in some of the papers in this
section. It is always a good idea to discuss potential methods with those investigators currently using them and
perhaps visit a site where the methods are being used.
USDA Forest Service Proceedings RMRS-P-16. 2000
Whereas the intensive approach might be considered
superior because it provides the most information, logistical
constraints may override its usefulness. For those facing
such practical considerations, it is perhaps useful to consider the two standardized methods, nest-monitoring and
constant-effort mist-netting, to be components of the intensive approach. Nest monitoring and constant-effort mistnetting complement each other to some extent (Nur and
Geupel 1993a). Thus, one might start out by using either
BBIRD or MAPS protocols, depending on preference, and
endeavor to integrate the other approach into the study at a
later date.
A cautionary note is also in order. There are problems
associated with estimation of demographic parameters
that potential investigators need to be aware of. Survival
estimation is problematic because of the tendency of longdistance migrants to disperse far from the site where they
were first captured. Although still alive, they are indistinguishable from dead birds if not resighted. This problem,
discussed here by Marshall and others, is so severe for
hatching-year birds dispersing from their natal site that
first year survival is hardly ever estimated. Estimation of
productivity also is problematic because it can vary greatly
both within and among years (e.g., Wilson and Cooper
USDA Forest Service Proceedings RMRS-P-16. 2000
1998). Reproductive success estimated from nests monitored during only a small part of the breeding season, or
from only one year, can therefore be very misleading.
Using standardized approaches is valuable for several
reasons. Large data sets from nationwide sources promote
collaboration. Indeed, some demographic questions are only
answerable over large scales by the joint analysis of several
data sets. For example, variation in population parameters
across a species’ range, regional patterns of productivity or
survival, or other regional comparisons are only possible
with large data sets such as these. Robinson and others
(1995b) provide an excellent example of what can be accomplished by combining data sets from several locations
(Robinson and Morse discuss linking local and regional
demographic studies in this section). In this case, the authors verified what had long been suspected, that reproductive success of forest interior bird species is inversely related
to the degree of fragmentation in the surrounding landscape. However, this result was based on data from the
midwestern U.S.A. only. It remains to be seen if the rest of
us can work collaboratively to combine data for other
regions of North America. Standardized databases such
as these can facilitate this objective if the daunting organizational obstacles can be overcome.