Wildlife Ecology Unit
Goshawk Bioregional Monitoring Design
A Brief Description of the Proposed Bioregional Monitoring
Design For Northern Goshawks
Christina Hargis and Brian Woodbridge
Information on northern goshawk (Accipiter gentilis)
status is generally obtained by observing nesting activity
at local scales. Although nest observations provide
breeding information for specific territories, this
approach is unable to provide information on the abundance
of goshawks over broad spatial extents. To address the
need for information on goshawk population trends, the
Forest Service assembled a working group in 2002 that
developed a design for monitoring goshawks at a "bioregional"
scale (e.g., northern Rockies, Cascade-Sierra, Intermountain
Great Basin). The working group consisted of statisticians
and goshawk researchers within and outside of the Forest
Service. The group was chartered to create a monitoring
design to be implemented on National Forest Service
land, but the Forest Service is interested in collaboration
with other landowners and state natural resource agencies
to provide a more complete picture of goshawk status.
The working group created a monitoring design with the
following objectives: 1) to estimate the relative abundance
of territorial adult goshawks within a bioregion; 2)
to assess changes in goshawk relative abundance over
time; and 3) to determine whether changes in relative
abundance, if any, are associated with changes in habitat.
The sample population for each bioregion is a grid of
square, 688 ha primary sampling units (PSUs) across
all potential goshawk habitat owned or managed by the
collaborating parties. The sampling frame is stratified
to obtain a reasonable estimate of goshawk relative
abundance with an efficient use of funds. The monitoring
indicator is the proportion of sampled PSUs with goshawk
presence, based on the broadcast acoustical survey method.
After the sampled PSUs are surveyed two times (nestling
and fledgling periods), the frequency of goshawk presence
within the bioregion is estimated using a maximum likelihood
estimator. Changes in frequency of goshawk presence
will be assessed after a minimum of five years, using
a logistic model with habitat parameters entered as
covariates. Information from bioregional monitoring
will help determine the status of goshawk populations
and their habitats over a spatial extent that is meaningful
for goshawk conservation.
For each bioregion, the first step in implementing this
design is to identify a bioregional coordinator to oversee
the entire program. The responsibilities of the bioregional
coordinator are to establish the sampling frame, determine
sampling intensity, oversee data collection, conduct
data analysis, prepare annual reports, and administer
the budget. The coordinator can be affiliated with any
agency, research facility, or university.
To establish the sampling frame, the bioregional coordinator
acquires a base map with layers for vegetation, roads,
and landownership. A grid of PSUs can be automated in
GIS using a random UTM as a starting point. All PSUs
that fall on lands owned or managed by the Forest Service
and any other collaborators are identified, and from
these, all non-habitat (non-forested) lands are removed.
The remaining PSUS are assigned unique identifiers and
are the sampling frame for the bioregion.
The PSUs are then stratified into 4 strata:
1) primary habitat, easy to access
2) primary habitat, difficult to access
3) marginal habitat, easy to access
4) marginal habitat, difficult to access
Criteria for primary and marginal, easy and difficult,
are developed by the bioregional coordinator with assistance
from the literature, expert opinion, or modeling. It
is expected that there will be errors in assigning PSUs
to these 4 strata, especially if goshawk habitat is
poorly understood in a bioregion and/or if accessibility
is unknown. Nevertheless, even crude stratification
can provide a more efficient design than simple random
sampling. Systematic or simple random sampling would
result in a large commitment of monitoring funds in
areas that are likely not used by goshawks, with the
inherent risk that little would be learned about goshawk
population status. The purpose of stratification is
to provide a reasonable estimate of goshawk abundance
with an efficient use of funds.
After the sampling frame is stratified, a specific algorithm
(Jim Baldwin, Pacific Southwest Research Station) is
used to 1) calculate sample size and 2) allocate the
sample size among the 4 strata. The equation does not
result in proportional sampling among the 4 strata.
Rather, it allocates the majority of the sample to primary
habitats and to habitats that are easy to access, while
ensuring that marginal habitats and habitats that are
difficult to access are also included. Areas that are
currently surveyed for goshawks tend to be restricted
to primary habitat in places that are easy to access,
so this stratification will ensure that the sample includes
areas with lesser habitat potential and less accessibility.
Each PSU is sampled twice using the broadcast acoustical
survey method developed by Kennedy and Stahlecker (1993)
and Joy et al. (1994). Transects are 250 m apart and
call points are 200 m apart. The surveyors begin with
the best habitat in the PSU and survey until a detection
is made or all suitable habitat is surveyed, whichever
comes first. Surveyors do not need to survey unsuitable
habitat such as cliffs, talus slopes, non-forested areas,
and water bodies. We anticipate 1-7 person days to survey
Each survey results in one of two possible outcomes:
presence (1) or absence (0). After both surveys, each
PSU has four possible combinations of presence or absence
outcomes: 00, 01, 10, or 11. The frequencies of these
combinations over all sampled PSUs are used in an equation
called the "likelihood function", along with
the detection rate coefficients and the probability
of presence. The values of the detection rate coefficients
and the probability of presence that maximize the likelihood
function are used as estimates for those parameters,
hence the name "maximum likelihood estimates."
The estimated probability of presence is the estimate
of how many of the total number of PSUs in the sampling
frame are likely to have goshawks present.
The bioregional plan also describes the collection of
habitat data. A number of variables are collected at
the landscape scale for every sampled PSU, whether or
not a goshawk was detected. From these, an assessment
can be made of landscape variables in PSUs with and
without detections. The variables are: 1) number of
vegetation patches; 2) number of vegetation cover types;
3) size of largest vegetation patch (including patch
area that extends beyond the PSU boundary); 4) percent
of PSU in primary, marginal, and non habitat as defined
by the initial PSU stratification process; 5) estimated
proportion of PSU that has been thinned and/or burned
under prescription in the last 20 years; 5) estimated
proportion of PSU that has been harvested in the last
20 years (commercial thinning, overstory removal or
clearcut) and 6) straight-line distances from the PSU
center to the nearest permanent water (including springs),
road (regardless of use status), trail, and meadow edge.
Forest Inventory Analysis (FIA) data are also useful
for evaluating habitat across the entire bioregion.
It would not be appropriate to correlate goshawk detections
with the nearest FIA point, because the FIA point might
be too distant from the goshawk detection to be meaningful.
Each FIA point covers approximately 6,000 ac. However,
changes in bioregional FIA data over 5 years can be
compared to changes in goshawk prevalence over 5 years,
to see if there is any correlation between habitat data
and goshawk abundance over time.
An important caveat is that the bioregional monitoring
plan provides an estimate of goshawks for the entire
area, but not for any individual forest. The sample
size on any individual forest would be too small to
estimate goshawk prevalence. Because of this, there
is concern that a bioregional monitoring plan might
not meet the legal requirement for monitoring MIS at
the forest scale. However, goshawks within a specific
national forest are not isolated from goshawks on adjacent
forests and other neighboring lands, so "population"
trends for a given forest are likely not meaningful.
We feel that the bioregional monitoring is the best
scale for evaluating goshawk population trends. If each
national forest contributes to the bioregional monitoring
effort, it could be argued that they are meeting their
monitoring requirements. Each national forest in the
bioregion should continue nest observations, however,
at least for a sample of known territories.
This monitoring plan was tested in 2003 on the San Juan
and Rio Grande National Forests in southwestern Colorado.
Twenty PSUs were surveyed, resulting in two detections.
The average cost to survey a PSU was approximately $1,000,
which included training, pre- and post-field mapping,
filling out forms, and report writing, as well as the
actual field work.
There is substantial momentum for implementing the bioregional
design, both within the Forest Service and in several
state agencies. Planning efforts are underway in the
Forest Service Northern Region, in Nevada (a collaborative
effort between Nevada Department of Wildlife and the
Humboldt-Toiyabe National Forest), in portions of the
Forest Service Rocky Mountain Region, and in the Great
Lakes area (multi-states, multi-forests). These efforts
include establishment of the sampling frame and field
testing to provide estimates of detection probabilities.
The goshawk bioregional monitoring design will be distributed
as a Forest Service report, The Northern Goshawk Inventory
and Monitoring Technical Guide. A condensed version
will also appear as a chapter in a forthcoming book,
The Northern Goshawk: A Technical Assessment of its
Status, Ecology, and Management, edited by Michael Morrison
and published in the series, Studies in Avian Biology.
Both publications will be available in 2004.
Kennedy, P. L., and D. W. Stahlecker. 1993. Responsiveness
of nesting Northern Goshawks to taped broadcasts of
3 conspecific calls. Journal of Wildlife Management
Joy, S. M., R. T. Reynolds, and D. G. Leslie. 1994.
Northern Goshawk broadcast surveys: hawk response variables
and survey cost. Pp. 24-30 in W. M. Block, M. L. Morrison,
and M. H. Reiser, (editors). The Northern Goshawk: Ecology
and management. Studies in Avian Biology No. 16, Cooper
Ornithological Society, Allen Press, Inc., Lawrence,