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State of the Birds Report

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Surveys & Analysis Methods

Introduction

In our analysis of the State of the Birds, we rely on results from three major surveys that collectively provide estimates of population change for almost 500 species of North American birds over the past 45 years at a continental scale.  The Waterfowl Breeding Population and Habitat Survey (WBPHS, Smith 1994) has been routinely used to provide status information for duck species for over 50 years (e.g, U.S. Fish and Wildlife Service 2014, http://mbdcapps.fws.gov/devp_new/mbdc/databases/mas/disclaimermas.html). The North American Breeding Bird Survey (BBS, Sauer et al. 2013) provides estimates of long-term population change for over 420 species, and trend estimates, summaries for groups of species of interest such as grassland-breeding birds or Neotropical migrant birds, and other results are available on the survey website (www.mbr-pwrc.usgs.gov/bbs). The Christmas Bird Count (CBC, Butcher 1990) has only recently been analyzed using modern statistical methods (Link et al. 2006), and analyses have been conducted for selected species and species groups (e.g., Niven et al. 2004); some basic results are provided on a website (www.audubon.org/bird/cbc).

For species that occur in the three primary surveys, we conducted summary analyses to provide a time-series of composite population change for a variety of species groups. Although incomplete (summaries are limited by lack of coverage or limited data for many species), these analyses document change for species that are presently monitored in North America. Here, we provide a summary of the data and methods used in the summary analyses.

Background: State of the Birds Summaries

In North America, a variety of approaches have been used to evaluate the status of birds. Most commonly, status has been assessed through summary of individual species trends from a variety of surveys (e.g., The National Audubon Society State of the Birds Reports: http://stateofthebirds.audubon.org/, Bildstein et al 2008). The U.S. Fish and Wildlife Service provides yearly status assessments for waterfowl and other harvested species, and presents a “total ducks” estimate that sums the counts from survey results for selected species to provide a composite population trajectory (e.g. U. S. Fish and Wildlife Service 2008). For the BBS, both individual species change estimates and composite summaries of long-term population trend for a variety of species groups (See http://www.mbr-pwrc.usgs.gov/bbs/trend/guild12.html for estimates of change; and see http://www.mbr-pwrc.usgs.gov/bbs/guild/guildlst.html for a list of the groups) are routinely presented. Sauer and Link (2002) described a hierarchical model that accommodates uncertainty in the estimation of population change for each species and is fit using Bayesian methods.

Gregory et al. (2005), Buckland et al. (2005), and Gregory (2006) describe methods for monitoring change in biodiversity and apply the methods to United Kingdom bird populations. They define groups of species thought to be indicators of a specific aspect of the environment, and estimate composite change for the indicator group from a “base year” (usually the first year). Change in population from the base year to each subsequent year is calculated from survey data for each species in the group, and composite change is estimated as a geometric mean of the individual species change estimates. These yearly estimates of total change from the base year provide a comprehensive view of change in status for the group. The population in the base year is scaled to 100%, and the total percentage change from that year (usually the first year of the survey) to any other year is displayed as a graph. Each point in the graph is a composite estimate of change relative to the base year.

In the State of the Birds report, we adopt the Gregory et al. (2005) approach of providing a composite summary for species indicator groups, and we estimate a similar quantity (i.e., change from the base year to any subsequent year for the collection of species). However, to accommodate the variation in quality of estimates of change among species, we use a hierarchical model, applied to estimates of change from the base year to each subsequent year, to estimate overall change for species groups.

Indicators and Species Groups

Gregory et al. (2005) provide a rationale for selecting species as indicators; in the United Kingdom, they developed indicators for farmland and woodland birds. For the North American analyses, we developed indicators for a variety of species groups. The primary groups were associated with major North American habitats:  arctic and alpine, aridlands, forests (further subdivided into boreal, eastern, western, and subtropical), grasslands, oceans, and marsh. For these broad habitats, we defined all species that occur in these habitats and a subset of species that are restricted to these habitats (obligate species). In the State of the Birds report, the estimated overall change for these obligate species is used as the indicator for these habitats. We also estimated change indicators for a few groupings of other species of management interest such as arctic landbirds, arctic geese, wintering coastal birds, waterfowl, wetland birds, and urban birds. Wetland and urban bird categories are large and heterogeneous, comprised of birds that occur in wetland or urban settings within their primary habitat types.

In the 2014 report, we focus on several of these indicator groups, and also discuss other groups of management interest.

We recognize that any grouping of species is bound to stimulate discussion (e.g., Mannan et al. 1994, Sauer et al. 1996). Our groupings were based on a consensus of opinions of the science team for the State of the Birds, and represent an initial attempt to summarize bird species into groups that capture relevant patterns of population change in North American birds. We encourage further discussion about alternative groupings.

Survey Descriptions and Estimation of Population Change

The hierarchical model for State of the Birds summaries requires estimates of population change (and their variances) from the base year (which we chose as 1968 to correspond to the year in which the BBS was fully implemented in the continental United States) for each species. Estimation of population change was conducted using methods best suited for each survey.

Waterfowl Breeding Population and Habitat Survey

The WBPHS is an aerial survey, in which an extensive sample of transects across the north-central United States, Alaska, and Canada are surveyed by observers in fixed-wing aircraft ( http://www.fws.gov/migratorybirds/NewReportsPublications/PopulationStatus/Waterfowl/StatusReport2014.pdf). Over most of the survey area, these results are augmented by an intensive survey of segments of the transects by ground- or helicopter-based survey crews, and a double-sampling estimation procedure is used to estimate total number of indicated breeding pairs for the surveyed area (Smith 1994). We used survey results estimated for the regulations process (U.S. Fish and Wildlife Service 2008) in this analysis. To generate estimates of population change from 1968 to all subsequent years, we used the yearly estimates from the published survey results, generating change estimates and precision from the ratios of the yearly indexes and their precision.

Christmas Bird Count

The CBC is a winter survey, in which volunteer observers conduct a day-long count within a 15 mi diameter sample unit, or “circle.” Counts are conducted within the period 14 December to 5 January. See Butcher (1990) for a summary of survey methods for the CBC. Link et al. (2006) described a hierarchical model for summary of CBC data. In this hierarchical log-linear model, variation in effort is controlled using a 2-parameter adjustment that varies by species. The model is fit using Bayesian methods implemented in program WinBUGS (Lunn et al. 2000). Niven et al. (2004) and Sauer et al. (2009) used this approach for summary of a variety of species from CBC data. Calculation of the population change for the survey from 1968 to any subsequent year can be calculated directly during the analysis as ratios of the estimated annual indices of abundance.

As the CBC is a winter survey, the population indices are indexed at a different time of year than the BBS and WBPHS. For the coastal wintering indicator that is based only on CBC data, the index refers to the winter count. However, to integrate CBC data with other survey data, we needed to use CBC data to predict changes that would have occurred between the breeding season counts. We interpolated change to the breeding season from the CBC data. Our CBC analysis was based on data through 2012.

North American Breeding Bird Survey

The BBS is a roadside survey conducted in June. An observer drives a 24.5 mi route, stopping each 0.5 mi and conducting a 3-min point count. The sum of the counts by species is used as an index to species abundance on the route. The BBS was implemented in the continental United States in 1968, and is routinely analyzed using route-regression methods (Sauer et al. 2013). In this analysis, we used a log-linear hierarchical model that was developed for BBS data (Link and Sauer 2002) and has been implemented for several years in conjunction with the older route-regression analysis. Comparative analyses of the long-term trend results based on this hierarchical model show consistency with the historical analysis (Sauer and Link 2008a, 2008b); implementation of the new analysis method will occur using the 2008 data. Use of the hierarchical model allows convenient estimation of the total change from 1968 to any subsequent year. Data were available to 2012 for the BBS. We note that BBS data from Alaska is not of sufficient quality for inclusion in the summary analysis, due to limited data prior to 1990.

American Woodcock Singing-Ground Survey

The American Woodcock Singing-ground Survey (SGS, Cooper and Rau 2012)  is a single-species survey conducted by the FWS and CWS to index American Woodcock. It is conducted earlier than the BBS (in April), and although it is also a roadside survey the routes are surveyed in the evening, and have only 10 stops to allow the survey to be conducted during the primary activity time for woodcock breeding displays. The survey was initiated in 1968, and data through 2012 were used in the State of the Birds analysis. The analysis methods used for the BBS were also used for SGS analyses.

Shorebird Data

This report contains a composite index of population change for migratory shorebirds based on data collected from monitoring sites in southern Canada and the United States. These data were only available from 1974–2012; 1974 is the base year for the composite analysis. Data analyses for these species was conducted by the Canadian Wildlife Service, following the same general methodology as for other habitat indicators (Adam Smith, Canadian Wildlife Service, Personal Communication).

Which Survey Provides the Best Information for Each Species?

All of the surveys used in this analysis provide information of varying quality for many species. Relatively few species are surveyed over their entire range during the breeding season, which is the generally preferred period for surveying birds, and many birds migrate south of the CBC survey area in winter. J. E. Fallon (USGS Patuxent Wildlife Research Center, Unpublished data) estimated the proportion of the breeding range of each species that is covered by the BBS and the proportion of the wintering range that is covered by the CBC, and Niven and Butcher (unpublished data) evaluated the relative efficiency of the BBS and CBC in terms of (1) portion of range covered by each, (2) relative sample sizes of species in each survey, (3) the mean abundances estimated from the survey and (4) the precision of the estimates from a long-term trend analysis of each survey. Smith (1995) provided a qualitative evaluation of the precision of estimates from the WBPHS, categorizing results from “excellent” to “poor.”

We used the Smith (1995) and Niven and Butcher (unpublished data) analyses to categorize the survey to use for each species in the analysis, with a few exceptions. First, we reevaluated the Niven and Butcher results to be more conservative in the use of CBC data, as the interpolation process needed to combine CBC data likely decreased the accuracy of the population change estimates for that survey. Of course, for most species these considerations are trivial (e.g., the BBS contains no data regarding Arctic species); the choice of survey is obvious. For other species, it is clear that the choice is arbitrary, and several reasonable estimates are available for each species. Unfortunately, no simple methods exist for combining information among surveys, although method development is occurring (e.g., Link et al. 2008, Niven and Butcher, unpublished analysis). As we note below, we expect that future reports will contain joint analyses of survey data.

Many additional surveys could contribute to the State of the Birds summary. A variety of waterfowl surveys, for example, contribute information for yearly regulations setting (U.S. Fish and Wildlife Service 2008). Although the report incorporates information from many surveys, we restricted our composite estimates in this report to the consistently collected information from the three primary surveys. 

We also note that adequate survey data sets are lacking for many species, and the three primary surveys used here–BBS, CBC, WBPHS–will never adequately sample these populations (e.g., arctic-nesting shorebirds, secretive marshbirds, ocean birds, some colonial waterbirds). Data to derive trend lines comparable to those generated by the primary surveys awaits the development of new surveys, and comprehensive data management and distribution systems for existing survey data.

Estimation of State of the Birds Summaries

Gregory et al. (2005) and Gregory (2006) describe the rationale for the summary analysis. Because the collection of species in our analyses tends to vary greatly in quality of information, we used a hierarchical model similar to that described by Sauer and Link (2002), but assuming a lognormal distribution of counts. To fit this model to a collection of species, we first estimated the total change from the base-year (1968) to all successive years for each species. We then applied the model to data for each collection of population changes (e.g., for all species for the interval 1968-1969, for all species for the interval 1968-1970,… for all species for the interval 1968-2012), to estimate a mean change for the interval. As the analysis was conducted using Bayesian methods, the analysis results in estimation of the Posterior Distribution of the mean change; we used the median, the 2.5% and the 97.5 percentiles of the distribution as our estimate of the composite change and its credible (Bayesian confidence) intervals.

This analysis provides estimates of change that are consistent with earlier evaluations of composite change for groups such as grassland-breeding birds (Sauer et al. 2013). Analyses using this approach have been presented for BBS data (Butcher et al. 2008a, Sauer et al. 2008a, Sauer et al. 2008b), and for CBC data (Butcher et al. 2008a, Sauer et al. in press).

Cautions and Concerns

It is our hope that the State of the Birds analyses will stimulate additional research and method development. Here, we identify issues that we believe would benefit from additional research:

• The process of developing relevant groupings of species needed to adequately show the state of North American Bird populations is very much an ongoing one.  This State of the Birds report has slightly modified species groupings from those presented in the first report, reflecting advances in the selection of indicator species.

• Many species are presently poorly monitored in North America, and often little information exists regarding the species of greatest concern. However, the summaries only incorporate information from species with adequate monitoring data. Clearly, bird conservation requires information for all species, not just the easy-to-monitor collection from which we presently draw inference. Bird conservation initiatives have focused their efforts on identifying species for which limited data exist and initiating new monitoring efforts to increase our understanding of the populations of these species (e.g. Dunn et al. 2005).  As these efforts provide additional information, they should be incorporated into State of the Birds reports, and this report includes data from surveys of migrating shorebirds.

• The time period we use (1968-2012) is constrained by available data. Our base year of 1968 has no particular relevance, and we caution users that this year may not be a particularly relevant year for comparison of later change. More consideration should be given in future reports to choice of base years that have management or ecological relevance.

• More detailed consideration of selection of the best survey results to use for species, and development of methods to aggregate information among surveys, should provide better estimates of change for many species. There are many surveys that collect data on birds in North America; our focus on the WBPHS, BBS, and CBC reflects our need for consistent, long term data that were adequately managed and analyzed. Species-by-species reviews of existing data such as that recently conducted for birds of prey (Bildstein et al. 2008) and comparative analyses of survey efficiency (e.g. Niven and Butcher, Unpublished analyses) assist in judging the most reliable data for each species and should provide better guidance for selection of survey results to include in the State of the Birds summary.

• Development and enhancement of statistical and computational methods for summary and display of primary survey results is also needed. Summaries of most recent BBS and CBC results are in progress, and will result in revised websites that better display patterns of population change from these surveys.  This State of the Birds project has demonstrated the need to jointly analyze and evaluate data from these important surveys.

• It is our hope that the publication of the State of the Birds report will encourage discussion about metrics for summary of bird population change. We chose to apply the Gregory et al. (2005) approach in this summary, but a variety of other possible metrics could be used. For example, the “total ducks” summary used in Waterfowl Status reports (U. S. Fish and Wildlife Service 2008) is a useful metric that reflects the number of total ducks available for harvest. Other metrics include the number of species in the group with increasing populations (Sauer and Link 2002). It would be beneficial to provide several alternative metrics that address different components of population change and appropriately display the uncertainty in the estimates.
 

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