Summaries of Population Change for Birds by Habitat
Following the approach developed by Gregory and van Strien (2010), State of the Birds reports focus on composite summaries of population change for collections of species that share common characteristics such as a common primary habitat biome or taxonomic affinity. In this report, we provide composite indexes for biome obligate species as defined in earlier reports (Grassland, Aridland, Eastern Forest and Western Forest), for several taxonomic-based groups (Shorebirds, Waterbirds, Geese and Swans, Sea Ducks, and Dabbling/Diving Duck species), and for species on our Tipping Point list with adequate data. Lists of species included in each group, surveys used for their analysis, and summary trend estimates are presented as supplemental material.
This report describes population change for 259 species of North American birds summarized from 5 surveys: the North American Breeding Bird Survey (BBS, 176 species, Sauer et al., 2020), the Christmas Bird Count (CBC, 60 species, Meehan et al., 2020), the Waterfowl Breeding Population and Habitat Survey (WBPHS, 14 species, U.S. Fish and Wildlife Service, 2021), the American Woodcock Signing-ground Survey (SGS, 1 species, Seamans and Rau, 2021), and International Shorebird Surveys (ISS, 9 species, Smith and Smith, 2022). All of these data sources were used in Rosenberg et al. (2019), and we refer readers to that publication for additional details on the surveys. For each species, annual indices of abundance were obtained from published sources (BBS, WBPHS, SGS) or from data managers (CBC, ISS). In this analysis, we used results from the time period 1970–2019 for all surveys except for the ISS, for which results were only available for 1980–2019.
Statistical analysis of composite summaries follows methods used in earlier State of the Birds reports. A quantitative description of the statistical model was published in Sauer and Link (2011). It employs a hierarchical model, for which input data are collections of estimates of population change for a species (at the survey-wide scale of summary) from a base year (1970 or 1980) to each subsequent year. For each year, the collection of actual population changes for all species from the base year to the year of interest are assumed to be normally distributed on the log scale, and the latent mean change for the collection is estimated. The model is fit to all years post-base year, and the resulting model-based means form the composite trajectories of change for the species group. The ratios of the annual indexes of each year, divided by the annual index of the base year, was used to estimate the cumulative change in the species population for that year. See Sauer and Link (2011) for additional details regarding the model and its fitting to BBS and other survey data.
On Alert and Tipping Point Species
Scientists for the Road to Recovery initiative have identified 90 On Alert bird species in need of strong and immediate scientific action to pinpoint causes of declines and to support practitioners dedicated to recovering their populations. These birds have high vulnerability to extinction and steep population declines as described below, with 50% or more of their populations lost during 1970–2019.
Of the 90 On Alert species, 70 are Tipping Point species that show continued or accelerated recent declines that if continued could lead to the loss of 50% or more of their populations in the next 50 years (high urgency)—or they have perilously small populations, high threats, and insufficient monitoring data (presumed high urgency/data deficient).
We relied first on data already available in the Avian Conservation Assessment Database (ACAD)—a database maintained by Partners in Flight (PIF) and housed at Bird Conservancy of the Rockies. We categorized the species based on high vulnerability to extinction, steep population decline, and high urgency, as described below.
High Vulnerability to Extinction
Vulnerability in the ACAD is assessed by carefully scoring a series of independent factors (Population Size, Breeding and Nonbreeding Distribution, Threats, and Trend) that are combined into a single Combined Conservation Score (CCS) that ranges from 4 to 20 (see the ACAD Handbook for a thorough description). Species that meet a threshold of CCS > 13 are considered to be highly vulnerable and are placed on the ACAD Watch List. Species with CCS ≥ 16 show high vulnerability across multiple factors and constitute the Red Watch List.
Steep Population Decline
Based on the latest long-term population trend data for 529 U.S./Canada species (Rosenberg et al., 2019), we identified those species that are estimated to have lost 50% or more of their total adult breeding population since 1970. This group of species are assigned a Population Trend (PT) score = 5 in the ACAD and include many Watch List species. Trend data have been updated through 2019 for this report.
To assess urgency, we examined the most recent population trajectories for each species, based on the most recent analysis of BBS and other survey data. This analysis mirrors the survey data used to assess trends for 529 species in Rosenberg et al. (2019) and have been updated through 2019. Notably this analysis includes a complete re-analysis of shorebird trend data by Paul Smith (unpublished 2021). By comparing long-term trends (back to 1970 for most species; to 1980 for shorebirds) with the most recent population trajectories (using a 3-generation period to define “recent” period), and examining trend estimates in light of the 2022 State of the Birds composite results, we identified species in two urgency categories, as follows:
Species of High Urgency
species with large long-term population loss (>50%) and with continued or accelerated recent declines that exhibit a “half-life” < 50 years when projecting forward the most recent 3-generation trend.
Presumed High Urgency/Data Deficient
Poorly monitored species that are believed to be declining have been assigned PT scores of 5 or 4 in the ACAD via expert opinion; for some of these, the population trend is completely unknown. This additional set of species are defined by a combination of small population size (PS = 4, 5) and high threats (TB or TN = 4, 5) and have expert-assigned PT scores of 5, 4, or 3. These species are marked with an asterisk on the Tipping Point Species list.