by Chris Johnson

At some point in a birder’s journey, you may ask yourself the following question: Who is monitoring bird population trends and what might I learn from the collected data? The answer to the first part of the question is relatively easy. The Breeding Bird Survey (BBS) is the primary source of information on population change for most Northern American bird species and is the primary data source for conservation status assessments. The survey data has been collected annually since 1966 by experienced observers on preestablished, roadside survey routes between April and July.

The Patuxent Wildlife Research Center (PWRC) has a user-friendly website for interacting with the survey data and examining trends by state or region (see Figure 1 below)[1]. Simply select the species, region, and year, and hover over the map and desired region to get the trend information. Right-clicking on the map will provide annual indices for the selected species and whether the trend is positive or negative within the selected timeframe. To determine whether the trend is significant, check that the 95% Confidence Intervals (CI) are both above or below zero.

Figure 1 Example BBS Trend Map (Black-headed Grosbeak, 1966-2019)

What are the annual indices mentioned above measuring? The simplest way to think about them is that they represent the average number of birds counted by an observer on a single survey for a given year. In reality, the indices are an estimate based on complex data modeling that controls for observer experience and observer differences across geographical regions over time[2]. Although analysis methods for survey data can be controversial, these methods are peer reviewed and exist in published literature.

One of the questions I personally hoped to answer with this data is what has been happening to western migrant passerines over the short- and long-term? Are some migrant species surviving the Anthropocene better than others? I began by selecting 28 western migrant species and pulling annual indices for them. I then tested for trend significance over three time-intervals in the U.S./Canada region (or the U.S./CA subregion depending on the species’ breeding range). Table 1 provides the results and identifies whether a species had a statistically significant positive or negative trend between the listed year and the latest year data was available (2019)[3]. For example, we see that Townsend’s Warbler declined by 46% between 2000 and 2019 across its breeding range, and this trend was statistically significant.

Table 1 Trends for Western Migrant Passerine Species

Some tentative conclusions can be reached upon inspection of this table. Only four species show a positive trend, including Warbling Vireo, Cassin’s Vireo, House Wren, and Ash-throated Flycatcher. Fifty percent of included species show a statistically significant negative long-term trend, with the average decline being around 48 percent (ouch!). Over the last 20 years (2019-2000), there is also a significant negative trend for 43% of included species.

Figure 2 Townsend’s Warbler Showed a Negative Trend in Recent Years (© Keith Bauer)

What does this mean for the average birder? It might indicate birders will be seeing fewer western migrants each year. In fact, in the western BBS region, “63% of migratory bird species were categorized as declining” between 1980 and 2006[4]. A literature review on the topic of neotropical migrant bird population declines indicates the cause is “mainly because of the loss and fragmentation of breeding, wintering, and migratory stopover habitats”[5]. Indeed, long-distance migrants “tend to be more vulnerable to habitat loss and fragmentation than birds that are resident or those that migrate only short distances within North America”5.

However, it’s not all gloom and doom for birds in general. Many other groups of birds are doing well and show positive trends. For instance, Sandhill Crane, Bald Eagle, Osprey, Brown Pelican, Pileated Woodpecker, some Vireos, and Hooded Oriole are all trending up. In addition, a good number of eastern warblers are trending up in their breeding ranges, including Ovenbird, both Waterthrush, Hooded, Northern Parula, and Magnolia. Unfortunately, other species of eastern warblers are trending down, such as Kentucky, Cerulean, Prairie, Canada, and Golden-winged. Some birds simply have adapted to the Anthropocene better than others, while some others have received special protections.

Investigations of breeding bird trending data provide insights that can help birders and researchers explore how bird populations are changing around them and how these changes may result in significant impacts over time in the frequency with which species are observed. They are also an important conservation tool and can be used to measure how populations are changing due to conservation actions taken or other interventions and threats. In addition, breeding bird survey data can be combined with Ebird data to serve as validation for models that attempt to estimate annual variation in local migrant patterns. They are also just a fun way to geek out with some data and satisfy individual curiosity.

[1] BBS Survey Link: https://www.mbr-pwrc.usgs.gov/

[2] https://www.mbr-pwrc.usgs.gov/bbs/Link_sauer_niven_2020.pdf

[3] Orange indicates a statistically negative trend. Green indicates a statistically positive trend. No color indicates a lack of a significant positive or negative trend. Note that a large positive or negative percent change may occur without being statistically significant (perhaps due to the relative scarcity of a species in its range).

[4] http://datazone.birdlife.org/sowb/casestudy/north-american-monitoring-schemes-are-revealing-declines-in-migratory-species

[5] https://www.nj.gov/dep/fgw/ensp/neomigrant_info.htm