Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems. Using multiple and independent monitoring networks, we report population losses across much of the North American avifauna over 48 years, including once-common species and from most biomes. Integration of range-wide population trajectories and size estimates indicates a net loss approaching 3 billion birds, or 29% of 1970 abundance. A continent-wide weather radar network also reveals a similarly steep decline in biomass passage of migrating birds over a recent 10-year period. This loss of bird abundance signals an urgent need to address threats to avert future avifaunal collapse and associated loss of ecosystem integrity, function, and services.

Climate change elevates conservation concerns worldwide because it is likely to exacerbate many identified threats to animal populations. In recent decades, grassland birds have declined faster than other North American bird species, a loss thought to be due to habitat loss and fragmentation and changing agricultural practices. Climate change poses additional threats of unknown magnitude to these already declining populations. We examined how seasonal and daily weather conditions over 10 years influenced nest survival of five species of insectivorous passerines native to the shortgrass prairie and evaluate our findings relative to future climate predictions for this region. Daily nest survival (n = 870) was best predicted by a combination of daily and seasonal weather variables, age of nest, time in season and bird habitat guild. Within a season, survival rates were lower on very hot days (temperatures ≥ 35 °C), on dry days (with a lag of 1 day) and on stormy days (especially for those species nesting in shorter vegetation). Across years, survival rates were also lower during warmer and drier breeding seasons. Clutch sizes were larger when early spring temperatures were cool and the week prior to egg‐laying was wetter and warming. Climate change is likely to exacerbate grassland bird population declines because projected climate conditions include rising temperatures, more prolonged drought and more intense storms as the hydrological cycle is altered. Under varying realistic scenarios, nest success estimates were halved compared to their current average value when models both increased the temperature (3 °C) and decreased precipitation (two additional dry days during a nesting period), thus underscoring a sense of urgency in identifying and addressing the current causes of range‐wide declines.

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Both the Breeding Bird Survey (BBS) and the Christmas Bird Count (CBC) have been used for several decades to determine the status and trends of birds in North America. We propose new methods for assessing and comparing the reliability of these two surveys for estimating continental bird trends. In addition, we propose a new method for combining BBS and CBC continental trends to produce a combined continental trend for any species with useful trend information from both surveys. METHODS Breeding Bird Survey (BBS)The BBS, administered by the U.S. Geological Survey; http://www.pwrc.usgs.gov/BBS/, is the primary source of status and trend information for North American birds during the breeding season. The BBS is a roadside survey that includes 50 3-minute stops one-half mile apart, at which experienced individuals count all birds seen and heard. Surveys are done between late May and early July beginning 30 minutes before dawn. Surveys have been done on more than 4,000 routes; about 3,000 routes are done each year. Data are aggregated by BCR and by state. The survey began in 1965, so our analyses begin with that year. Audubon Christmas Bird Count (CBC)The CBC (http://www.audubon.org/bird/cbc/) is the primary source of status and trend information for North American birds in early winter. Each individual CBC occurs within a 15-mile diameter circle on a single day within two weeks of Christmas. Participants join groups that survey subunits of the circle during the course of the day using a variety of transportation methods (mostly on foot, in a car, or watching at a feeder). Just over 2,000 circles are surveyed each year. Like the BBS, data are aggregated by BCR and by state. The first CBC was done in 1900. We begin our analysis of CBC trends with the winter of 1965-66 for comparison with the BBS (which began in 1965) and because earlier CBC data are less comparable to current CBC data due to changes in methods and intensity of effort. Combining CBC and BBS for Continental Bird Population TrendsTrend analysis methods BBS trends and annual indices are estimated using the route-regression methods described by Geissler and Sauer (1990). In this analysis, trend is estimated first, and annual indices of abundance are used to assess higher levels of pattern in the context of the trend.Route trends are estimated as multiplicative trends using the estimating equations estimator described by Link and Sauer (1994). Observer effects are incorporated in the model to prevent bias associated with increases in observer quality over time . Regional trends are found as weighted averages of route trends. Regardless of variability in the counts on the route, missing counts (from years when the route was not surveyed) and observer changes (that modify the quality of the data) both tend to make route data less reliable. Consequently, it is necessary to weight the route trends by a measure of the consistency of counting on the route. This weight is proportional to number of years run and number of observer changes, but because it does not ...

Grassland birds have suffered dramatic population declines and are under threat of further grassland conversion. Simultaneously, grassland regions are projected to have high rates of future climate change. We assessed the vulnerability of grassland birds in North America under scenarios of global climate change reflecting the objectives of the Paris Agreement. The assessment incorporated model‐based projections of range losses and gains as well as trait‐based information on adaptive capacity. Nearly half (42%) of grassland birds were highly vulnerable during the breeding season under a 3.0°C increase in global mean temperature scenario representing current commitments under the Paris Accord. This proportion declined to 13% with a 2.0°C increase and to 8% with a 1.5°C increase over preindustrial global mean temperature. Regardless of scenario, more than 70% of grassland birds had some vulnerability to climate change. Policy actions beyond the present‐day national commitments under the Paris Accord are needed to reduce vulnerability of grassland birds in a changing climate.