The second Pennsylvania Breeding Bird Atlas arrived in my postbox in Dublin on April 9, 2026. It is a heavy book — six hundred and forty pages, four pounds, cloth-bound, with a hermit thrush on the cover.
The first atlas, published in 1992, covered the 1984 to 1989 field seasons. The second atlas, this one, covered 2018 to 2023. The thirty-year gap is significant. Pennsylvania has changed.
Atlas projects work like this. The state is divided into survey blocks, typically nine square miles each, derived from a grid laid over US Geological Survey topographic quadrangles. Pennsylvania has 4,937 such blocks. Volunteers are assigned to blocks and asked to record, for each species observed during the breeding season, the strongest evidence of breeding they encountered.
The evidence categories are standardized across atlas projects worldwide. A bird heard singing is possible breeding. A pair observed in suitable habitat is probable. An adult carrying food to a hidden nest is confirmed. The atlas asks not whether a species was present but whether it was breeding, which is a different question.
Confirming breeding is harder than it sounds. Most songbird nests are not visible from a road. Adult birds in summer are often silent. Volunteers must learn to read behavior — the carrying of nest material, the agitated alarm call near a nest, the fledgling pursued by a parent — as evidence.
The Pennsylvania project recruited 2,103 volunteers across the five years. They contributed over a million records. The coordinating committee, based at the Pennsylvania Game Commission and Powdermill Avian Research Center, reviewed the records, vetted unusual reports, and produced the species accounts that fill the book's central section.
Comparing the two atlases is the point. For each species, the new volume includes a distribution map for the 2018-2023 period, the 1984-1989 distribution map for comparison, and a change map showing blocks where the species was added, lost, or persistent.
The headlines are unsurprising. Forest interior species are retreating. Grassland species, where grassland remains, are declining sharply. Several southern species — black vulture, fish crow, summer tanager — have expanded northward, consistent with climate models predicting range shifts of approximately ten to fifteen miles per decade for the eastern United States.
Black vulture is the starkest case. In 1989, it was confirmed breeding in seven blocks, all in the southeastern corner of the state. In 2023, it was confirmed in three hundred and four blocks, including the northern tier. The map shows the species advancing like a slow tide.
Cerulean warbler tells a quieter story. The species was always uncommon in Pennsylvania, restricted to mature deciduous forest on shaded slopes. The first atlas confirmed it in 187 blocks. The second confirmed it in 84. The loss is concentrated in the central Allegheny region, where the species' preferred habitat has been fragmented by gas-extraction infrastructure.
The atlas is careful in how it presents these losses. It is a reference book, not a polemic. The species account for cerulean warbler notes the decline, references the relevant literature on Marcellus Shale development, and lets the maps speak.
What I want to emphasize, because it is what these projects are for, is the human labour underneath the numbers.
A volunteer in Tioga County, a retired postal worker named Glenn Hartzler, surveyed his assigned blocks for five consecutive years. He started in May and finished in July each season. He walked, by his own count, over twelve hundred miles of forest roads, hayfield edges, and stream corridors. He recorded ninety-three species across his blocks, confirmed breeding for sixty-one, and identified four nesting locations of black-throated blue warbler that had not been previously documented in the county.
Hartzler is not a professional ornithologist. He took a six-hour training session offered by the project, bought a pair of Vortex Diamondback binoculars, and showed up. He is one of two thousand one hundred and two such people.
The atlas's species account for black-throated blue warbler cites his observations.
The atlas movement, broadly, began in Britain in the early 1970s. The first British Breeding Atlas, published in 1976, established the methodology. Atlases were then conducted across most European countries, in southern Africa, in Australia, and, beginning with Maryland in 1983, across most US states and Canadian provinces.
Second atlases, like Pennsylvania's, are now common. Some states are on their third. The temporal comparison is what these projects exist for. A snapshot of distribution at one moment is interesting. A pair of snapshots thirty years apart is science.
There are concerns about whether the volunteer base for atlas work can be sustained into a third Pennsylvania atlas, which would be due to begin around 2049. The participating volunteers in the second atlas were, on average, in their sixties. Many had also participated in the first atlas. Recruitment of younger birders has been the constant project of the atlas coordinators.
The atlas does not solve this. It only documents what was, while there were still people willing to walk the blocks.
A second atlas like Pennsylvania's costs, in inflation-adjusted dollars, somewhere between four and six million to produce. Most of that is in-kind contribution from the volunteers. The cash budget, raised through state agency support, foundation grants, and individual donors, was approximately one and a half million.
For that money, the state now has a definitive distributional record for two hundred and twelve breeding species. Conservation planners will use the maps for the next two decades. Researchers will mine the change maps for papers I cannot yet imagine.
The book itself is a peculiar artifact. It is too heavy to read in bed. It is too detailed to read quickly. It is, in a discipline that publishes mostly in journals, one of the rare ornithological books that will be on a shelf in 2050 and still useful.
Glenn Hartzler's copy arrived a week before mine. He told me on the phone that he had spent the first evening turning to the species accounts for the warblers in his blocks, reading what the maps said about the birds he had spent five summers watching.
He said it was strange to see his observations turned into a printed page. He also said he was already thinking about whether the cerulean warblers were still on the same slope this year.
