Rosita Corey was a Massachusetts schoolteacher who recorded spring bird arrivals for decades; her old notes now help show that some birds are arriving about 7 days earlier

A notebook kept by a Massachusetts schoolteacher is now part of a long record showing how spring bird migration is changing. Rosita Corey, a retired schoolteacher and birdwatcher in Concord, Massachusetts, maintained records of first arrival dates of birds from 1956 to 1973 and again from 1988 to 2007. Researchers combined her observations with older records and, more recently, modern bird sightings to study changes in migration timing. A 2025 study, "Integrating historical records and citizen science data to understand bird responses to climate change in Concord, Massachusetts: Thoreau to eBird", found that 18 migratory bird species were arriving in Concord approximately seven days earlier, on average, in the modern period than in the historical records.

It started with observations made more than 170 years ago

Concord has a long history of bird arrivals in the spring. The story begins with the naturalist and writer Henry David Thoreau, who kept bird notes in Concord between 1851 and 1854. Observations of bird arrivals were recorded by ornithologist William Brewster in 1886 and from 1900 to 1919, and by ornithologist Ludlow Griscom in 1930, 1931, and from 1933 to 1954. Corey then provided another set of observations for the periods 1956–1973 and 1988–2007.


These records were compiled in the 2010 paper published in The Condor: Ornithological Applications. The resulting data set spanned 157 years, the longest known record of bird-arrival dates in North America at the time. The researchers also note that the observers' methods were similar in important ways: they walked around Concord several times a week, visiting different habitats and recording newly arriving birds. They also found that some details, like how many hours each observer spent watching birds and the exact area they covered, were unknown.

The first study didn’t show that all the birds were arriving earlier.

And that’s where the story gets more interesting. The 2010 research examined 22 species of migratory songbirds. For all 22 species combined, there was no average shift in the timing of arrival over the entire period from 1851-2007, researchers found. But the individual species did not all do the same thing.

Three species arrived significantly earlier; four species arrived significantly later. Eight species showed significant relationships between arrival date and temperature, with seven arriving earlier in warmer years. In Concord, the researchers found that birds’ arrival times appeared to be less responsive to temperature than were plant flowering times. That finding is important because it shows why it is too simplistic to say climate change has caused earlier arrival for all migratory birds. Species may respond differently, and long historical datasets have their own limitations.

New data changed the picture

The newer study used the same long-running record, but added observations from eBird, an online birding platform. Researchers combined historical first-arrival records for 18 species of birds with eBird observations from 2013 to 2024 in Concord. Since the exact historical effort in hours and area wasn't recorded, the researchers instead used a subset of the eBird data to approximate the general pattern of historical observation, several visits a week across different habitats, rather than matching it precisely.

The new analysis took a different approach. The researchers found that the average of the 18 species was arriving about seven days earlier now than in the past. This, they said, provided evidence of a stronger response to warming temperatures and changes across more species than were identified in the earlier study. Importantly, this is an average across the species studied and does not mean that every bird species now arrives exactly seven days early.

Why Corey's notebooks are important

Corey’s observations are especially valuable because they fill an important gap between the records of early naturalists and today’s digital birding data. Her first set of observations was made in 1956, and her second set lasted until 2007. That gives researchers decades of observations by one person, over a time period that spans the older historical record and modern datasets. Such records have long been known to be of value to researchers. A 2012 study published in BioScience explains how naturalists’ journals, field notes, museum specimens, photographs, and other historical observations can provide useful evidence of ecological change. It also states that these records need to be used with caution, since the conditions in which historic observations were made were not always the same as for modern scientific data.

That makes Corey's records especially relevant today. She was merely recording what she observed: when certain birds returned to Concord each spring. Decades later, those notes can be compared with observations made by thousands of modern birdwatchers with digital devices. The larger lesson is not that all the birds have suddenly changed their schedule. The point is that long-term records can show patterns that would be almost impossible to see from a few years of observations. Thoreau, Brewster, Griscom and Corey have provided a rare glimpse of changing seasons in Concord. Today, modern citizen-science records are extending that window into the present.

For Corey, it may have been a simple habit of watching and recording the birds around her home. For today's researchers, those observations are part of a much longer scientific record, one that is helping them understand how spring migration is changing over time.