Dr. Haas is using the collection to study how agricultural practices have changed bird diets. She said that during the 1950s, farmers in the South were encouraged to replace native grasses with cool-season grasses, which could provide food for livestock earlier in the springtime. Now several grassland birds are declining, and Dr. Haas suspects that it is because of the switch to nonnative grasses.

Jean-François Ouellet, a doctoral student at the University of Quebec at Rimouski, is using the collection to study evolutionary relationships between a bird’s size and the quality of what it eats. He said the collection was helpful for its detailed information about hundreds of birds across the country over different time periods.

The collection may also be useful in reconstructing historical environments, or understanding migration habits. Dr. Haas said that as she was sifting through the card collection, the notes about a group of meadowlarks from Florida caught her eye. “Normally we think of meadowlarks as a grassland bird,” she said, “but these had all eaten longleaf pine seeds. Then I noticed they were all female.”

The males and females of some species look alike and cannot be distinguished unless they are dissected. Since the Patuxent scientists had already cut open the birds to take out the gullet and gizzard, it had been easy to determine the sex of each specimen as well.

So Dr. Haas was able to use the card collection to find a second meadowlark population farther north, on Long Island, that was 80 percent male. She thinks this is evidence that the species may have differential migration patterns. In endangered species, information like that could help with conservation efforts.

Over the years, various Patuxent scientists have served as stewards of the collection. Mr. Droege inherited it from Mr. Perry, and Mr. Perry inherited it from a scientist named Francis M. Uhler. Who will care for it next is unknown; Mr. Perry says most scientists just aren’t interested.

As it happened, an art gallery was. The gallery, Proteus Gowanus in Brooklyn, has been pursuing the theme of migration, and its director, Tamara Pittman, put out a call for interesting objects on that theme.

The jars were recently replaced by a new exhibition, “Future Migrations.” The jars are now lined up in Ms. Pittman’s living room, awaiting their own migration back to Patuxent and an uncertain future.