"Egg collections are such a fascinating tool for us to learn about bird ecology over time," says John Bates, curator of birds at the Field Museum and the study's lead author. "I love the fact that this paper combines these older and modern datasets to look at these trends over about 120 years and help answer really critical questions about how climate change is affecting birds."

Bates got interested in studying the museum's egg collections after editing a book about eggs. "Once I got to know our egg collection, I got to thinking about how valuable that collection's data are, and how those data aren't replicated in modern collections," he says.

The egg collection itself occupies a small room crammed full with floor-to-ceiling cabinets, each containing hundreds of eggs, most of which were collected a century ago. The eggs themselves (or rather, just their clean, dry shells, with the contents blown out a hundred years ago) are stored in small boxes and accompanied by labels, often hand-written, saying what kind of bird they belong to, where they're from, and precisely when they were collected, down to the day.

"These early egg people were incredible natural historians, in order to do what they did. You really have to know the birds in order to go out and find the nests and do the collecting," says Bates. "They were very attuned to when the birds were starting to lay, and that leads to, in my opinion, very accurate dates for when the eggs were laid."