It might be hard to tell by looking at songbirds visiting your backyard birdfeeder, but birdwatchers often notice that migratory birds tend to have “pointier” wings than birds that don’t have to migrate.

Like the pointed wings of an airplane, pointed wings in birds are thought to provide efficiency during flight. University of Michigan researchers wondered if that held true within a species: if migrating subspecies within a group had differently shaped wings than their nonmigratory cousins.

Now, they have found that the wings of migratory yellow warblers are indeed more pointed. But thanks to a special collection of yellow warblers at the U-M Museum of Zoology, the study also found that their wings are not any longer than the wings of nonmigrating warblers. Instead, the wing itself is shaped differently—it is narrower at the base.

Documenting these changes will allow researchers to better understand how and why different wing shapes evolve across birds more generally, according to senior author Ben Winger, associate curator at the Museum of Zoology and associate professor of ecology and evolutionary biology.

Teresa Pegan, lead author of the study who completed the study as a postdoctoral researcher at U-M, says she couldn’t have done the work without the yellow warbler collection, part of which was meticulously collected and prepared by U-M alumna Nedra Klein in the 1990s. The collection underscores the rich breadth of data—and the scientific work it can support—held by the Museum of Zoology, Winger says.

“The legacy of the specimens is more enduring than any scientific article,” Winger said. “We are working hard to curate these specimens and make them available to the next generations of researchers.”

A researcher’s passion

Born in 1951, Klein completed her Ph.D. at U-M in 1993. Her yellow warbler project documented the genetic structure of the species—but alongside genetic samples she also prepared what are known as voucher specimens. Klein’s specimens are unusually complete because each bird specimen was preserved in full, including a study skin with feathers and a skeletal specimen with bones. Because saving both a skeletal and a skin specimen is time-intensive, not all specimens are preserved in this way—which made the collection especially valuable for researchers studying wing shape. Klein continued her work at U-M as a postdoctoral researcher, studying the relationship between finches and indigo-birds, which parasitize finch nests, according to an obituary published in the journal The Auk.

Klein, who survived Hodgkin’s disease as a teenager, died in 2001 at the age of 50 as a result of organ and tissue damage suffered during radiation treatment. She remained an advocate for museum collections throughout her career, according to the obituary.

“Often with our collections, we’re dealing with specimens that are maybe 100 years old, and we know the name of the collector, but they’re from a past era and feel a little less accessible to us,” Winger said. “But in this study, the generational distance between Nedra Klein and Teresa felt a little closer. Nedra died way too young, but she left specimens that are still inspiring to the next generation of students.”

The value of collections

Pegan in particular is interested in wing pointedness, called the hand-wing index. Previous studies have shown that migratory birds have pointier wings, but Pegan wanted to understand what specific morphological change was responsible for the pointedness. The difficulty in studying this was that most collections preserve either the bones or the skin and feathers, but rarely both.

Teaming up with U-M undergraduate Vera Ting, who was part of the Undergraduate Research Opportunity Program, Pegan says she “serendipitously realized that there’s this really special collection at the University of Michigan Museum of Zoology that would let us answer something I had always wondered about hand-wing index, but that nobody I knew of had ever looked at before.”

Yellow warblers have a vast breeding range, Pegan says. They range from Alaska, down through Michigan, with populations in Mexico, the Caribbean and the coasts of Central and South America. The populations in Alaska and the northern United States are migratory, but those in lower latitudes don’t migrate for winter.

“Because we had access to these great specimens, we were able to take it a step further and ask, ‘What is it actually about the wing that makes it more pointed? Is it a change in the feathers? Is it a change in the bones? Or both?” she said.

To make the measurements, the researchers carefully attached tiny lead pellets to the tip of each specimen’s wing feathers, a technique developed by study co-author Brett Benz, collection manager and assistant research scientist at the Museum of Zoology. Then, they used an X-ray machine to take an image of the wing and its base. The lead allowed them to measure the wing from the very tip of its feathers to the base of the wing.

What the researchers found surprised them. It wasn’t the length of the wing that made it pointy. The length of the wing did not vary between the migratory and nonmigratory warblers. Instead, the base of the migratory warblers’ wings were narrower.

“This was satisfying. It was something I was curious about for many years, and something I would argue provides some important context for thinking about how avian wings are evolving,” Pegan said.