The Secret Perfume of Birds cover
Courtesy Johns Hopkins University Press

The Secret History of Bird Smells

Evolutionary biologist Danielle Whittaker tells us about the little-known science behind birds’ sense of smell.
By Brittany Dennison
August 4, 2022
7 minute read

Danielle Whittaker was never particularly fond of birds. She started her studies as an evolutionary biologist by focusing on the monogamous mating patterns of gibbons in Indonesia, but the small sample size to observe in a remote area wasn’t fulfilling as a scientist, so she switched to the most monogamous (and plentiful) species: birds. In her years of examining bird mating at the Beacon Center for the Study of Evolution in Action at Michigan State University, she discovered not only a love of birds and their particularities, but patterns of behavior that showed birds have a more highly keen sense of smell than what was established by scientists as respected as John James Audubon himself. Whittaker examines all this and more in her new book, The Secret Perfume of Birds: Uncovering the Science of Avian Scent (Johns Hopkins University Press). We recently spoke with her about her research on bird behavior and how their sense of smell and their own unique scent might be used to help conservation efforts in the future.

How did you get started studying birds and their sense of smell?

I was doing a project that involved birds that were reproducing, and another student in the lab was working on something called preen oil. Birds have this one major gland at the base of their tail that secretes this oil that they use when they’re preening their feathers that helps protect the feathers from exposure to light and parasites. All of that helps keep them clean. We were awestruck to find that this oil gave off an odor. The hypothesis she had started with was that they would produce less odor when the birds were nesting because they wouldn’t want predators to find the nest. She actually found the opposite. She found that birds and breeding conditions produced more odor. That made me think, Wow, maybe that’s important in attracting or choosing mates. First I needed to know if they could actually smell it, so I took preen oil from various birds and I put it on the nest of the birds that I was watching. I took videos, and I discovered that if I put their own preen oil on the nest, it didn’t affect their behavior at all. But, if I put preen oil from another bird of the same species, they had a little bit of a reaction. If I put oil from a different species entirely, they got up and left the nest much more quickly. I was like, Wow, they can actually sense this! What’s in that oil, and what information does it communicate about the bird? I went from there to looking at how it affects their behavior.

What sort of olfactory discoveries have you unearthed?

The research I was most excited about was with dark-eyed juncos. They’re little gray and white sparrows that are common across North America. I found that the odor of their preen oil early in the breeding season correlates very strongly with the number of offspring they will produce. Basically, it predicts how successful they’re going to be. I don’t know if that’s because it’s an odor that attracts mates, and if they’re successful at finding mates, then they’re producing offspring. In another study, we found that odor was related to how aggressive birds would be in response to an intruder on their territory. Birds can really detect a lot about each other just through scent.

They can detect odors, but the scent of a human is not going to be enough to put them off of their nest. They’re not going to abandon their young if a human touches a baby bird or their nest. But they can detect odors of known predators. Other people have done work on this. They have taken things like urine from weasels and put it in nest boxes. Birds will avoid using nest boxes that have that odor near them, showing us that they can assess the threat of other animals through their sense of smell.

They can also use scent for foraging for food. There are some great studies about seabirds showing that they can detect the scent given off by phytoplankton in the ocean. When there’s a large aggregation of phytoplankton, they give off a chemical called DMS. Where there’s a lot of phytoplankton, there’s a lot of fish, and so albatrosses will use the DMS scent to know where to hunt for food.

How is a bird’s sense of smell different from our own?

The reason that human memory is involved in smell is because of the way the olfactory nerves pass through certain parts of our brain. Bird brains are structured a little bit differently. We’re still learning a lot about it. For a long time, people have ignored the olfactory bulb in the bird brain because it’s relatively small, and they just assumed that they didn’t use it. Birds do have nostrils, but they’re not flexible the way that mammalian nostrils are. You can’t tell if a bird is sniffing something. You don’t see that kind of behavior. They’re just these fixed parts of the beak. I think people thought that they were just for breathing and not for smelling.

What kinds of applications can your research be used for?

I’ve always approached this just as really basic research into a hole in our knowledge and just for the sake of understanding the world around us. In terms of where I’m going with it next, I’ve become interested in bacteria. It turns out that these odors are produced not by the birds’ own genetic processes, but by bacteria that live in the pineal gland, that gland that produces preen oil. So I’ve gotten interested in what controls that population of bacteria. Why do they produce odor that tells you information about the bird? That kind of microbiome research can extend to all kinds of things because we’re still learning about microbiomes and humans and how it affects our biology in some pretty profound ways, smell being one of them.

In terms of practical applications, there have been some interesting ideas out there about how to use scent in conservation management. If odor is important to an animal’s biology and, say, you’re trying to release animals from the zoo back into the wild, smell might be one thing we need to take into account for making sure that they can communicate with each other and maybe to attract mates.

Do you have a favorite-smelling bird?

I can recognize the smells of some birds now that I notice it. But it’s subtle. I’m not going to find a bird in my backyard based on their smell because they’re usually not that strong smelling. But once I handle birds, I can smell them. Different species do smell different to me. In fact, my favorite-smelling bird is the brown-headed cowbird, which smells like cookies. My chemist collaborators who have worked with the perfume industry before had some interesting thoughts after I discovered that cowbirds smell like cookies. We wonder if we can synthesize eau de cowbird….