This post was contributed by Brooke Goodman, co-author of a recent paper in The Wilson Journal of Ornithology on the coloration of urban Northern Cardinals.
The first time I held a Northern Cardinal, I immediately regretted it. A lot of fire fits in that tiny body, and they aren’t afraid to express it with a nasty bite to your finger. Then Dr. Daniel Baldassarre, his fingers covered in Band-Aids for bite protection, calmly handed me a miniature plastic finger for the cardinal to bite on instead of my own. At that moment I knew I had gotten myself into something fun.
I was a freshman at the State University of New York at Oswego when I joined the Oswego Bird Behavior Lab, headed by Dr. Baldassarre. I knew I loved birds, but I didn’t really know what I could do with that passion. Lucky for me, Dr. Baldassarree needed students to work on various summer projects focused on cardinals, which is how I found myself on the business end of a cardinal beak. We study cardinals not because we are masochists, but because we want to explore how urbanization is impacting birds, and cardinals are fantastic urban adapters.
The cardinal’s willingness to breed in disturbed areas and its generalist nature make it well suited to persevere in the urban areas most birds steer clear of. They’re also ready and willing to take advantage of anything that the invasive species that thrive in disturbed areas have to offer. They are successful in both rural and urban areas, meaning we can study and compare populations in both contexts. For the two summers that I worked on the project collecting data in the field, we spent our weeks traveling between our rural site in Oswego and our urban site in Syracuse, New York.
Despite the eager adaptation of urban cardinals, they face more novel dangers than their rural counterparts. Light and noise pollution, feral cats, and windows pose severe threats to these birds. At the Bird Behavior Lab, we look at how these pressures may be altering cardinal behavior and what it could mean for their evolutionary future.We have investigated differences between populations in neophobia, disease, song, and most importantly for our recent paper in The Wilson Journal of Ornithology, carotenoid-based signals.
Color is clearly important to cardinals. Their bright red plumage is the star of many picturesque holiday cards. What many people aren’t aware of is the effort these birds put into maintaining that vibrant red. The “good genes hypothesis” tells us that ornaments may signal an animal’s condition; that is, a bright red cardinal is theoretically healthier, perhaps displaying to a potential mate that they will have super-healthy offspring. That may be why the metabolic energy they spend converting the yellow carotenoids in their environment into the red they deposit into their feathers is worth it.
But urban cardinals have an advantage: one of the invasive species that thrives in their disturbed habitat, honeysuckle (Lonicera sp.), has naturally red carotenoids. Cardinals that eat honeysuckle berries don’t need to spend energy converting them from yellow to red, because they can go right into the bird’s feathers.
To examine carotenoid color patches, we looked at the bills (male and female), underwing feathers (female), and chest feathers (male) of urban and rural cardinals. We used reflectance spectrometry (chest feathers) and photos taken against color standards (bill and underwing) to evaluate the hue, saturation, and brightness of the color patches of interest. We also wanted to account for the sensitivity of the avian visual system during our analysis—birds can see a lot of things that humans can’t, including UV light, and we weren’t interested in seeing if humans could tell the difference between the rural and urban color patches, we wanted to know if a cardinal could! We also measured the bird’s body condition in the field by taking measurements of mass and tarsus.
Our results, in contrast to previous studies, found enhanced carotenoid-based signals in urban birds. Specifically, urban males had redder chest feathers in one year of our study and redder bills in every year. Urban females had more saturated underwing color than rural females in every year. Urban cardinals had significantly reduced mass-related body condition, but despite this poor condition, they could produce these presumably costly signals. The abundance of honeysuckle at the urban site may explain not only the enhanced signals, but the stability of the signals.The color of our rural birds seemed to undergo larger year-to-year fluctuations than the color of the urban birds.
It is disturbing but fascinating to consider that urbanization could be disrupting traditionally honest signals in cardinals. For millions of years these birds may have been relying on plumage quality to choose mates, and now that signal is no longer reliable. Of course, there are other reasons why carotenoid-based signals could vary between the urban and rural populations, such as different lighting conditions, but honeysuckle availability is likely playing a role in this difference.
Despite the challenges they face, I know our urban cardinals will persevere. We only visited our urban site once a week, but I always looked forward to it the most. I watched urban cardinals avoid feral cats, attempt to sing over obnoxious lawn mowers, and make nests in places so close to human activity that I could check them without leaving the busy trail. A moment I’ll never forget is watching (via trail cam) a cardinal nest get mowed down by an over-eager landscaper, only for the female to begin building a new nest just a week later. The toughness of cardinals reflects the harshness of the environments they choose to inhabit, and I am excited to explore more of these behaviors with the Bird Behavior Lab in the future.