Can Aggression Be Determined by Female Birds' Markings?

Researchers have found that the markings of female birds can tell us more than just their species and gender. Repeating a previous study has yielded a surprising discovery refuting what was thought to be a link between their markings and aggressive behavior when defending their nests. Published in The Auk: Ornithological Advances, the study reveals that the meanings behind the markings of female birds may change from one place to another despite a similarity in species.

Caitlin Winters and Jodie Jawor of the University of Southern Mississippi based their research on a previous study that related the brightness of Ohio's female Northern Cardinals' facial markings to their aggressive nesting behavior. But when Winters and Jawor repeated the study in Mississippi's longleaf pine forest, they were surprised to learn that the variation among females' facial masks in their southern study population had no link to their aggressive behavior.

To gather data, Winters and Jawor collected female cardinals early in the breeding season and measured the brightness of their face masks with a color reflectance spectrometer. They observed aggressive nest defense behavior by waiting until a female left for a break in incubation and then placing a female Northern Cardinal decoy near the nest. They then observed the bird's reaction upon its return.

A vital differences between the northern and southern cardinal populations studied is that unlike in Ohio, the researchers did not observe any evidence of brood parasitism, where one female cardinal sneaks an egg into another's nest, among cardinals in Mississippi. The Mississippi birds also had more habitat available to them and defended larger territories, which meant female cardinals weren't as hard-pressed to defend their nests. "This is an indication that selection pressures vary between northern and southern populations and that the information a female in the north needs to convey to other cardinals differs from what a female in the south has to say." concluded Jawor.