Date: November 23, 2016
Source: Central Ornithology
Publication Office
Both male and female birds use
traits like plumage brightness to size each other up, but a new study on
Northern Cardinals in The Auk: Ornithological Advances shows that the meanings
of female birds' markings may vary from one place to another, even within the
same species.
Though they're often not as showy
as the males, female birds have plumage ornaments that can convey information
to other members of their species. A previous study found that among Northern
Cardinals in Ohio, the brightness of females' facial markings indicated how
aggressive they would be in defending their nests. However, when Caitlin
Winters and Jodie Jawor of the University of Southern Mississippi repeated the
study in Mississippi's longleaf pine forest to determine if the same held true
there, they were surprised to learn that the variation among females' facial
masks in their southern study population had no relationship to their
aggressive behavior.
One of the key 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, leaving female cardinals there with less need 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," explains Jawor, who has since moved on to New Mexico
State University. "The ornament and behavior are both malleable."
To collect their data, Winters
and Jawor captured female cardinals early in the breeding season and measured
the brightness of their face masks with a color reflectance spectrometer. They
tested 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, observing the bird's reaction when it returned.
