Galvan, I. 2008. The importance of white on black: unmelanized plumage proportion predicts display complexity
in birds. Behav
Ecol Sociobiol, 63: 303-311.
This
study looked at the potential relationship between complexity of visual
displays and plumage patterns in 20 species of birds from the order Pelecaniformes.
Pelicaniform birds were used as a model for
this study because of their almost entirely melanized
and unmelanized feather patterns that create white
and black (or brown) patches (with a few exceptions) and due to the detailed
studies that exist on the complexity of their terrestrial social displays and
phylogeny. Two ritualized displays
were examined by Galvan: �male advertising� and �threat,� each with different
degrees of complexity. �Male
advertising� was described as a sexual display used to attract females towards
males, while the �threat� display is presented by both male and females to keep
other animals away. Previous studies
established 5 levels of complexity for the �male advertising� display and 8
levels for the �threat� display.
Galvan
examined illustrations of the pelecaniforms, estimated
the proportion of white feathers that existed in each species, and assigned relative
discrete scores for each species ranging from 0 to 5. A score of 0 was indicative of a total absence of white
feathers while a score of 5 meant that the bird had completely white plumage.
The same scale of 0 to 5 was used for the display complexity variable; 0
corresponding to an absence of display.
Scores for both unmelanized plumage and
display complexity were plotted on a log scale and Pearson�s correlation
coefficient was calculated between the two variables. Galvan also calculated the critical value from the
correlation coefficient from the data simulated on the taxa
phylogeny using PDAP software and computer simulations with PDSIMUL. Galvan utilized the comparative method
to infer the phenotypic trait evolution of melanized/unmelanized
plumage by deriving a phylogenetic tree from
independent data obtained from past studies.
The
results of this study found a negative and significant correlation between the
complexity of the male advertising and threat displays and the proportion of
white plumage (r=-0.66, n=20, P=0.002 for male advertising; r=-0.74, n=20,
P<0.001 for the threat display).
There was a significant relationship when considering the effect of
common ancestry, and when data on the taxa was
removed indicating that the variation of the phenotypic trait of unmelanized versus melanized
plumage has been evolved and favored by the benefits gained by signalers during
the ritualized social displays.
The results of this study supported Galvan�s hypothesis that dark and
white contrasts increase the efficacy of visual displays because they enhance
the detection of movements and/or presence of the signalers to receivers. Galvan proposed that the stereotyped
displays are conserved across the phylogeny and can vary in complexity across
species because natural selection has conserved melanized
plumage forms due to its adaptive benefits and the various tasks in visual
signaling.
I
chose this study because I found it intriguing that the ritualized displays in
these birds has a direct correlation to contrast patterns of their
plumage. This relates to material
covered in lecture and in the textbook concerning signal ritualization. In lecture, we discussed the flight
intention and courtship displays in pelecaniforms as
an example of sender precursors of visual signals. All information presented in this study supported material
learned in lecture and extended on topics such as contrast patterns for coding
schemes and signal ritualization related to
complexity and efficacy between sender and receiver (reducing message
ambiguity). Galvan proposed that
the colored patches found in the pelecaniform�s
plumage increases conspicuousness and this has coevolved with ritualized
movements to increase the efficacy of stereotyped visual displays. This study demonstrated that the
greater the number of white patches each bird has, the less complex they had to
make their visual displays to communicate because they were more readily
perceived by the receiver due to the increased contrast.