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.