Common waxbills with the highest social ranks aren鈥檛 necessarily larger or more intelligent than their peers 鈥� but they do have chest feathers that are a richer shade of red. This may be because individuals are so healthy that they can spare resources on accentuating their colours.
at the University聽of Porto in Portugal and聽her colleagues discovered this聽by evaluating dozens of common waxbills (Estrilda astrild) that were captured as adults in a聽large outdoor netted area.
The researchers measured the birds鈥� body size and then used digital photography and reflectance spectrophotometry to determine the size and saturation levels of the聽red-feathered chest patches.
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They also ran standard behaviour tests on each bird to judge their intelligence, stress tolerance and level of aggression or passivity. Then, they monitored bird feeders in聽the netted area, recording when a聽bird recognised another as higher-ranking by giving up its聽place聽at the feeder.
The researchers found that the聽only obvious factor linked with聽rank and dominance was the聽saturation level of the red chest聽plumage, says Beltr茫o.
If a bird has more saturated red colouring, it could indicate that it is聽healthy enough to spare nutrients in food for pigment use, so the feathers could act as a 鈥渂adge of聽honour鈥�, says Beltr茫o. 鈥淏ut that鈥檚聽just a hypothesis.鈥�
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