This Article by National Geographic describes the recent discovery of the cause behind the mysterious changing of colors in Yellow-shafted Northern Flicker’s feathers, as well as demonstrates the ripple effects of invasive species.
In the last few decades, ornithologists noted that some yellow-shafted northern flickers had their yellow feathers turned red. At first they thought it was due to interbreeding with the red-shafted northern flicker in the west yet the oddly red birds were found in areas extremely distant from them. Apparently new research has discovered that it actually had to do with their diets (as red, orange, and yellow pigments in bird feathers tend to be attributed to) and due to the importance of coloration in signaling it could have massive impacts on populations (including in finding mates).
One of the first clues to finding the truth was based on a study of the changes in cedar waxwing’s coloration that occurred in the 1960s. The yellow tips of some of the cedar waxwing’s feathers were turning orange and due to lack of red-feathered relatives they looked into the diet. Apparently it was due to the birds eating invasive honeysuckle berries (imported by horticulturalists in the late 1800s for landscaping, bird habitat and food then spread rapidly to become an invasive species) which appealed to their voracious appetite in regards to berries.
In their study regarding the northern flickers, they analyzed the chemicals in the pigments of both the red feathers and honeysuckle berries and confirmed that the red hue did in fact come from honeysuckle berries. This issue may lead to problems for the northern flicker’s ability to find mates and it is unknown how many other bird species are experiencing similar problems as well.