|Adult Ross's Gull showing obvious pink tones on underparts . Cherry Creek Reservoir, CO. 21 NOV 2010.|
The earlier literature suggested that this pink coloration was a result of red preen oil containing carotenoids that were applied to the feathers via preening (Stegmann, 1956). Grant also used this explanation in his identification guide but this explanation overlooks the fact that some white-headed gulls (such as Ring-billeds) show this coloration on parts of the head that are impossible to reach with the bill while preening.
|Adult Ring-billed showing uniform pink coloration all throughout. Tinley Park, IL. 11 March 2012.|
|Adult Little Gull with pink cast on underparts. Carlyle Lake, IL. 21 January 2012. Photo courtesy of Dan Kassebaum.|
Most interesting is Hardy's suggestion that as of 1998, more and more Ring-billeds (up to 30%) in the Pacific Northwest have been found with traces of pink in their plumes. This was two years after salmon hatcheries in Washington state began providing synthetic astaxanthin as a diet supplement to their young fish. A natural question I've asked is why Mew and Glaucous-winged Gulls don't show these pink tones. I don't know of any gull species that consumes more salmon than Glaucous-wingeds - this species thrives on the spawning streams of salmon all throughout the Pacific Northwest. Could it be Glaucous-wingeds (and Mews) don't sufficiently accumulate, utilize and/or produce these carotenoids?
Observing this pink suffusion in the field is not very difficult, but it can be tricky. Overcast conditions are most favorable for observing and photographing pink gulls. Strong sunlight significantly washes out these pink tones. Take for example this Franklin's Gull:
|2nd cycle FRGU. Chicago, IL. 25 March 2011.|
As for the implications this pink coloration has within conspecifics, McGraw and Hardy suggest the following, "Because carotenoid colors often are sexually selected indicators of mate quality in adult birds, there may be signaling benefits to producing pink plumage in these gull species". Although this may hold with Ross's and Franklin's Gulls, I've found that Ring-billeds with pink tones are at the bottom of the feeding hierarchy and they seem to be slight social outcasts among their flocks.
|White and "pink" adult Ring-billeds. Oak Forest, IL. 27 March 2012.|
These "pink" individuals almost always seem to be more dirty and untidy, giving me the impression that it's difficult to remove dirt from their feathers via bathing and preening. Do these feathers possess an adhesive-like property? That would indeed be disadvantageous to any bird. Tentatively, I suspect the pink found on Ring-billeds, at least outwardly, is sufficiently "different" than what is observed on Ross's and Franklin's.
|"Pink" adult Ring-billed. Chicago, IL. 24 April 2010.|
Carolyn Marsh and I were recently discussing the increase in Ring-billeds exhibiting an exaggerated amount of pink feathers and she feels a steel factory in East Chicago, where now sits the largest Ring-billed colony on southern Lake Michigan, might be the cause. Her unverified thoughts on this is that some type of iron deposit or chemical is acting as an external dye in the water.
|Calumet Park. Chicago, IL; 14 April 2012.|
Although more is known about this colorant now than just one decade ago, there are still many questions that remain to be answered. I'd appreciate learning about your experiences and observations with these "pink" gulls.