Every spring and summer, people watching gulls on Lake Michigan will see pink ring-billed gulls. There is some confusion about what causes this color, whether it is from carotenoids in diet, or staining. As I started looking into this question, I found it very likely that both factors are at play. Here I will outline the evidence I found for each cause and how you might be able to tell them apart, at least some of the time.

1 - Carotenoids

Ring-billed gulls. March 12, 2012, IL. Photo credit Amar Ayyash original blog.

The article “The Peculiar Puzzle of the Pink Ring-billed Gulls” by Lisa Hardy in the October 2003 issue of Birding outlines what is known about diet-related carotenoids in birds and gulls. Hardy went on to coauthor a 2006 study that found a single carotenoid obtained through diet, astaxanthin (pronounced ah·stuh·zan·thin), to be the cause of the pink color in both ring-billed and Franklin’s gull feathers in the western US. There are several points from these articles that are relevant here:

1) Most of the observations of pink ring-billed gulls discussed in the 2003 article come from March and April.

2) The body and head feathers show pink color in these birds.

3) Ring-billeds with pink body feathers often show a reddish tint to the legs, making them more orange than yellow. It is clear from photos that the same is true for the bills of these birds.

Using these features and focusing especially on the bare parts, i.e. the bill and legs, I started to see obvious differences in photos of pink ring-billeds from March and April compared to pink birds later in the summer that are likely stained. I’ll discuss those differences below, but first here are some examples of ring-billed gulls that I suspect are showing color from ingested carotenoids rather than staining.

As noted by Hardy, some of the pink ring-billeds in March and April also show a more reddish hue in the legs. The bill is also noteworthy – this pattern of an orange base with yellow tip shows up in a number of these early spring birds.

March 25, 2012, IL. Photo credit Amar Ayyash original blog. Note the solid orange legs and bill with orange base and yellow tip.

February 17, 2016. ON, Canada. Photo credit Jean Iron, with more individuals from the same date and location here. These observations are notable for the early time of year, during which all the individuals still showed some basic plumage head feathers. The bill and legs of this bird also show more yellow and green areas, which may change as the bird changes hormonally closer to breeding season–or not.

March 18, 2012, NV. Photo credit Fugle original source – this photographer also includes some info here on the timing and frequency of pink birds at this location, usually between 1-2% in late February - early March. This bird shows that not all pink birds during this period have intensely orange bare parts. They still might be more reddish than average here, and there is still time for them to change as the breeding season approaches.

March 20, 2014, CT. Photo credit Keith Mueller. Original source here. This bird has very intense orange color in the bill and legs, even extending into the tip of the bill.

It’s important to note that a significant number of ring-billed gulls show orange bare parts without showing pink body feathers. This is a normal and expected variation during the breeding season. It’s probable the pink color wears away with time, as is observed in adult Franklin’s gulls, and so some of these birds with orange bare parts may have had pink body feathers in the past, but that is not necessarily true. The point is that many, perhaps a majority, of ring-billeds that get pink in their body feathers from carotenoids also show more reddish legs and bills.

2 - Staining

Evidence for staining

For a while now, people have strongly suspected that staining, specifically with iron oxide, plays a part in the pink ring-billeds we see on Lake Michigan. One strong piece of evidence for staining comes from looking at birds near the breeding site in East Chicago, Indiana.

Satellite image (Google Maps) showing a central portion of the area hosting what may be the largest concentration of breeding ring-billed gulls in lower Lake Michigan. Numbers from breeding bird surveys are typically in the tens of thousands. It is also the site of a steel fabricator and a coal distributor.

The reddish color in the image above is likely iron oxide, which is used in the manufacturing of steel. It can be piled up for storage, it can color the byproducts of the steel-making process (slag), or it might settle from dust in the air or be deposited from water runoff. Breeding bird surveyors have noted the proximity of breeding gulls to the red substance, but the exact route of exposure is uncertain.

July 3, 2024, IN. Notice the pink juvenile (left of center), a much more common sight at this beach than at locations farther north on the lakefront and in Illinois.

The image above, while not a true representation of the actual proportions of pink gulls present, highlights a very real increase in these birds close to the breeding grounds here. I did some counting on one of my visits to this beach and consistently came up with 32-35% of the *close enough* ring-billed gulls that showed some visible pink color–I realized early on that my ability to perceive more subtle color was affected by, among a number of factors, distance. The number of birds counted was on the order of ~35/100. This is about 3x the proportion of ~10% pink ring-billeds figured by Terry Walsh on the North Side of Chicago–I encountered this same percentage in a couple different locations in the city.

While studying the birds at the beaches neighboring these breeding sites in late June-early July, I think I solidified some consistent differences in these pink ring-billeds from the birds in early spring:

1) The amount of color can vary, but in more colorful birds the color can be seen on the upperparts (gray in adults), not just the head and underparts.

2) The color is often less uniform and is more concentrated and irregular on the head.

3) At least at the time of these photos (early July), most stained birds have bare parts that appear to be yellow and variably covered in a red substance, especially concentrated near the feet and lower legs, rather than solid orange in color.

July 3, 2024, IN. Closeup of the legs in the previous photo. This is a very common appearance in these stained birds, one that is altogether different from the uniform orange legs of the individuals shown previously from March and April. One explanation is that the legs were covered in some substance, and then water washed the substance off to a degree, causing it to accumulate on the feet and lower legs.

Stained birds show variable color in bare parts, but in a closeup most of the color appears to be yellow legs covered in red stuff.

July 3, 2024, IN. Notice the pink wash is obvious in the upperparts, something not seen (to my knowledge) in any gull that is pink due to carotenoids.

July 8, 2024, IN. Notice the irregular spots of color on the head.

… and a problem

You might be wondering, what happened to those subtle pink birds from the early spring with the solid orange legs? Hardy (2003) suggested that perhaps the pink feathers are seen in the early spring as they are grown in prealternate molt and wear away in the following months. I don’t know if this has been verified, but my observations of ring-billed colonies in June and July would support this. Either way, if there are still carotenoid-pink ring-billeds around in the summer, there is a serious problem with trying to tell them apart from certain stained birds like the one on the left here, at least in lower Lake Michigan. As I said earlier, just because a ring-billed gull has orange legs doesn’t mean it will have a pink wash. The bird on the left shows solid orange legs and bill, suggesting carotenoids are involved, not staining, and yet there is no way to know if the subtle pink in the body and head are from staining, carotenoids, or even both. If pink from carotenoids wears off of feathers by July, it is likely staining, but what if a bird like this showed up in April or May? If there is an overlap in the timing of carotenoid-caused pink and pink from staining, it’s entirely possible that a ring-billed gull could show color from both sources at the same time.

Pink Juveniles

I’ve heard that occasionally pink juveniles show up elsewhere on the lake, less frequently than adults, but during my search in early July, I only saw one outside of the East Chicago beach. That was at relatively nearby Rainbow Beach. I saw pink juveniles at the East Chicago beach on each of my four visits. My assumption is that juveniles take some time before they wander far from where they were born, even after they improve their flight ability. If this is true and the pink is from staining, one would expect the occurrence of pink juveniles to be more concentrated near sources of the staining during the summer, and then for that concentration to dissolve later in the year as breeding birds and juveniles disperse. I went to see other locations with breeding ring-billeds in Chicago to check if I could find any pink juveniles, which would disprove this guess.

Du Sable Harbor/Navy Pier colony

On a couple of visits to the breeding birds at Du Sable Harbor and Navy Pier in Chicago, I did not find any pink juveniles, including juveniles that could fly and so possibly wander from another breeding colony.

I did find pink adults at this site, and they showed some interesting patterns. These adults were never seen in the rocky area in which many young juveniles (and presumably their parents) were seen. This rocky area contained old nests and all of the flightless, downy juveniles present, and so it may be a territory heavily guarded by parents still protecting their chicks. The pink adults were all some distance away on the cement, and even there they were more often chased off than the other adults.

July 9, 2024, IL. Pink adults like the one top and center were always absent from the rocky nesting area below which contained younger, flightless chicks.

When I compared the number of pink individuals to the total number that I could see well here, the proportion was very small (there were none in the 600+ birds on the rocky area), but when I compared the pink birds to the total sitting on the cement wall, the proportion was ~5-8%, similar to the number reported by others in the city. My thinking here is that the birds on the cement wall contain more individuals that are wandering, not specifically attached to this breeding site, raising chicks, etc. If that’s true, that would explain why the cement wall here might resemble any random beach in the city in terms of the number of stained birds.

July 5, 2024, IL. Pink adults at the Du Sable colony stuck to areas without many juveniles and were chased off more frequently than other adults.

Apart from the increased prevalence of stained juveniles, I also observed pink likely due to staining in herring gulls and Caspian terns (both adult and juvenile) at the East Chicago beach in Indiana, something I have not observed elsewhere. I have not found any information on whether herring gulls or Caspian terns can show pink due to diet.