A couple of days ago I was out photographing Sooty and Pied Oystercatchers on the Sir Richard Peninsula in South Australia’s Coorong. The two species were intermingled on the sandy beach. Usually, the sooty species prefers a rocky shoreline, but not on this day, when they shared the Pied Oystercatchers preferred sandy shoreline. Indeed, they interacted as if they were a single species. Despite 30-degree temperatures (and heat shimmer), I got some reasonable images that I was pleased with, but all the time whilst shooting and editing I couldn’t help wonder; what was the biological basis and origin of their magnificent, striking orange bills and eyes? These visually stunning attributes are as amazing as pretty much anything you’ll find in the avian world.
If you read my recent article “An Essay on Bird Colour for Photographers”, you’ll know that I have a personal and professional interest in how genetic and dietary factors can influence bird colouration. However, when I started digging around trying to find answers to these questions, I couldn’t find any substantive information on the biochemical pigments, carotenoid types, dietary origin, genetic pathways, or evolutionary history of the orange/red bill and eye coloration in Pied (Haematopus longirostris) or Sooty (H. fuliginosus) Oystercatchers. There are plenty of sources describing the appearance of the coloration, but not its biochemical or genetic basis.
The most assertive comment we can make is that the coloration is ontogenetically acquired, likely condition dependent, and probably diet linked, but nothing goes beyond this level of vagueness. At this point, I should ask that if anyone knows more than me on this question of oystercatcher colour, please let me know and I’ll update/correct this narrative accordingly.
Although it’s not supported by specific sources in the scientific literature (at least as far as I can see), the following points are well established in avian pigment biology and help frame what is also likely true for oystercatchers (unless anyone knows otherwise):
The red/orange bare part coloration in shorebirds is almost always carotenoid based, typically derived from dietary carotenoids (e.g., lutein/zeaxanthin) that are enzymatically converted into red ketocarotenoids (e.g., astaxanthin, canthaxanthin).
The gene CYP2J19 encodes the major ketolation enzyme in many birds, producing red coloration.
SCARB1 and other lipid transport genes regulate carotenoid uptake.
BCO2 regulates carotenoid degradation and can influence whether yellow/orange/red pigments accumulate in bare parts.
Ontogenetic colour change (brown to red) strongly suggests carotenoid deposition increasing with maturity and condition.
The above five points are general avian principles, not oystercatcher specific findings. Indeed, it seems that there are no HPLC pigment profiles available, no identification of specific carotenoids, no histologic pigment-deposition studies, no oystercatcher genomic studies linking colouration to known carotenoid genes, and no comparative pigment work between Pied and Sooty Oystercatchers. However, even though Haematopus hasn’t been studied directly, shorebirds as a group have, and their bare-part coloration follows consistent biochemical rules.
The likely pigment class is red ketocarotenoids; In shorebirds, red/orange bare part coloration is almost always produced by:
Astaxanthin (crustaceans are a rich source)
Canthaxanthin
Adonirubin / Adonixanthin
3-hydroxy echinenone
These are all produced by ketolation of dietary yellow carotenoid precursors (lutein, zeaxanthin) obtained from bivalves, crustaceans, marine worms and algae-associated invertebrates. Given oystercatchers’ heavy reliance on molluscs and crustaceans, the carotenoid pool available to them is consistent with the pigments seen in other red billed shorebirds.
When it comes to genetics, again we can only speculate on what is likely to occur in oystercatchers. CYP2J19 encodes the red ketocarotenoid enzyme, which converts yellow carotenoids into red ketocarotenoids (it is responsible for red bills in zebra finches and red plumage in many birds and is highly conserved across birds). My inference here is that Haematopus almost certainly uses CYP2J19 for red bill and eye coloration. Similarly, SCARB1 encodes the carotenoid uptake receptor, regulating carotenoid absorption from the diet. Mutations in this gene alter beak and skin colour intensity. A gene (BCO2) that encodes an enzyme that breaks down carotenoids can be down-regulated aiding carotenoid accumulation. Now, there are no published genomic or transcriptomic studies linking these genes to coloration in Haematopus, BUT I think it is fair to infer that 1. the presence of bright red bare-parts implies active CYP2J19 (true for 95% of birds studied – others accumulate keto-carotenoids directly); 2. Geographic variation in Sooty Oystercatcher eye ring colour (yellowish in the northern subspecies) suggests regulatory variation in carotenoid uptake or conversion; 3. The dull juvenile Haematopus coloration implies developmental regulation of SCARB1 and CYP2J19.
I think that this is as far as we can go with inference regarding biochemistry and genetics. However, I think it’s reasonable to make an evolutionary interpretation as to why oystercatchers have orange/red bills and eyes? The following points have merit:
1. Species recognition: Orange eyes/bill afford high-contrast signalling, long-distance visibility in coastal environments, and help reinforce species boundaries.
2. Sexual selection: In many shorebirds, red bare-parts are usually condition-dependent, linked to foraging efficiency, and used in mate choice. The fact that coloration intensifies during the breeding season in many oystercatcher species supports this.
3. Ecological divergence: Pied and Sooty Oystercatchers differ in habitat (open sandy beaches vs rocky shores) and diet (bivalves vs limpets/rocky shore prey). The bottom line is that different diets mean different carotenoid availability and hence a possible divergence in pigment intensity or hue. It’s worth mentioning that no other species of bird (nor indeed humans) can open and extricate flawlessly armoured shellfish as well as oystercatchers can – a skill that is acquired over several years. Locally on the Sir Richard Peninsula, large cockles (pipi) are a favourite.
4.Phylogenetic inertia: The distribution of traits across extant species suggest that the ancestral Haematopus might have had black plumage and a red bill and eye suggesting the trait is ancient.
So, to summarise I think it’s fair to say that:
Oystercatcher bill and eye coloration is carotenoid based (inferred from shorebird biology).
Likely pigments include astaxanthin, canthaxanthin, adonirubin, etc.
Likely genes include CYP2J19, SCARB1, BCO2.
Colour is condition dependent and increases with age.
Evolutionary drivers include species recognition and sexual selection.
Before ending, I should just mention that the Eurasian Oystercatcher fits the same pattern, with the bare-part phenotype being essentially the same.
If anyone has better more accurate information, let me know and I’ll update this article. Shame I’m now retired; the fact that we know a lot about the core machinery for red carotenoids in birds, but that no one has sequenced, or functionally linked colour genes to bare-part coloration in Haematopus would make a great PhD project for a graduate student.
If the narrative is too reminiscent of school chemistry classes and leaves you with hives, I hope that at least you enjoy the images of South Australia’s two lovely oystercatchers. If I zoom out from the specifics, my inference would be that Pied, Sooty, and Eurasian Oystercatchers almost certainly share the same ancestral, CYP2J19 based, red ketocarotenoid system for bill and eye coloration; what differs between them is not the fundamental pigment machinery, but how strongly, where, and when that shared system is expressed. A final point that should be mentioned is that Haematopus species hybridise surprisingly often, but I don’t think this weakens any argument I’m trying to make.
If you want to photograph these birds, then my recommendation would be to use a lens of at least 400mm focal length (ideally a 400mm on APS-C or 600mm on full frame). A zoom or prime will be fine and it doesn’t need to be fast glass as smaller apertures should still allow fast enough shutter speeds, even for flight shots, since beaches tend to be bright places with plenty of light. In fact, most of my images had an extra +0.3EV dialed in to cope with the bright/harsh beach lighting that otherwise led to underexposure (fooling of the cameras TTL meter). If you want to shoot oystercatchers in flight, I’d start at 1/3200sec providing that doesn’t push your ISO up too much. I’ve written the basic settings of each image in the captions as a guide for you.
This article feels a bit like a citizen science venture, but in any event, I hope some of you have enjoyed reading the article – have fun shooting oystercatchers, and if you’re a budding Masters or PhD student – feel free to use my thoughts as the basis of a project. – remember, everything we know about oystercatcher coloration is inferred from general avian carotenoid biology, because Haematopus remains almost entirely unstudied at the biochemical and genetic level. 😊
