I'd like to give a different perspective on this topic by reviewing the literature, especially Jehl (1985), and questioning some of our assumptions. The photo (left) is from Jehl (p. 495) showing a mixed pair of oystercatchers from Scammons Lagoon, Baja California. The purpose of his paper "is to clarify species limits by documenting the nature of variation in size and plumage among populations of oystercatchers on the west coast of North and Middle America, with special reference to the zone of overlap."

The population of American Oystercatcher in Baja California and west Mexico is "Frazar's Oystercatcher" H. p. frazari. Jehl created a "character index" in which a "pure" Black Oystercatcher would score 0 and an American Oystercatcher in eastern North America [H.p. palliatus] would score 42. Ten plumage characters would be scored from 0-5 or 0-7 "to classify the range of variation." To represent BLOY, Jehl chose his scores of birds from Oregon northwards; to represent AMOY he used "populations of frazari in the northern Gulf of California and the coasts of Sonora and Sinaloa" (p. 490). Using just that set of AMOY, he proposed to "consider character scores of 30-38 (mean of 33.6) as typical of frazari. For purposes of this paper, any individual with a score of 10-29 is considered to be a hybrid;" (p. 491).

The first assumption I wish to disavow is that Jehl's character index, available on-line, was meant to define what was actually a hybrid or not. He had no such intent — indeed, in 1985, he had no genetic information at all. Nor is the Jehl score meant to categorize the birds of western Baja California. He intentionally used scores from specimens far from the area of species' overlap to use as a tool to analyze what he found in the overlap zone. The Jehl score is not an evaluation of Baja oystercatchers, and the borders drawn within the index (e.g., 10-29 as hybrid) is not meant to identify hybrids. Still, it is a very nice tool to evaluate "color variation as expressed in the degree to which dark coloration invades the white areas" of AMOY (p. 499).

Direct hybridization (i.e., F1 hybrids) can lead to multiple back-crossing introgression, and the flow of genes expressing such introgression, through a population of organisms. In birds it is one method by which new species develop (Grant & Grant 1997). It is neither a good thing or a bad thing — it is a normal part of evolution as an organism adapts to environmental changes. Some introgression that occurred eons ago may remain in the organism's genome. In humans, up to 4% of the genome contains Neanderthal ancestry as a direct result of admixture with these archaic species (Green et al. 2010). As Neanderthals went extinct about 50,000 years ago, that is surely very ancient hybridization [no jokes here about your ability to pick out those people currently with the most Neanderthal genes].

The parasitic blood fluke Schistosoma haematobium, the one most commonly the cause of the disease schistosomiasis in humans, may hybridize with livestock schistosomes, including S. bovis. A recent study in Niger and Zanzibar found no evidence for contemporary hybridization between S. bovis and S. haematobium in recent samples. However, all S. haematobium genomes from Niger revealed genetic evidence of an ancient introgression event that occurred at least 108–613 generations ago; Pratt et al. (2019). These worms live 3-5 years on average, and lay eggs throughout their lifespan. I'm not sure how long in time 108-613 generations would take.

We know that oystercatchers are long-lived, have slow reproductive rates, and have low annual productivity success, often below 20% (i.e., reports of the California Central Coast BLOY Project for the Monterey Bay region [fide H. Hanks]). Jehl (1985) reported that "some examples of H. unicolor (New Zealand; photo right), H.p. durnfordi (Argentina), and H.p. galapagensis (Galapagos Is.) may be distinguishable from frazari only on the basis of locality;" (p. 499). From this fact he deduced that each population had gone through ancient periods of hybrid swarms, or in the case of galapagensis, a Middle American ancestor had been "derived from a hybrid population" (p. 499).

My point is that ancient hybridization and introgression, occurring millions of generations ago, can be expressed currently from the archaic genes in the genome. For the Galapagos AMOY, that must have occurred more than 3-5 million years ago, as the islands are geologically "new" and didn't appear until about then. The Baja Peninsula split from the mainland about 12-15 mya, providing an overlap with BLOY for millions of years. We cannot assume that the current plumage of any oystercatcher represents recent genetic hybridization — it could well be an expression of ancient hybrids. Surely there is something like a "statute of limitations" to be applied to archaic expressions of long-distance gene flow?

The results of Jehl's study are summarized in the Abstract to his paper (below left), and shown nicely in figure 7 (below right, from p. 497)

The ranges of the Black Oystercatcher (Haematopus bachmani) and the American Oystercatcher (Haematopus palliatus frazari) overlap for approximately 480 km along the Pacific coast of Baja California. Phenotypes are variable, especially in coloration, and hybridization has long been known. Yet, at most localities parental morphs predominate, hybrids are uncommon, and assortative mating occurs when possible. The historical record is sufficiently detailed to allow demonstration of changes in the hybrid zone. At the turn of the century — or whenever they were first studied — most populations were composed largely of parental morphs. After intensive collecting, resulting in the virtual extermination of some populations, recolonization occurred from both north and south. The reconstituted populations differed from the original ones, and by the 1920s and 1930s the frequency of hybridization was high. Recently, however, many populations have returned to their original composition. The prevalence of parental forms, a demonstration of assortative mating, and the resumption of stability in the zone of hybridization after a period of dynamic change, all show that there is selection against hybridization in this zone of secondary contact and that the two forms are specifically distinct. The nature of the selective forces remains to be determined.

Figure 7 shows the scores of about 270 oystercatchers within the range of BLOY-AMOY overlap, which is approximately the northern half (300 miles) of the Baja Peninsula. Jehl (pp. 497-498) says the figure provides evidence of mate preference in the zone of overlap: "If matings occurred at random, the result would be a hybrid swarm and the distribution on character scores would approximate a normal distribution [meaning, a standard bell curve graph]. The opposite is the case (fig. 7). The distribution is strongly bimodal, clustering near "pure" scores for each of the parental forms; intermediate scores are poorly represented. I interpret this to mean that assortative mating is prevalent" [emphasis added].

You will recall that Jehl's character index, established from just northern Gulf of California AMOYs, had a mean of 33.6. Later (p. 499), he states that if scores from southwestern Baja to southern Mexico are included, the "mean scores vary from 28.3 to 38.6 [overall range 20-42], suggestive of hybridization and introgression with bachmani to the north and palliatus to the south." Those areas are all outside the overlap zone in northern Baja, where there was active hybridization in the 1920-1930s as populations from the north and south took over empty territories after the extirpation of oystercatchers from that zone. Thus, such a revised index would include expressions of ancient hybridization but exclude those from 100 years ago. I've moved the "mean" on a revised figure 7 [right] with a new AMOY mean set at 28 (red arrow) and a bell curve distribution for each bimodal distribution (blue box for BLOY, red box for AMOY. I suggest that these boxes better represent an index for hybridization than the Jehl chart comparing Gulf of California birds to overlap zone.

It is apparent in my comments in 1993 that, at that time, I was a fan of Jehl's character index, and possibly thought it actually indicated what birds were likely to be hybrids. Yet he scored the specimen — which is much easier than scoring photos — at 27. His acceptance suggests, to me, that his "character index" border at 30 was meant for analyzing his Baja data for his 1985 paper, and not an effort to try to identify "pure" frazari, whatever that may mean.
      The current Monterey County bird (right) has been independently scored six times: 3 by observers (including a scoring that I did with my wife Rita Carratello), and 3 outside experts who used a wide selection of photos. The scores were (low to high): 22, 24, 23-25, 26.5, 29, 29. Let's throw out the low score and one high score (i.e., we'll throw out my score), and let's restate the score that has a range (23-25) as 24, and the one with a partial score (26.5) to adjust downwards to this set of four: 24, 24, 26, 29. The average of those scores is 25.7. It is on the darker side of the ledger but is within about 2.5 points of Jehl's lowest mean (28.3) for frazari's full range (excluding northern Baja). The most prominent differences among scorers related to width of wing stripe, thighs, and undertail coverts — parts of the anatomy that can be hard to review in photographs — and further illustrate why the Jehl scale was originally intended to apply primarily to specimen review.

About midway down the Baja Peninsula "the predominantly rocky shores and islands of western North America give way to the light colored sandy beaches of southern Baja California. It is in this region that the ranges of the two oystercatchers overlap;" (Jehl, p. 491). There are many barriers to producing hybrid young in today's conditions. Oystercatchers lay eggs in nests on the ground. To avoid mammalian predators, nearly all nests are on islands or islets off the mainland.

Jehl explains some problems faced: "First, the number of nesting sites on some islands is limited, oystercatchers are long-lived, and pairs remain intact for many seasons, perhaps for life. Thus, it is probably difficult for young birds to gain a territory and to enter the breeding population near their natal site.
     Second, because the reproductive life of an oystercatcher is very long, it follows that annual production of breeders or survivorship of non-breeders is low. And, because pair bonds are persistent, generation times are long, the opportunities for backcrossing and introgression are reduced, and selection to establish or reinforce potential isolating mechanism acts slowly. . .
     [In addition], there is a general correlation between substrate color and adult phenotypes, although it is obscured on [some islands]. Oystercatcher chicks are polymorphic, being dark-colored in bachmani and pale with dark marking in frazari (Jehl 1968). In broods of mixed parentage some chicks will not be cryptically colored (see Bancroft 1927). The probability that those will be quickly discovered by Western Gulls (Larus occidentalis) seems high;" (pp. 497-498).
     Copied here is the photo in Bancroft (1927), showing two fledglings from one nest at Scammons Lagoon in 1926. He did not state the plumages of the parents, but these fledglings may well by F1 hybrids. After the photo, Bancroft collected these young.

So, in summary, my conclusion are:

• Isn't this surely an obvious hybrid from the "hybrid swarm" of oystercatchers in Baja California?
        Answer: No, there is no "hybrid swarm" at any recent time in Baja California. The most recent "swarm" was about 100 years ago. To quote Jehl's response to that question: "The opposite is the case" (Jehl 1985, p. 498).
• Can't one just use the "Jehl index," get a score, and label a specific bird by its score?
        Answer: You may, but the Jehl index was not devised for that purpose; was not devised from specimens in Baja California so does not address their plumage distribution; is quite subjective to use; and Jehl himself identified a specimen as an acceptable frazari to establish California's second record, despite a score in the "hybrid" zone of his index.
• Can't one classify a "pure" frazari from a high score on the Jehl index?
      Answer: Probably not. Jehl opined (p. 499) that "frazari [and several subspecies elsewhere in the world] are not classical subspecies but originated as hybrid swarms in zones of secondary contact." Even the race galapagensis, isolated in the Galapagos Islands, may have "shared a common ancestor [with frazari] and that the Galapagos residents were derived from a hybrid population in Middle America." Unless one finds a way to ignore ancient hybridization within a genome, there are no "pure" birds in any of these populations, including frazari.
• Is the Monterey bird a hybrid?
        Answer: Maybe, we don't know. There is no genetic evidence one way or the other. It is probably not a recent hybrid, and is unlikely to be an F1 hybrid or the result of any recent mating. It shows many characters that have long been embedded in the frazari genome, which establishes genetic introgression at some point in the past in this bird's family history, but that could have been hundreds or thousands of years ago. If your preferred definition of a "hybrid" includes ancient hybridization stretching back into millennia, you will prefer to call it a "hybrid." In fact, as discussed above, you should probably consider the entire oystercatcher population within the range of frazari as "hybrids." By that criteria, no pied oystercatcher from Baja should ever be accepted in California.
• Is the Monterey bird a frazari American Oystercatcher?
        Answer: It is surely an oystercatcher hatched within the known range of frazari, and very likely from Baja California. Whatever it is, it is a rare vagrant to central California. Jehl found that in the zone of overlap in northern Baja, and after decades to recover from the over-collecting there, there was a "high frequency of assortative mating ... as well as a low frequency of hybrids." I think the Monterey bird's plumage score likely expresses the standard frazari genome with its long history of archaic hybridization. Because hybrids are scarce, and there are many barriers to produce a hybrid, the rule of parsimony suggests to me that this is most likely a normal darkish-end frazari. The likelihood of it being a recent hybrid is comparatively small.

So what do you do when a named taxon, defined by a suite of characters that suggest a long history of hybridization in the distant past, goes through another period of active hybridization a hundred years ago, but has re-established assortative mating and isolating mechanisms that select against hybrids? What if the characters that might suggest recent hybridization are the same characters for which the taxon was defined centuries ago? I suggest that the most reasonable approach is to accept "it is what it is." One would try to weed out F1 hybrids or clear intermediates (e.g., an all black oystercatcher with white vent and partial white wingstripe; there are photos of birds like this on-line from Baja). Otherwise, if the oystercatcher is clearly on the AMOY side of the ledger, with dark smudging invading the white areas "to a variable degree, especially on the rump, undertail coverts, wing lining, margin of chest band . . and having the amount of white on the primaries reduced or absent," as described by Brewster (1888) and Murphy (1925), I'd identify it as frazari and be content. The amazing fact is that it reached California from a distant and definable population. I don't see the point in trying to select some "whitest" end of a cline described long ago.

I was going to discuss the 11 extant species oystercatchers (6 pied, 4 black, and one with pied and black morphs), but this is too long already. You can find a review of the Oystercatcher family [Haematopodidae] on this web site.

Finally, for fun, here are two famous California records of American Oystercatcher. First, an adult frazari on Anacapa Island, Ventura Co., present from 24 May 1964–19 Oct 1980 (below left; photo in spring 1975 © Dave Rudholm), that remained for 16 years (!) and represents the long-staying of all State AMOYs. Second, a photo of two of 3 juv American Oystercatchers at Salton City, Imperial Co., from 14-19 Aug 1977, and then at Salt Creek, Riverside Co., from 20-30 Aug 1977 (below right, photo © Jeri M. Langham), that are the only interior record for the State. Patten et al. (2003) reviewed photos of the Salton Sea birds, and suggested that these birds may have been nominate H. p. palliatus, perhaps from the Gulf of Mexico. If all California records of frazari were rejected on grounds of ancient hybridization, these would represent the only State record.

Acknowledgments: I thank Curtis Marantz for providing a copy of Jehl (1985) for this study. I'm grateful to those who provided expert comments, discussion, or scoring of oystercatchers for this project — thanks to Bonnie Burgess, Rita Carratello, Curtis Marantz, Joe Morlan, Andrew Spencer, and Brian Sullivan. Rita Carratello and Michael Rieser graciously read an earlier version of this page, and suggested many corrections. I thank Jeri Langham and Dave Rudholm for the old CA photos, and thank all the photographers of the Monterey individual. Some particularly useful photos were provided by Bonnie Burgess, Terence Degan, John Drum, Bill Hill, Carole Rose and Brian Sullivan; quite a number of these are posted in eBird. I also reviewed photos from eBird of oystercatchers in Baja California and Baja California Sur, and appreciate the efforts of those photographers. The opinions expressed here are mine, and not necessarily shared by those acknowledged here. There is a wide variety of opinions on these topics.

Literature cited or consulted:

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California Bird Records Committee (R.A. Hamilton, M.A. Patten, and R.A. Erickson, eds). 2007. Rare Birds of California. West. Field Ornithologists, Camarillo, CA.

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