Monday, November 5, 2007

"Great White" Heron - not just a color morph

Great White Heron Ardea herodias occidentalis

updated 13 Nov 2007, thanks to all those who have commented publicly and privately. I've backed off a bit from my criticism of the TBRC decision, the more I learn the less clear-cut this seems, although I still think it's at least a good subspecies. Shaibal Mitra sent me a copy of a paper he and John Fritz published in the Kingbird a few years ago, which reaches the same conclusion that Great White Heron is a distinctive subspecies, but points to my book as one of the sources unfortunately labeling the Great White Heron "simply a color morph". Oops, I guess it does. That's not quite what I meant!

This post is about the debate over whether the "Great White" population of Great Blue Heron is "simply a color morph" (TBRC 2006, Butler 1992), a subspecies (Mayr 1956, Meyerriecks 1957), or a full species (McGuire 2002). A few days ago in the first draft of this post it seemed clear-cut, now with additional information from many sources it seems less so. Much of what I've written here has been said before by Mitra and Fritz (2002) and by Tony Gallucci in 2004 on TexBirds here.

Butler (1992) dismisses the white population with almost no discussion, and unfortunately I labeled this the "white morph" in my field guide (Sibley 2000) even though I recognized that it was more than just a color morph. The Texas Bird Records Committee (TBRC) decided in 2006 to drop "Great White" Heron from the state review list, saying that it seemed to be just a color morph and not a distinct subspecies. This decision was apparently prompted by two records of white nestlings in Great Blue nests in Texas - an old photo from Galveston County (presumably from McHenry and Dyes, 1983) and an unpublished 2006 photo from Aransas County showing a white and dark nestling together in a nest tended by two dark adults!

I am fascinated by these records of white nestlings in Great Blue nests in Texas, but I disagree with the TBRC decision. I have always considered Great White Herons distinctive and I can't accept that this is "simply a color morph". Mayr (1956) did some actual research to confirm that "The Great White Herons are not merely albino specimens of Ward's [Great Blue] Heron, but form a mangrove population in the Key West area which differs from Ward's Heron on the mainland not only by the white coloration, but also by shorter plumes and an average larger bill." (some nice Great White photos are here).

Mayr (1956) and Meyerriecks (1957) studied the white and dark herons of south Florida and found mixed pairs, no clear differences in behavior, and subtle differences in morphology. Zachow (1983) found that measurements of Great Whites are significantly larger than Great Blues from the Florida peninsula, which in turn are significantly larger than Great Blues from farther north. Mayr and Meyerriecks both argue that the "Great White" Heron is not a separate species, but they never question the fact that it is a valid subspecies.

Looking at the measurements from a field ID perspective, however, suggests that they may not be as diagnostic as has been assumed. The following graph shows Mayr's bill/wing data in graphic form. Obviously there is lots of overlap between Great White and Ward's Great Blue from the Florida peninsula, even though there is enough difference for most birders to take away the impression that the Great White is a "much larger-billed" bird.
McGuire (2002) in a more detailed study actually does suggest that "The great white heron appears to be a good biological species". McGuire found that although some mixed dark-white pairs occur in the Florida Keys, there are fewer than would be expected by chance. DNA analysis suggests that the herons of Florida Bay and the Keys are isolated to some extent from the Great Blue Herons of the Florida Peninsula. [McGuire suggests that one possible isolating mechanism is time of breeding, with the peak of nesting in the Keys from October to April, and the nesting season on the mainland beginning in Feb-Mar].

The map below shows the breeding range as recorded in the Florida Breeding Bird Atlas. I added the green color to show the Great White records. Note that the green dot far north on the Gulf Coast represents a solitary Great White among Great Blues. The red dot at the upper end of Key Largo might represent one or more nests of true Great Blue Herons or an intermediate "Wurdemann's-type". Interesting to note on this map is the small but obvious gap between breeding Great Blues and Great Whites.

One of the most interesting facets of this is that the dark birds in the keys are intermediate in plumage and known as "Wurdemann's Heron". These are found only in the Florida Keys with Great White Herons, and according to McGuire, Mayr, and Meyerriecks all of the dark birds breeding in that area are typical of "Wurdemann's" rather than the mainland subspecies of Great Blue Heron. So when researchers in the Keys report dark-white pairs and also dark-dark pairs with some white offspring, the dark birds are "Wurdemann's" and not typical dark mainland Great Blues. Among nesting colonies in Florida Bay and the Keys, white birds (Great White) outnumber blue (Wurdemann's) about 4:1 (McGuire 2002).

McGuire shows that "dark" birds in the keys are slightly smaller than white ones, but not significantly, and emphasizes that color of dark birds varies continuously from Great-Blue-like but (always?) with more white on the head (photo here) to mostly white with pale gray wings and back, so that it is not possible to classify the non-white birds into subgroups. In size measurements and in DNA the dark birds of the Keys are slightly but not significantly different from Great Whites, but they are significantly different from the mainland Great Blues (McGuire 2002). McGuire takes the color and size difference as evidence that "Wurdemann's" are intergrades, but it would be helpful to know if measurements are correlated with size. That is, are the birds with the most Great-Blue-like plumage in the keys also the smallest? Assortative mating supports the intergrade hypothesis.

I may not go so far as to endorse McGuire's view that the Great White Heron is a separate species, but there does seem to be plenty of evidence that this population is distinctive and at least somewhat isolated. A vagrant outside of the normal range should be identifiable with a high degree of certainty, and Great White and "Wurdemann's" can be reliably distinguished from albino Great Blue Herons.

Birders in Texas and elsewhere should be encouraged to watch for this distinctive subspecies, and the Texas Bird Records committee should put it back on the state review list. That of course reopens the question of what to make of the white nestlings photographed in Texas. They should not be accepted as "Great White" Herons just because they're white. Similarly, their mere existence does not negate the distinctiveness of true Great Whites from the Florida Keys. The true status of those white nestlings will have to remain a mystery for now, awaiting further study.

It is interesting that white nestlings have been found twice in Texas but full-grown white birds have been seen very rarely there, and only as brief visitors. We still don't know what these white nestlings look like as adults.

Have white nestlings been found elsewhere in Great Blue nests?

White morph Great Blues are also said to occur in Cuba, Jamaica, the Yucatan, and off Venezuela but are apparently smaller than the Keys birds and scarce (not a majority). What do these birds actually look like and what is their status?

Just how big and short-plumed are Great Whites? I didn't do a thorough search but couldn't find a good set of published measurements. I found no published measurements of head plumes, only the repeated assertion that Great White has shorter plumes. So I can't confirm the identification features, only that I have the impression that Great Whites are distinctive, and should be more distinctive the farther one gets from Florida (as the size of Great Blues decreases clinally).

Does it make more sense to consider the variable "Wurdemann's" Heron as an intergrade swarm, or simply as the dark morph of Great White Heron - making Great White a dimorphic, large, short-plumed subspecies of Great Blue Heron?

There are isolated records of Great White Heron nesting north to the Tampa area (Bancroft, 1969; Florida Breeding Bird Atlas map), and nonbreeders wander regularly to northern Florida (not mapped) and less often but still regularly to coastal Georgia.

This map shows the resident range (purple), distribution of vagrant records (green), and general areas of reported occurrence outside the US (yellow). The two red dots represent multiple records at a single location, which might be more likely to represent color abnormalities of local Great Blues rather than wandering Great Whites (Pymatuning Lake, PA: three birds in 1938 and another in 1961); South Holston Lake, VA/TN: single bird in fall 1990, 1991, 1994, and 2002). But in general the distribution of records appears consistent with a south Florida origin. On the other hand, Marshall Iliff (pers. comm.) points out that this is a surprising number of vagrant records given that the total breeding population of Great White Heron is under 1000 breeding pairs.
Aberrant "Wurdemann's-like" herons:
A bird photographed in Washington County, PA in 2004 and present every year since then is clearly not a "Wurdemann's" Heron, and likely a Great Blue x Great Egret hybrid.

Another odd bird photographed in MA in Sep 2005 was clearly a leucistic Great Blue based on size and plumage details, and not a "Wurdemann's". (Thanks to M. Rines for the photo)

Bancroft, G. 1969. A great white heron in great blue nesting colony. Auk
86:141–142. pdf here

Butler, Robert W.. 1992 . Great Blue Heron (Ardea herodias), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online

Mayr, E. 1956. Is the great white heron a good species? Auk 73:71–77. pdf here

McGuire, H. L. 2002. Taxonomic status of the great white heron (Ardea herodias occidentalis): an analysis of behavioral, genetic, and morphometric evidence. Final Report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA. pdf here

McHenry, E. N., and J. C. Dyes. 1983. First record of juvenal “white-phase”
great blue heron in Texas. American Birds 37:119.

Meyerriecks, A. J. 1957. Field observations pertaining to the systematic
status of the great white heron in the Florida Keys. Auk 74:469–478. pdf here

Mitra, S. S. and Fritz, J. (2002) Two Great White Herons (Ardea (herodias) occidentalis) in NewYork,Sept-Nov 2001.Kingbird 52 (1):27-34.

Sibley, D. A. 2000. The Sibley Guide to Birds. Chanticleer Press.

Texas Bird Records Committee. 2006. Minutes of Annual Meeting.

Zachow, K. F. 1983. The great blue and great white heron (Aves: Ciconiiformes: Ardeidae): a multivariate morphometric analysis of skeletons. Thesis, University of Miami, Coral Gables, Florida, USA.


cyberthrush said...

I would think this is the sort of case where one would want to lean on DNA evidence as much as possible (physical features can be deceptive in their differences, and subspecies could have distinctive physical features without being fully separate species). You state that "in DNA analysis the herons of Florida Bay and the Keys cluster apart from the Great Blue Herons of the Florida Peninsula" -- can you expand on that at all, or what sample sizes are involved? "Cluster apart" is a little vague (any overlap?) -- it may simply be that the current DNA analysis is too weak to be very helpful, but maybe it's clear enough to strongly point in the direction you're suggesting.

Chris said...

There are several records from Tennessee, including several with photo documentation.

David Sibley said...

Thanks for the comments. I've made some changes to the post in response. There is a lot about the DNA work in McGuire's paper, and I'll simply refer you there for all the details. In my quick read I think it shows that the Florida Keys herons are genetically isolated to some extent, and supports the same pattern of variation that the measurements show - the white and dark herons from the Florida Keys are slightly but distinctly different from the dark birds of the Florida peninsula, which are different from Great Blue herons farther north.

I just read what I think is a helpful metaphor in the book "1491" by Charles Mann. Quoting Hugo Perales (about variation in Maize) "The varieties are not like islands, carefully apart. They are more like gentle hills in a landscape - you see them, they are clearly present, but you cannot specify precisely where they start."

I'd say that across most of North America we have a rolling or steady incline of subtle variation in Great Blue Herons, punctuated by this sudden conspicuous peak in the Florida Keys. Whether the Great White Heron is more like an island or a hill, my main point is simply that it is "clearly present".

Tony Bledsoe said...

If I remember correctly, Payne and Risley published a fairly extensive morphometric analysis of Ardeidae. It might be worth checking to see if they looked at Great White Heron specimens. The McGuire article probably cites the paper.

Floyd Hayes said...

David, thanks for posting the McGuire (2002) pdf file. I'm up to my neck with academia and haven't had a chance yet to read the entire article by McGuire (2002), but the abstract sums it up nicely (assuming the conclusions are supported by the data within the paper):

"I observed more white/white and blue/blue pairs and fewer mixed pairs than expected in a randomly mating population, suggesting that premating isolating mechanisms exist within the Florida Bay breeding population... The great white heron appears to be a good biological species."

So we have two sympatrically breeding morphs that differ in more than one morphological trait, plus they differ genetically as well. But most importantly, and this trumps all else in my opinion, they mate assortatively. If the birds do not view each other as a randomly mating, freely interbreeding population, why should we? As far as I'm concerned they're different biological species.

By the way, I like the "comprehensive biologic species concept (CBSC)" proposed for birds by Johnson et al. (1999:1478): "An avian species is a system of populations representing an essentially monophyletic, genetically cohesive, and genealogically concordant lineage of individuals that share a common fertilisation system through time and space, represent an independent evolutionary trajectory, and demonstrate ESSENTIAL BUT NOT NECESSARILY COMPLETE REPRODUCTIVE ISOLATION [emphasis supplied] from other such systems."

Here is the citation:

Johnson, N. K., J. V. Remsen Jr., and C. Cicero. 1999. Resolution of the debate over species concepts in ornithology: a new comprehensive biologic species concept. Pp. 1470-1482 in Adams, N. J., and R. H. Slotow, eds. Proceedings of the 22nd International Ornithological Congress, Durban. BirdLife South Africa, Johannesburg.

David Sibley said...

Klaus has posted on this on his blog, with great photos of all three types - Great Blue, Great White, and Wurdemann's.